Selective electrochemical reduction of CO2 into energy-dense organic compounds is a promising strategy for using CO2 as a carbon source. However, efficient and selective earth abundant metal catalysts for the two reactions typically required for efficient overall CO2 electrolysis, namely the oxygen evolution reaction (OER) and CO2 reduction, are still scarce. We will present here an array of strategies inspired from biological systems to promote these reactions with high selectivity and efficiency. The importance of tackling these challenging reaction at multiple scale will be illustrated by a series of molecular and heterogeneous catalysts replicating enzymatic features, from their active sites including secondary features such as hydrophilic and hydrophobic domains to the overall shape of organs involved in gas trapping.[1-7]                                                                              

The direct CO₂ electrochemical reduction reaction (CO2RR) is a potential technology to convert waste CO₂ streams into valuable chemicals, using renewable electricity as a driving force. During this process a variety of carbon-based products such as CO, HCOOH and hydrocarbons can be formed. The selectivity of the CO2RR is determined by the nature of the catalyst interphase, which is dependent on the catalyst material, its morphology and the working electrolyte, among other factors. Therefore, it is necessary to get a holistic approach to fully understand the CO2RR and to get optimal catalytic results.

For this contribution we look at the influence of proton concentration on the selectivity of carbon based catalysts. Unlike hydrogen or methane generation, CO production has been shown to be independent of pH concentration on the NHE scale. Therefore, the proton concentration can be used to tune the CO/H₂ ratio. In aqueous electrolytes, for instance, the selectivity towards CO on Fe nitrogen-doped carbon is clearly enhanced at neutral pH whereas acidic conditions favored methane and hydrogen production. [1]

For this contribution we studied the CO2RR in aprotic media aiming to suppress the competing hydrogen evolution reaction (HER). We first studied a pure carbon electrode which in aqueous electrolyte selectively produced H₂, yet traces of CO were also obtained. By contrast, in an aprotic electrolyte (0.1M NBu4PF6 in acetonitrile) CO was obtained with a faradaic efficiency higher than 90%. Density functional theory (DFT) simulations confirmed this is attributed to the absence of protons, showing that certain carbon defects can reduce CO₂ into CO in both media. Nevertheless, carbon sites are predicted to be more active towards the HER and thus in aqueous media the formation of H₂ is the predominant process.

While it is remarkable that the pure carbon can selectively reduce CO₂, the process takes place at  high over potentials, this can be improved by intruding dopants. Metal nitrogen doped carbons, in particular, have been shown to be highly active towards the CO2RR in aqueous media. [2] Working in a non-protic electrolyte, resulted in suppression of both the HER and the CO2RR, as both reactions require protons. Nevertheless, the presence of small amounts of water resulted in a clear enhancement of the CO2RR while keeping a low HER activity. These results highlight the importance of proton concentration on determining the reaction selectivity. On one hand H^₊ concentration has to be limited to suppress the competing process of the HER. However, protons are also necessary to carry out the reduction of CO₂ into CO and methane. 

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO₂ emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO₂-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen- and sulfur- based reactants as products from water/CO₂ electrolysis do not cover the full scope of industrial needs.

To this end, this talk focuses on my lab’s efforts in the co-electrolysis of CO₂ with additional small molecule reactants (NH₃, NO₃^-, SO₃^2-…) in generating products with C-N and C-S bonds like amides, urea, and sulfonates that are important as commodity and fine chemicals in the chemical industry.  In particular, I will discuss several new reaction pathways discovered towards several of the above-mentioned products and the application of operando techniques to understand the key C-N/C-S coupling steps in the reaction process.

Electrocatalytically driven CO₂ reduction reaction (CO₂RR) to produce alternative fuels and chemicals is a useful means to store renewable energy in the form of chemical bonds. in recent years there has been a significant increase in research efforts aiming to develop highly efficient CO₂RR electrocatalysts. Yet, despite having made significant progress in this field, there is still a need for developing new materials that could function as active and selective CO₂RR electrocatalysts.

In that respect, Metal–Organic Frameworks (MOFs), are an emerging class of hybrid materials with immense potential in electrochemical catalysis. Yet, to reach a further leap in our understanding of electrocatalytic MOF-based systems, one also needs to consider the well-defined structure and chemical modularity of MOFs as another important virtue for efficient electrocatalysis, as it can be used to fine-tune the immediate chemical environment of the active site, and thus affect its overall catalytic performance.

Our group utilizes Metal-Organic Frameworks (MOFs) based materials as a platform for imposing molecular approaches to control and manipulate heterogenous electrocatalytic systems. In this talk, I will present our recent study on electrocatalytic CO₂RR schemes involving MOFs, acting as: a) electroactive unit that incorporates molecular CO₂RRelectrocatalysts, or b) non-electroactive MOF-based membranes coated on solid CO₂RR catalysts (see illustrative TOC figure).

The electrochemical CO 2 reduction reaction (CO 2 RR) is an attractive method to produce renewable fuel
and chemical feedstock using clean energy sources. Although copper (Cu) is a well-known electrocatalyst
for the electroreduction of CO 2 to multiple hydrocarbons, it suffers from poor selectivity, efficiency, and
stability. In this work, a novel bimetallic Cu-Pd alloy anchored on two-dimensional Ti 3 C2 Tx -MXene
nanosheets was further converted to highly conductive three-dimensional Cu-Pd/MXene aerogel, which
acts as an excellent electrocatalyst to reduce CO 2 to formate with high selectivity of 93% and current
density of 150 mA/cm 2 . Compared with Cu-Pd aerogel, the newly designed Cu-Pd/MXene aerogel could
effectively decrease the overpotential and enhance selectivity and current density with great stability
during a long-term reaction. This simple strategy represents an important step toward experimental
demonstration of 3D-MXenes-based electrocatalysts for CO 2 RR application and opens a new platform for
the fabrication of macroscale aerogel MXene-based electrocatalysts that can be used for a range of
applications, including nanocomposites, electronic devices, and all-liquid microfluidic devices.

The electrochemical reduction of CO₂ (CO₂RR) with Cu-based catalysts has a rather poor selectivity towards C₂₊ molecules, such as ethylene and ethanol. Up to 16 different products can typically be formed during the process. [1] However, copper is the only known element with the ability to make C-C bonds and produce the desired C₂₊ products.[2] Therefore, it is key to gain a better understanding of the active sites of Cu-based electrocatalysts in order to improve the design and consequently the selectivity towards molecules such as ethylene and ethanol. This can be done, for example, by using well defined surfaces such as single crystals, providing insights into the reactivity of different exposed planes. It has beed shown that  ethylene is mainly produced on Cu (111) facets while a higher selectivity for methane was observed for Cu (100) facets. [3],[4]

In this study, cubic and octahedral Cu₂O nanocrystals predominantly exposing (100) and (111) facets were synthesized, respectively. Polyvinylpyrrolidone (PVP) was added during the synthesis as a capping agent to delicately control the ratio of (111) over (100) planes, yielding different shape Cu₂O nanostructures. [5] The presence of preferential cubic or octahedral shapes on the different synthesis was also confirmed by X-ray poweder diffraction (XRD), Moreover, the ratio of diffraction intensity for the (111) and (200) reflections showed an increase for the octahedral structure. Additionally, the synthesis procedure allowed the control of the size of the nanostructures between 400 and 800 nm as confirmed by SEM. Chronoamperometric measurements revealed that the cubic Cu₂O nanocrystals are more selective towards ethylene compared to the octahedral shaped particles. The cubic Cu₂O nanocrystals achieved a Faradaic efficiency for C₂₊ products of approximately 60% at a potential of -1.0 V vs. RHE. These results are in agreement with the observations on Cu single crystals, showing that the surface structure dependence for CO₂RR is maintained for Cu₂O catalysts.

Our sustainable future requires finding affordable and green routes to prepare nanostructured materials for more efficient energy conversion reactions. Compared to bulk materials, nanocatalysts show improved electrocatalytic activity due to their high electrochemically active surface area (ECSA) combined with reduced loadings of noble metals. Metal electrodeposition in deep eutectic solvents (DES) has emerged as a versatile, green and affordable alternative to prepare bimetallic nanostructures. DES are non-toxic solvents which show wide potential limits, enough conductivity, high solubility of the majority of the metals and do not require surfactant agents or additives for controlled growth of the nanostructures.^[1] In this work, we investigate the preparation of high surface area nanostructures of Cu-Au^[2] and Pd-Au^[3] with tunable structure and composition by electrodeposition in choline chlorine plus urea DES. We combine electrochemical methods with microscopy and spectroscopy techniques to characterize the electrodeposited nanostructured materials. We assess the increase of the electroactive surface area through the analysis of metal underpotential deposition (UPD) on the prepared films and, CO stripping on the Pd-Au bimetallic films, to investigate the effect of the Pd loading on the electrochemical response. Finally, Pd and Pd-Au nanostructures were tested for formic acid oxidation in comparison to extended polycrystalline surfaces. Overall, I show a simply and eco-friendly method to prepare mono and bimetallic nanostructures for different electrocatalytic reactions.

Global energy demand is increasing exponentially and the main source of energy is still based on fossil fuels, which are scarce and responsible for the majority of carbon dioxide (CO₂) emissions into the atmosphere. Green hydrogen has emerged as a promising energy vector to respond to this increasing energy demand and to decarbonize transportation, heating and fine chemicals sectors. On the other hand, electrochemical CO₂ conversion into energy-rich fuels and chemicals has gained significant interest as a potential strategy for simultaneously mitigating increasing global CO2 concentration.

Here we present a cost-effective copper-based electrocatalyst to evolve hydrogen with faradaic efficiencies close to 100%. Moreover, this electrocatalyst in presence of CO₂ is able to reduce it in formic acid with faradaic efficiencies close to 70%. Beyond the easy synthesis of this electrocatalyst and its high efficiencies for hydrogen evolution (HER) and CO₂ reduction reaction (CO₂RR), what really makes this material interesting is its behavior during the electrocatalysis. This copper-derived electrocatalyst enhance its currents as a function of time when evolves hydrogen increasing too the active surface area almost proportionally. While in its application for CO₂ reduction, its activity was studied when the electrode is reused several times, showing an increasing selectivity trend toward formic acid in each use.

Integrating the alkaline capture of CO₂ from the air with the electrochemical conversion of the obtained (bi)carbonate solution in one and the same system is among the most promising strategies in the field of Carbon Capture & Utilization (CCU) technologies. Thus far this approach has received little or no attention as a consequence of the challenging conversion of CO₂ from bicarbonate solutions as it mostly results in low Faradaic Efficiency (FE) and partial Current Density (CD) towards carbon products during the electrolysis and owing to the parasitic hydrogen evolution reaction (HER). Very recently, thanks to the advances in reactor design and in the understanding of the mechanism of bicarbonate electrolysis, promising results were obtained in terms of performance (i.e., >60% FE towards formate or CO at >50 mA cm^-2) [1] and as such provided us with the required knowhow to for the first time, construct and validate a proof-of-concept experimental setup where CO₂ is captured from the air with KOH, in the form of a 0.7 M (bi)carbonate solution, through Direct Air Capture and then converted to formate or CO in a zero-gap flow electrolyzer using a bipolar membrane in a Membrane-Electrode Assembly (MEA). The presented results provide a new opportunity towards upscaling the electrochemical conversion of CO₂, since integrating the capture and the conversion steps is a crucial step to enhance the economic feasibility of the CCU technology (energy-intensive CO₂ separation can be avoided) and thus increase its chances of industrial implementation.

The electrochemical reduction of carbon dioxide (CO₂) to hydrocarbons provides storage of renewable electricity into energy dense molecules while also neutralizing CO₂ emissions. Last decade has shown an unprecedented development towards its industrial application by reaching CO₂ reduction reaction efficiencies (>95 % for CO and 87 % for C₂H₄) thanks to the fine engineering of novel catalyst and membrane types. In chorus, the gas diffusion electrodes (GDE) and flow-type reactors have played a key role to overcome mass transfer limitation and steer the current densities up to industrially relevant values, e.g. ~1 A/cm².

Currently, most of the gas diffusion layers forming the backbone of the GDEs have been adopted from the proven processes such as fuel-cell and chlor-alkali systems. However they are not ideal for the large scale implementation and long term stability of CO₂ electrolysis due to the diverse nature of the reaction. In this work, we will show the main advantages and limitations of the three most common gas diffusion layers employed in the field of CO₂ electrolysis which are; nonwoven-, paper and cloth-GDEs with an identical catalyst layer obtained by sputtering of 150 nm thin copper film. By using an operando IR-thermography technique (filed patent), we mapped the temperature distribution profile of the GDEs until their failure by flooding. Thermal imaging results showed a smaller gradient and uniform temperature profile along the Cloth-GDL, which was attributed to the continuous fiber structure forming a straight electron path. On the other hand, the agglomerated PTFE flakes, large pockets and cracks of Paper-GDL were found as failure points to flooding by electrowetting and displayed a resistance to the electron path, evident from the higher potential and temperature value at elevated current densities. Conversely, very compact form of carbon nanoparticle and fiber structure of Nonwoven-GDL underwent premature flooding by the salt deposits and blocked CO2 diffusion to the catalyst layer. Coupled with an advanced ex-situ electron microscopy for post-mortem analysis , we have localized and matched the failure mechanisms of each GDE with a focus on the changes observed in the operando analysis during an hour long electrolysis at 250 mA/cm². Our results will provide the strategies for inspection and improvement for better performing GDEs which would be beneficial not only to CO₂ electrolysis but also to the upscaling of electrical energy conversion and storage devices.

The field of CO₂ electrolysis witnessed rapid development in recent years. At the same time, the role of the anodic half-reaction has received considerably less attention. Iridium is almost exclusively used as the anode catalyst in polyelectrolyte membrane water electrolyzers. This practice quickly became a standard in CO₂ electrolysis due to its similarity to water electrolysis (oxygen evolution - OER is the anode process in both cases). An alkaline electrolyte solution is typically recirculated in the anode compartment to ensure a high reaction rate during CO₂ electrolysis. However, iridium is known to slowly but continuously dissolve in alkaline solutions. Additionally, while using iridium as the anode catalyst on a laboratory scale is acceptable, economic reasons urge the exploration of non-noble alternatives.

In my presentation, I am going to discuss alternative anode electrocatalysts that have been tried already in CO₂ electrolyzers. A closer look will be taken at Ni, which should be stable at least under the initial process conditions. We found that while Ni showed high activity (similar cell voltage to that with iridium) at the beginning of our experiments, it instantaneously started to decrease over time. If the anolyte is recirculated (typical scenario), the continuous carbonate transport through the anion exchange membrane decreases the bulk pH of the anolyte. In the case of the zero-gap electrolyzer cells employed in our study, the carbonate ion flux directly hits the anode catalyst layer, causing a high carbonate ion concentration in the close vicinity of the electrode. In parallel, H⁺ ions formed at the anode as the result of OER are not neutralized instantly (unlike in water electrolysis), which leads to an acidic surface pH. These two effects together alter the local chemical environment at the surface of the anode electrocatalyst leading to a gradual dissolution of Ni.[1] Based on our measurements, a set of criteria will be described that have to be fulfilled by an anode catalyst to achieve high performance.

In the second part of my talk, I am going to provide an outlook on replacing OER at the anode with various alternative anode reactions (such as electrocatalytic alcohol oxidation) to deliver high-value products in parallel with increasing the energy efficiency for the whole process.[2] A closer look will be taken at issues regarding the selectivity and stability of these systems along with discussing possible mitigation strategies and future research directions.

Cu is the only monometallic catalyst that can reduce CO producing a large variety of valuable compounds. However, the efficiency and selectivity towards specific products is highly affected by the structure properties of the catalyst and the composition of the electrolyte.[1]

Herein we have investigated the effect of the pH and the anion specifically adsorbed on the surface, on the structure and properties of the Cu(111) and Cu(100)-aqueous electrolyte interface.[2–4] Experiments under potential control were conducted on well-defined Cu single crystalline electrodes, and in presence of CO, to assess the electrolyte and surface orientation effect on the onset potential for the CO reduction.[3] We specifically used cyclic voltammetry in combination with other electrochemical approaches such as the CO displacement technique and laser induced temperature technique, in order to shed lights into the distribution of the surface charge and adsorbed electrolyte species across the whole potential window.[2–4] We experimentally showed that the structure of the Cu-electrolyte interface, at different pH and in presence of different anions, controls the onset potentials of the CO reduction. Lastly, we have investigated lead underpotential deposition to assess the different facets and domains in nanostructured copper, which is relevant to design more efficient electrocatalysts for the CO reduction. [5]

Figure 1 shows the cyclic voltammogram (CV) of Cu(111) in contact with 0.1 M phosphate buffer solution at pH 5 and in presence of CO, recorded at a short potential window (blue line) and long potential window (red line), 50mV/s.

Efficient conversion of solar energy into molecules for energy storage and other purposes is one of the most important challenges for sustainable energy systems of the future. Particularly directly coupled photovoltaic (PV) assisted electrochemical (EC) processes, like e.g., PV-assisted water splitting, are of interest. In most practical cases, PV and EC devices are developed separately and then merged at a later stage to verify their performance as a PV-EC system. Enormous efforts of many groups in catalyst development produce increasing number of new catalysts every year. In turn photovoltaic community continues optimization of variety of PV technologies. In this situation, it would be useful to be able evaluate potential of one or other catalyst pair in combination with different PV technologies without building and optimizing an experimental device. To address this issue, we have recently developed a simple method to assess limit of solar-to-hydrogen efficiency (STH) of a specific electrolyzer based on the “reverse analysis” of its polarization curve [1]. We show that despite the complexity of the parameter space, there is a surprisingly simple way to estimate the limit of efficiency in any PV-assisted electrolyzer system.

The maximum STH for a specific electrolyzer is achieved when the current-voltage (IV) characteristics of the PV device crosses polarization curve of the EC device at maximum power point (MPP). Conversely, each point on the EC polarization curve can be considered the MPP of a PV device optimally coupled to the EC device. Therefore, at each point on the polarization curve, the minimum PV efficiency and maximum EC efficiency can be calculated for a specific irradiance. The product of both efficiencies generates the STH limit that can be attained at that specific point on the polarization curve. The result of the transformation is the dependence of the STH limit on the PV efficiency at the assumed irradiance level and optionally ratio of the active areas of PV and EC devices. This "reverse analysis," carried out with elementary math, does not involve any modeling or analysis of PV IV characteristics and provides a quick simple way to quantify the potential of any electrolyzer.

In our paper we present the principle of the reverse analysis using an example NiMo/NiFeO_X catalyst pair. Next, we show how using this analysis losses in an experimental PV-EC combination can be identified and quantified. We present extended set of results of reverse analysis applied to a variety of PV-EC combinations described in the literature. Finally, we discuss how this analysis can be applied to different electrochemical devices coupled to PV like e.g., PV-assisted CO₂ reduction.

TOC Figure. Principle of the reverse analysis: (a) at each point on the polarization curve, minimum PV efficiency and maximum EC efficiency are calculated; (b) the product of both efficiencies generates the STH limit at the specific point; (c) the result of the transformation presented as STH limit vs. PV efficiency.

Energy surplus, if not utilized, are dumped as waste energy in the form of heat, not to mention adding carbon footprint to our daily operations. Yet, developing countries struggle to meet their demand and distribution schedule of energy leading to rotational brownouts. Research on energy generation, storage, and release has been continuously rolling. Hydrogen, generated from water splitting, is postulated as one of the most promising alternatives to fossil fuels. In this context, direct hydrogen generation by electrolysis and fixation to graphene oxide (GO) in an aqueous suspension could overcome storage and distribution problems of gaseous hydrogen. Herein, approaches of hydrogenating graphene oxide were studied primarily by time-resolved Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR).

GO films, utilized as electrode for hydrogen evolution reaction with dynamic modulation of the potential difference, favoured 160% increase of C-H bond formation. Epoxide ring opening leading to hydroxyl groups suggests that these groups play a key role in hydrogenation. FTIR revealed characteristic -CH₂ and -CH₃ vibrations. This shows that hydrogenation is significantly also occurring in defective sites and edges of the graphene basal plane, rather than H-C(sp³). Partial reversibility was observed by in-situ Raman when applying a cyclic voltammetry caused by a reversible reaction present in the 0.34 to -1 V_RHE range.

Photocatalytic hydrogenation was also explored. Metal organic frameworks (MOF) of graphene oxide, Ni²⁺, Cu²⁺ were found to acquire -CH₂ and -CH₃ moieties upon irradiation by a 75 W xenon lamp with TiO₂ photocatalyst. The dispersing media and hole scavengers (water, methanol, and sulphite) were also found to affect the fixation of hydrogen in the (MOF). This method has demonstrated successful evolution of H-C(sp³) in the GO MOF whereas graphene-like structure (loss of oxygen functional groups) is produced when GO was subjected to the same irradiation technique.

One of the major limitations of renewables is their intermittent character which  depends on  climate conditions. The problem is a complete disconnection between energy production and demand. Consequently, the storage of energy surpluses from renewables becomes a necessity. To tackle this problem, in this work we propose an efficient way of energy storage based on using green hydrogen as an energy vector. The aim is to develop efficient systems for hydrogen storage in the form of chemical bonds based on Liquid Organic Hydrogen Carriers (LOHCs) [1].

From a chemical view, the key point of efficient LOHCs lies in the design of suitable catalysts and processes for the conversion and reconversion of hydrogen into chemical bonds. In this work, we describe our preliminary results in the field [2][3] and the last results in electrocatalytic dehydrogenation of amines and the catalytic hydrogenation of nitriles as efficient systems for hydrogen  storage in the form LOHCs.

The production of green hydrogen for water electrolysis is very promising for the development of a zero-carbon-emission energy structure. However, proton exchange membrane (PEM) electrolyzers, which operate in acidic media, are limited by the slow kinetics of the oxygen evolution reaction (OER) at the anode. Additionally, the anode requires high amounts of scarce precious metals like Ir as catalysts¹. An inherent need for developing preparation methods to decrease mass loadings and enhance activity and stability has therefore arisen. We introduce a feasible and easy way to prepare IrO_x catalysts by galvanic displacement of electrodeposited Co and Ni on polycrystalline Au surfaces. We performed the electrodeposition of Co and Ni on gold beads in a deep eutectic solvent (DES) – a non-toxic solvent with a broad potential window and high solubility of many metals². We fabricated Ir nanoparticles by employing galvanic displacement of Co and Ni in aqueous solutions of IrCl_4, and then electrochemically oxidized the metallic Ir. Our preparation of IrO_x catalysts replaces complicated plating methods by the galvanic displacement technique and replaces the use of surfactant agents during electrodeposition by a green DES electrolyte. We evaluated the activity and stability of the IrO_x catalysts with state-of-the-art Ir-based catalysts. The IrO_x-Ni displayed enhanced activity and stability in relation to the IrO_x-Co in line with previous reports³.

The requirement of the scarce element iridium for PEM electrolyzer anodes is a potential bottleneck in the transition to a fossil-free society. Replacing iridium is difficult due to the harsh conditions at the PEM anode of high potential and low pH. Only a handful of elements are stable as a solid phase in the region of their Pourbaix diagram corresponding to these conditions, most of which are either not conductive and/or not active for water oxidation. Ruthenium, though less stable and almost as rare as iridium, is an interesting candidate because its oxides (RuOx) have higher activity for water oxidation than those of iridium (IrOx). This talk will describe trends in the activity and stability of ruthenium dioxide at low overpotential. We use in-situ detection of O2 to distinguish water oxidation from charging currents [1] and isotope-labeling to probe the mechanistic coupling of water oxidation and catalyst degradation [2]. Ruthenium and iridium are also interesting in that both form rutile oxides, like most of the other elements that are stable (but inactive) under PEM anode conditions. Insights from IrO2 and RuO2 can therefore be transferred to novel solid-solution rutile oxides of the form AxB1-xO2. This talk will also provide a perspective on this approach to noble-metal-free PEM anode electrocatalysts.

Producing electrodes that comprise highly active but low-cost catalysts for water splitting reactions is still a great challenge for developing new technologies for green energy. In the last few years, transition metal chalcogenides such as NiSe and CoSe were studied as electrochemical catalysts, displaying good electrocatalytic activity. Combining Ni and Co to form ternary compounds allows tuning the active sites for the reaction. In addition, NiSe and CoSe are photothermal materials, where irradiating with light leads to a substantial increase in their temperature.[1] Combining these two functionalities, these materials can be bifunctional, such that their heating contributes to the apparent catalytic reaction rate.

Ternary (Ni_xCo_y)Se nanoparticles were synthesized via a solvothermal method. The resulting nanoparticles were 15-30 nm in size, with a hexagonal crystallographic structure. The ternary nanoparticles demonstrated enhanced electro-catalytic activity towards the hydrogen evolution reaction in acidic conditions. (Ni_0.25Co_0.75)Se exhibited a significantly improved HER performance relative to the binary compounds with an overpotential of 190 mV at 10 mA cm^-2 and a Tafel slope of 53 mV dec^-1. As for the photothermal performance, (Ni_0.9Co_0.1)Se proved to be the most efficient photothermal material in the set. Future work will focus on utilizing them for photo-thermal assisted electrochemistry and evaluating the added value of such setups.

Nitrogen fixation has shaped our society over the past two centuries, providing ammonia derived fertilizers through the Haber-Bosch process to now feed more than half the world population. However, its production is responsible for >1% of our greenhouse gases emissions.[1] It is therefore essential to find alternative synthesis methods sourced by renewable energies.

The electrochemical Li-mediated nitrogen reduction to ammonia (NRR) provides a more sustainable alternative to this challenging reaction.[2,3] This system, operating in a non-aqueous electrolyte analogous to the ones used in Li-ion batteries, provides tangible selectivity towards NH3.[3–5] Just like in Li-ion batteries, a passivating Solid-Electrolyte Interphase (SEI), made of electrolyte degradation products, forms at the cathode of the NRR system. This SEI likely plays a role in controlled proton activity at the electrode surface, limited hydrogen evolution and thus the observed selectivity.[6–8] Consequently, characterisation of this interface (and of the whole system) is vital.

The work presented here expresses guidance for reliable and in-depth electrochemical characterisation of the NRR system and its SEI, all through the lens of a new non-aqueous electrochemical cell design. Drawing further inspiration from batteries, considerations on the cell’s three-electrode geometry,[9–11] construction of a true reference electrode[10,12–14] (as opposed to quasi-reference electrodes in current setups in the field),[3,5,15] and cathode/anode processes separation will be under investigation. As a result, we propose and demonstrate an ideal setup for potential-controlled NRR electrochemistry, capable of measuring accurately the different electrochemical processes occurring over the course of this elusive reaction. Improvements in the accuracy, consistency and reproducibility of the electrochemical measurements resulting from this cell design will be illustrated through the study of a variety of electrolyte architectures.

Amines are compounds very sensitive to oxidation, and a wide array of products may be generated depending on the oxidant.¹ A particularly challenging oxidation is the conversion of a primary amine to a nitrile. In this work, the electrochemistry of primary amines to nitriles is explored using porous Ti/Ni alloys electrodes. These electrodes were recently developed in our group and proved very active in the water splitting oxidation achieving very low overpotentials.² Here, the electrodes have been used for the oxidation of amines to nitriles. A selection of primary amines have been evaluated to understand formation of secondary reactions that reduced their yield. For example, attachment of the amines to the electrodes and degradation in the reaction media has been reported frequently.³ Several conditions were screened to reduce these secondary reactions such as the use of solvents and their mixtures or modification of pH.  Under optimized conditions quantitative conversions and high yields are obtained in the electrochemical reaction of primary amines to nitriles.⁴

Upgrading processes are required to improve the quality of raw bio-oil and thereby increase their viability of bio-based fuel usage. Electrochemical upgrading of bio-oil can be an appealing alternative to current upgrading technologies, as they operate at moderate reaction conditions and can employ the expected surplus in electricity due to increasing capacities of renewable energy installations. One way to lower the acid content in bio-oil and thereby improving the quality of the bio-oil is by Kolbe electrolysis. In Kolbe electrolysis, acids are converted to (non)-Kolbe products and carbon dioxide. The reaction conditions strongly influence the product distribution. The influence of electrolyte pH is studied in literature, however its effect on the product outcome is not yet fully resolved. Contradictory observations and conclusions with respect to performance stability have been reported. Therefore, this work investigates the influence of electrolyte pH on the reaction-time and pH dependent product selectivity during Kolbe electrolysis of acetic acid on platinum.
In this study Kolbe electrolysis of acetic acid on a platinum anode in aqueous electrolytes is addressed with a particular focus on electrolyte composition. We will first disclose that a pH similar to, or larger than the pKa of acetic acid is required to favor formation and homocoupling of methyl radicals towards the Kolbe product ethane. However, extended duration of electrolysis of acetate at basic pH results in loss of Faradaic efficiency to the Kolbe product (ethane), compensated by the formation of the Hofer-Moest product (methanol). We postulate that the observed change in selectivity is caused by the dissolution of CO₂, resulting in enlarged concentration of (bi)carbonate near the electrode electrolyte interface. During the presentation we present our current understanding of the electrode-electrolyte interface and we will provide evidence that the spatial spacing between two methyl radicals is increased which lowers the dimerization rate. Finally, we will discuss the implications of our observations for practical applications, like bio-oil upgrading.

Nowadays, the search and development of economic and environmentally friendly synthetic procedures, alternative to traditional ones energetically fed by fossil fuels, is essential to avoid not only climate change, but also the inexorable depletion of such energy resource. Within this context, electrocatalysis has emerged as a suitable synthetic methodology that avoids the use of harsh reaction conditions and fossil fuels. Besides, in electrochemical approaches, the application of an electric potential difference allows performing chemical reactions that in normal atmospheric conditions would not take place. 

The main chemical procedures studied in electrochemistry are water splitting and CO₂ reduction. In both cases, the oxygen evolution reaction (OER) takes place at the anode, providing the electrons and protons needed in the cathode for the generation of H₂ in water splitting or the conversion of CO₂ into valuable species, such as CH₄ or CH₃OH. The OER produces O₂ that despite being a very important compound, its price in the market is very low, which limits the economic viability of the process and ultimately reduces the interest in this technology.[1] Furthermore, this reaction exhibits large overpotentials when using catalysts based on earth abundant materials, limiting energy conversion efficiency.[2] For these two reasons, there is a growing interest to find alternative reactions to OER at the anode that, by one side, reduce the overpotentials needed and by the other, produce compounds with higher added-value and interest for the chemical industry.[1,3] In this framework biomass valorization, has emerged as an attractive substitute to the oxidation of H₂O.[4] Herein we present our studies in the electrochemical oxidation of 5-hydroxymethylfurfural, a species derived from biomass, as an alternative reaction to OER that allows obtaining 2,5-furandicarboxylic acid (FDCA), a useful building block in the pharmaceutical and polymer industry.[5,6]

‘Water-in-salt’ type of electrolytes are highly explored in the recent past for  its applications in aqueous batteries. Such electrolytes decreases the hydrogen evolution reaction (HER) by extending the water stability window which can be useful for other electrochemical reactions like CO₂ reduction reaction or N₂ reduction reaction (NRR) as HER is the main competitor reaction, where such processes are highly important in chemical industry. For example, development of methods for economically feasible greener ammonia (NH₃) production is gaining tremendous scientific attention due to its importance in fertilizer industry and it is envisaged as a safer liquid hydrogen carrier for futuristic energy resources. In this aspect, electrochemical reduction reaction of nitrogen to ammonia in aqueous electrolyte is a promising way. In our research, an aqueous electrolysis based NH₃ production in ambient conditions is discovered, which yields high faradaic efficiency (~12%) NH₃ via NRR at lower over potentials (~ -0.6V vs. RHE or -1.1V vs. Ag/AgCl) on polycrystalline copper (Cu) bypassing HER.¹ Li⁺ based aqueous electrolyte is used in varying Li⁺ concentration as electrolyte, where the role of Li⁺ in tuning the heterogeneous reaction is established by theory and experiments.^2-4 It is observed that the HER activities of metals such as Pt, Ir, and Pd are suppressed by increasing Li⁺ concentration whereas that of Au, Fe, and Ni augmented with increasing Li⁺ concentration. Here the tunability in the metal-hydrogen (M-H) bonding energy with Li⁺ is experimentally and theoretically established, and the studies show that tunability in the HER properties of both noble and non-noble metals can be achieved irrespective of the pH (0 and 13) and counter ions (TFSI^-, Cl^-, ClO₄^-, NO₃^-and OH^-) by tuning the M-H bond energy using Li⁺.

Transition metal phosphides (TMP) are a promising materials family that has been studied extensively. Still, synthesizing complex and ternary phosphides in a reproducible manner is a challenge, due to the various available oxidation states and crystallographic phases. The various reactivities of the precursors and phase segregation often produce structure of diverse morphologies and compositions. For elucidating the intrinsic properties of these structures as catalysts, such synthetic control is crucial. Here, I will describe a few methods to produce complex ternary structures and their electro-catalytic activity towards hydrogen evolution (HER), oxygen evolution (OER) and alcohol oxidation.

Specifically, I will present a comparative study that illustrates the catalytic activity of three Ni-P phases towards hydrogen production through electrochemical water reduction as well as hydrogen retrieval by hydrolysis of hydrogen storage materials (ammonia-borane and NaBH₄). I will show that Ni₁₂P₅ was recognized as a suitable platform for the electrochemical production of γ-NiOOH—a particularly active phase—because of its matching crystallographic structure. An additional incorporation of tungsten by doping produces surface roughness, increases the electrochemical surface area (ESCA), and reduces the energy barrier for electron-coupled water dissociation (the Volmer step for the formation of H_ads). We explored the three different phases of nickel phosphide also for the electro-oxidation of methanol, ethanol, isopropanol, ethylene glycol, and glycerol. Ni₁₂P₅ exhibits excellent activity (210 mA cm^-2 at 1.72 V vs RHE), durability, and mass activity (~4.2 A mg^-‍1), outperforming the state-of-the-art catalysts. The high selectivity of the reaction and the large suppression of further oxidation to CO₂ are a marker of a preferred bidentate adsorption configuration that conveys a specific O–H activation reaction path. The catalysts show excellent activity towards alcohol oxidation and durability, likely thanks to the mild conditions required for the process, which allow the formation of a regenerating thin active layer of oxidized nickel.

Current ammonia production via the Haber Bosch process requires high pressures, high temperatures and consumes >1% of our current fossil fuel production. To the contrary, nitrogenase in nature catalyses N₂ reduction at room temperature and atmospheric pressures.  The translation of the activity and selectivity of nitrogenase to a solid inorganic surface would enable the efficient on-site on demand synthesis of ammonia, powered by renewable electricity.

Amongst solid electrodes, thus far, only lithium based electrodes in organic electrolytes can catalyse N₂ reduction to NH₃.[1,2] Significant improvements are required to bring the lithium mediated system to an industrial reality; nonetheless, the field is progressing quickly, as we documented in a recent commentary.[3] Nonetheless, we lack comprehensive insight into the factors controlling the reaction.

In this talk, I will discuss why the lithium mediated system is unique amongst solid electrodes in its ability to reduce N₂ to NH₃. Our data using a combination electrochemical experiments, operando and ex-situ characterisation, and density functional theory calculations.[4] I will draw from concepts from enzymes, homogeneous catalysis and battery science. On this basis, I will discuss the most promising avenues towards more efficient N₂ reduction.

Electro-organic synthesis has attracted many attentions in the last years due to the possibility of using renewable feedstocks like CO₂, CO and N-based compounds to generate valuable products. Among many possible reactions, C-C and C-N bond forming reactions represent an important synthetic class because C2+ products and amine derivatives, respectively, play crucial roles in various applications in synthetic chemistry.

CO₂ emitted in the atmosphere has been used as a feedstock for the synthesis of chemicals and fuels, although C2+ products are more attractive because of they have wider applicability and higher energy density [1], while NH₃ derivatives are used to generate amines and amino acids [2] .

Here we studied both C-C and C-N coupling reactions using reductive electrochemical transformations.

The synthesis of C2+ products from the electrochemical reduction of CO₂ was investigated on Cu-oxide derived nanoparticles. Our results showed the reaction is dependent on the electrode surface structure and proton concentration. Using online GC, ethylene and propanol were detected as products on Cu cubic nanoparticles, whereas on Cu polycrystalline, such C2+ products were not detected.

In order to enhance the spectrum of products, CO₂ has been also considered in the coupling reaction with other organic molecules, i.e. aldehydes, to make carboxylic acids via electrocarboxylation. Our initial results showed CO₂ reacts with butanal via C-C coupling to yield the corresponding carboxylic acid, in absence of CO₂, the electrochemical reduction of butanal leads only to dimers.

We have also investigated the electro-reductive amination of benzyl alcohols to benzylamines in aqueous solvent. Using TEMPO-immobilised graphite electrode, benzyl alcohol was selectively oxidized to benzaldehyde (Figure 1), while in the cathode, t-butylamine was converted into imine, which acts as the main intermediate to the C-N bond formation. Using the electrochemical conditions displayed in scheme 1, MS and NMR data analyses showed faraday efficiency (FE) of 12% towards benzylamine.Despite the low FE, this work opens an opportunity to the synthesis of important fine chemical by using simple renewable feedstocks.

References:

[1] da Silva, A. H. M.; Raaijman, S. J.; Santana, C. S.; Assaf, J. M.; Gomes, J. F.; Koper, M. T. M. Electrocatalytic CO2 Reduction to C2+ Products on Cu and CuxZny Electrodes: Effects of Chemical Composition and Surface Morphology. Journal of Electroanalytical Chemistry 2021, 880, 114750.

[2] Jeong Eun Kim, Seungwoo Choi, Mani Balamurugan, Jun Ho Jang, Ki Tae Nam. Electrochemical C–N Bond Formation for Sustainable Amine Synthesis. Trends in Chemistry, November 2020, 2, 11.

Nitrate (NO2-) is the world’s most widespread surface and ground water contaminant that causes adverse effects on human health such as methemoglobinemia (“blue baby syndrome”) and cancer. Most abiotic nitrate removal strategies center on the use of ion exchange resins. Ion exchange resins are effective, yet are not sustainable the process generates a large amount of waste. Biological approaches are inapplicable for removal of high nitrate ions and large scale applications as well as possibility of bacterial contamination in the drinking water.  Electrocatalytic NO3- remediation; however, is one emerging approach for nitrate removal which does not produce waste as NO3- is converted directly to inert dinitrogen (N2) gas. The main challenge with electrocatalytic NO3- reduction is the low NO3- conversion yield, poor N2 selectivity, and lack of understanding regarding catalyst stability. The production of equally harmful contaminant intermediate species such as nitrite (NO2-) and ammonium (NH4+), has also limited the applicability of electrochemical routes for nitrate remediation. Electrocatalytic NO3- valorization is another emerging approach for nitrate removal. Here, NO3- is converted directly to ammonium (NH4+). The main challenge with valorization of NO3- is the low activity and selectivity of the NO3- to NH4+ and the structure sensitivity studies are insufficient. Here, we will describe an investigation on the electrocatalytic properties of palladium (Pd) and copper (Cu) which contains structured surfaces and coatings. The primary aim is to identify which facets and surfaces are highly active and selective for nitrate and nitrite reduction. Engineering the structure of Pd and Cu that enhances the NO3- removal efficiency and enables to steer the selectivity toward N2 and NH4+. To achieve the real-life applications, we also verify the long-term operations for the developed electrocatalysts.Nitrate (NO2-) is the world’s most widespread surface and ground water contaminant that causes adverse effects on human health such as methemoglobinemia (“blue baby syndrome”) and cancer. Most abiotic nitrate removal strategies center on the use of ion exchange resins. Ion exchange resins are effective, yet are not sustainable the process generates a large amount of waste. Biological approaches are inapplicable for removal of high nitrate ions and large scale applications as well as possibility of bacterial contamination in the drinking water.  Electrocatalytic NO3- remediation; however, is one emerging approach for nitrate removal which does not produce waste as NO3- is converted directly to inert dinitrogen (N2) gas. The main challenge with electrocatalytic NO3- reduction is the low NO3- conversion yield, poor N2 selectivity, and lack of understanding regarding catalyst stability. The production of equally harmful contaminant intermediate species such as nitrite (NO2-) and ammonium (NH4+), has also limited the applicability of electrochemical routes for nitrate remediation. Electrocatalytic NO3- valorization is another emerging approach for nitrate removal. Here, NO3- is converted directly to ammonium (NH4+). The main challenge with valorization of NO3- is the low activity and selectivity of the NO3- to NH4+ and the structure sensitivity studies are insufficient. Here, we will describe an investigation on the electrocatalytic properties of palladium (Pd) and copper (Cu) which contains structured surfaces and coatings. The primary aim is to identify which facets and surfaces are highly active and selective for nitrate and nitrite reduction. Engineering the structure of Pd and Cu that enhances the NO3- removal efficiency and enables to steer the selectivity toward N2 and NH4+. To achieve the real-life applications, we also verify the long-term operations for the developed electrocatalysts.

Electrochemistry plays a focal role in the development of better and more efficient catalytic processes for the use of electricity to form or break chemical bonds.

This discipline is closely associated with the development of new renewable energy solutions, such as energy storage (electrolysers, ammonia and other renewable fuels) and chemical production (from CO or biomass). In particular, electrocatalysis has allowed in recent years to achieve significant development on material development and reaction optimization for processes aiming at the synthesis of chemicals and renewable fuels. With this symposium, we want to highlight the scientific excellence in this field.

This discipline is closely associated with the development of new renewable energy solutions, such as energy storage (electrolysers, ammonia and other renewable fuels) and chemical production (from CO or biomass). In particular, electrocatalysis has allowed in recent years to achieve significant development on material development and reaction optimization for processes aiming at the synthesis of chemicals and renewable fuels. With this symposium, we want to highlight the scientific excellence in this field.

Materials and electrochemistry of N-containing compounds
Materials and electrochemistry of carbon dioxide reduction
Electrocatalytic synthesis of high-value compounds: new routes and materials
Electrochemistry for the valorization of biomass

Density functional theory (DFT) calculations at level of the generalized gradient approximation (GGA) are often used in computational electrocatalysis models [1]. This is because DFT-GGA functionals provide fair descriptions of metals at reasonable computational expenses. In this talk, I will first show that the common practice of using DFT-GGA functionals usually entails large numerical errors for the description of molecules, particularly those with multiple bonds, and how to mitigate them by means of a simple and intuitive semiempirical method [2, 3].

Furthermore, I will show that molecular errors have an impact on calculated equilibrium potentials and free-energy diagrams of electrocatalytic reactions, as observed for CO₂ reduction to CO, O₂ reduction to H₂O and H₂O₂, O₂ evolution, and reactions within the nitrogen cycle [2, 4-6]. Finally, I will show that molecular errors appreciably modify Sabatier-type activity plots [6], such that different guidelines for catalyst design are obtained depending on whether or not molecular errors are corrected. The main conclusion is that it is generally advisable to use gas-phase corrections in screening routines for electrocatalytic materials.

To further expand fuel cell technology, a full understanding of the processes taking place at the surface of the electrodes is needed. After decades of investigation, platinum (Pt) is still the single metal with the highest electrocatalytic activity for the reactions taking place within a fuel cell. It has been shown that its catalytic properties can be further exploited by tuning the surface structure and composition. Therefore, preparing and understanding bimetallic surfaces is of paramount importance if one is to improve fuel cell performance [1].

Bi-metallic catalysts with well-defined surface structure can be prepared by depositing adatoms on Pt single crystal electrodes  [2], through the formation of surface alloys by co-deposition and gently annealing [3], or, as in this study, by preparing bulk alloy single crystal surfaces [4]. Here, the preparation of Pt–Pd bulk alloy single crystal electrodes following a modification of the Clavilier bead method has been revised. The surface composition and structure of the electrodes have been analyzed by X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), respectively, pointing out that the surfaces prepared by this methodology are well-defined and give rise to surface compositions close to the nominal ones. Subsequent correlation between surface structure and composition and the voltammetric response has been assessed by performing cyclic voltammetry in different supporting electrolytes, displaying a characteristic and reproducible response for each surface.

Reportedly, very few studies have been accomplished for fuel cell reactions on well-defined bulk Pt-Pd alloys. Here, we will also show the electrocatalytic behavior of a collection of Pt-Pd single crystal surfaces for certain reactions of interest, such as formic acid oxidation and oxygen reduction reaction. Therefore, this contribution is a step forward towards the understanding of the combined role of surface structure and composition of Pt-Pd bulk single crystal electrodes in their catalytic behavior.

Electrochemical reduction of NO_x, CO₂, N₂ and combinations hold the promise to be a cornerstone for sustainable production of fuels and chemicals. Importantly, all reactions share a direct competition with hydrogen, and furthermore, several products are formed from each reactant of these reactants.

For electrochemical CO₂, a complex reaction, it has been shown to give multiple different products depending on the metal catalyst [1]. The unique copper catalyst, as the only metal catalyst has the ability to bind *CO without the coadsorption of *H, which results in a high-value multiple-carbon product distribution [2].

For electrochemical NO_x, also multiple products are formed; N₂O, N₂ and NH₃[3]. Uniquely again the copper catalyst stands out with the ability to bind *NO without the coadsorption of *H, which enables copper to produce ammonia [4].

For electrochemical N₂ reduction to ammonia (NH₃) the interest at ambient conditions is burgeoning [5-6]. Most interesting for the direct electrochemical N₂ reduction in aqueous there is not a “copper” catalyst [7], and instead, the reaction is limited to the non-aqueous lithium-mediated system.

In this talk I will give a unified approach to these reduction reactions versus hydrogen:

►  Show an original and simplistic view of reduction reactions versus hydrogen by investigating the *NO, *CO and *N₂ binding energies versus the *H binding energies. This gives direct insight into the product formation for NO_x and CO₂ reduction, and the challenge of the direct N₂ reduction reaction.

►  Show how one can use these molecular binding energies versus hydrogen in combination to form products beyond the typical reduction reaction products, which reduction reactions are possible based on this framework and how to challenge it [8].

Electrochemical reduction of CO₂ to fuels has attracted a great deal of attention in recent years as a potential solution to close the carbon cycle [1]. In search for novel catalyst materials, we present in this study density functional theory-based screening of heteroatom-doped transition metal nitrides [2]. We first examine zinc blend and rock salt polymorphs of transition metal nitrides and then consider doping the surface with 25% B, P, Sb, Bi, and C heteroatoms [3]. We fully assess the stability and activity of the examined catalysts. The catalytic activity is measured by the calculated limiting potential, and the stability is assessed against hydroxyl (*OH) poisoning as well as dissolution under CO reduction relevant potentials. Of the screened nitrides, many are predicted to be active for the CO reduction reaction but only a few are stable under electrochemical conditions. In particular, on nearly all of the metal nitrides with a desired catalytic activity for CO reduction, either the competing hydrogen evolution reaction prevails over CO reduction or *OH poisoning occurs at CO reduction onset potentials. We identify five promising candidates including P-doped NbN, C-doped VN, P-doped VN, TiN, and Sb-doped TiN. Ultimately, this study emphasizes the relevance of stability under electrochemical conditions and the importance of taking into account the competition between the CO reduction, hydrogen evolution, and *OH reduction reactions under electrochemical conditions.

The insertion of heteroatoms with different electronegativity into a carbon network can greatly tune its electronic, optical, and electrochemical properties along with its chemical reactivity. The introduction of nitrogen and sulfur that serve as electron donors or boron as an electron withdrawing group, into a carbon structure will alter its electronic states, (electro)catalytic properties, and chemical stability to oxidation and to high temperatures. The traditional methods of synthesizing heteroatom-modified carbon matrices are carried out by solid state reactions, chemical vapor deposition (CVD), and functionalization of the carbon materials. Although an impressive wide range of materials were synthesized by these methods, there are still some drawbacks that limit the progress in this field.

Herein we show a new, scalable, and easy way to synthesize heteroatom-incorporated carbon materials by means of molten-state intermediate. The molten-state intermediate is achieved by using molten precursors, polycyclic aromatic hydrocarbons as carbon source and other molten precursors for the various heteroatoms e.g. elemental sulfur or ammonia-borane complex (NH₃BH₃). These precursors once heated above their melting point form the above-mentioned molten-state intermediate which induces strong covalent/coordination bonds between the heteroatoms and the sp² carbon skeleton. This method enables the synthesis of BNCO, CS materials, transition-metal incorporated carbon/carbon-sulfur-oxide materials. The elemental composition is easily controlled within these materials (up to 30 wt.% boron, 20 wt.% sulfur, 30 wt.% nitrogen, while the transition metal ratio is kept within relevant materials) resulting in fine tuning of the materials properties. These materials show promising performance towards electrochemical OER, Na-ion and Li-ion batteries while illustrating the structure-activity relations.

The development of sustainable strategies for the production of added-value chemicals and fuels using renewable resources is particularly attractive to promote a transition towards a more sustainable energetic landscape, overcoming the dependence of fossil fuels at a global scale.[1] One of the most promising alternatives involves the use of renewable electricity (wind, solar, hydropower, etc…) to power electrochemical conversion processes, which convert abundant molecules (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia). In all these processes, (photo)-electrocatalytic water oxidation stands out as the preferred reaction to provide the protons and electrons needed for the target reduction reactions. In this context, metal oxides of earth-abundant elements (Fe, Ni, Co, etc…) are identified as excellent candidates, since these materials can fulfil most of the needed requirements, although in some cases their performance needs to be improved for a more realistic technological assessment. On the other hand, a clear mechanistic understanding of the physical-chemical processes taking place during operation is essential. In the present contribution, we will describe our efforts to understand the operation mechanisms of different metal oxides (mainly NiO_x)[2-5] and other systems (CoFe prussian blue derivatives) for electrochemical water oxidation, as well as other electrochemical processes, [6-7] with particular emphasis on enhancing their performance. Our studies focus on the correlation of the (photo)electrochemical response of the materials with a detailed structural, optoelectronic and photoelectrochemical characterization carried out by different microscopic and spectroscopic tools.

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers for green hydrogen production. Amorphous iridium oxides (IrO_x) show far higher activity than the rutile crystalline iridium oxides (IrO₂), but the underlying origin behind the discrepancy in activity for IrO_x and IrO₂ remains poorly understood. Density functional theory (DFT) calculations are usually used to explain the kinetic difference based on the thermodynamic barrier for forming *OH, *O, and *OOH intermediates on a single Ir site of well-defined model catalysts.^{[1, 2]} However, unambiguously correlating the theoretical free energy with experimentally observed kinetic is very challenging, especially for disordered and non-well defined nano-particle systems, such as amorphous IrO_x. In addition, the ambiguities in measuring the number of active states for determining the intrinsic activity per states has further rendered it difficult to understanding the correlation between activity and the observed physical/chemical discrepancies on IrO_x and IrO₂.

In this talk, I will present our results on using time-resolved UV-vis absorption spectroscopy to experimentally probe the energetics of intermediate states and correlating them with water oxidation kinetics on rutile and amorphous iridium oxides.  We have identified the same oxidised species for both IrO_x and IrO₂ and quantified their concentrations as a function of potential. By comparing the concentration of oxidised states to water oxidation reaction rates, we can directly measure turnover frequency (TOF) for IrO_x and IrO₂ and correlated them with the experimentally measured energetics of the states. Therefore, this study shines insights into the origin of activity difference between amorphous and rutile iridium oxides.

The electrocatalysis systems are very sensitive to electrode surface structure as well as the surface composition. A detail investigation of the nature of electrocatalysts is essential not only for the identification of reaction active sites or components, but also for the clarification of mechanism of electrocatalysts deactivation/activation, as the case of oxygen evolution reaction (OER)^1-2. X-ray photoelectron spectroscopy (XPS) coupled with electrochemical methods is a powerful technique for this purpose. However, it is still quite challenging to combine the conventional electrochemical measurement under ambient conditions with XPS measurement under ultra-high vacuum (UHV) conditions. The in situ electrochemical XPS methods can overcome the shortness of sample change during the transfer as it is commonly encountered in ex situ studies³. However, due to presence of both electrolyte and vapor, synchrotron technique is usually required. In addition, the electrochemical signals are usually distorted by poor mass transfer. We have developed a quasi in situ XPS which consist of electrochemical treatment at ambient pressure, air free transfer from ambient pressure to UHV, and detection under UHV conditions. It has great advantages in: 1) acquiring electrochemical signal of good quality that can be directly compared with those in conventional cells; 2) avoiding oxidation of electrocatalysts due to air exposure; 3) collecting high intensity XPS signals by only commercial X-ray source. In this presentation, I will introduction the detailed setup, benchmark practice, and the application of a nearly in situ XPS on obtaining information on oxygen evolution reaction on Ru based catalysts.    

Photoelectrocatalysis has emerged as a promising process to store solar energy into fuels and high added-value chemicals to decarbonise the energy and fine chemical sectors. In this process the generation of H₂, carbon-based chemicals or NH₃ from H⁺, CO₂ and N₂ reduction, is usually limited by the oxidation reaction taking place at the photoanode, in particular when using metal-oxide photoanodes. The photoelectrochemical performance of these photoanodes vary depending on their synthetic route and post-synthesis treatment that can lead to crystal defects such as oxygen vacancies. However, the chemical nature of such oxygen vacancies and their role in photoelectrochemical oxidation of water or organic substrates to produce high added-value chemicals is still in debate.

In this talk, I will present a spectroscopic, microscopic and electrochemical analysis of the chemical nature of light-induced oxygen vacancies in one of the most studied photoanodes such as BiVO₄. Oxygen vacancies in these BiVO₄ photoanodes were produced by light exposure treatments and are associated with the migration of Bi towards the surface forming nanoparticles.^[1] Additionally, I will show the role of oxygen vacancies in the photoelectrochemical behaviour of BiVO₄, WO₃^[2] and a-Fe₂O₃^[3] photoanodes and their role in the water oxidation mechanism as example.

Growing environmental concerns, as a result of fossil fuel utilization, constitute a driving force towards the development of sustainable technologies to produce chemicals. The electroreduction of CO (CORR) or CO₂ (CO2RR) to fuels are part of these technologies as C₁ (methane, methanol) and C₂₊ products (ethylene, propylene, ethanol, propanol etc.) can be obtained [1]. CO is a particularly interesting reactant because it is present in CO-rich waste streams of common industries such as steel manufacture. For instance, it may be valuable to convert CO contained in blast furnaces’ flue gases, considered an irreducible source of COx, into chemical building blocks such as olefins or alcohols. Converting CO to light hydrocarbons and oxygenates by means of renewable electricity could help reducing our dependence on fossil resources. Moreover, CO can also be obtained from the widely studied CO2RR. CO constitutes a key intermediate in C₂₊ products formation in the aforementioned reaction [2]. Alternative to classical CO hydrogenation (Fischer-Tropsch synthesis), we considered here an electrochemical variant to obtain products with C-C bonds (C₂₊ products).

In this study, we focused on the electrosynthesis of C₂₊ products from CO on Cu-based catalysts. We studied the influence of catalyst particle size on the product distribution obtained during CO electroreduction. Particles ranging from smaller than 7 nm up to 45 nm were investigated. Intermediate sizes (20 nm – 30 nm) and a Cu foil have been reported to be the most selective for the formation of C-C bonds (ethanol, ethylene, propanol, etc..) during electroreduction of CO₂ [3]. Our results show a different selectivity trend when using CO as reactant. The chronoamperometry data show a clear dependency of the selectivity on the particle size of the Cu electrocatalyst (Figure 1) with intermediate sizes (11 to 27 nm) showing the highest selectivity towards C₂₊ products while smaller particles favor hydrogen evolution. Among the used characterization methods, in situ XRD was employed to investigate stability of the particles under electrochemical conditions.

Electrolysis of water, CO₂ and nitrogen-based compounds presents the opportunity of generating fossil-free fuels and chemical feedstocks at an industrial scale. These devices are complex in operation, and their activity and efficiency metrics are usually reported as averaged quantities across an electrode. This heat generation, of usually 30-50% of the inputted power, is due to cell inefficiencies. In this work we seek to use these inefficiencies as a measure of spatial electrochemical activity by taking advantage of the link between heat generation and the activity-dependent transport of electrons and ions. To this end we use data from an infrared camera positioned at the backbone of a gas-diffusion electrode in an attempt to develop a 'thermal potentiostat' which provides spatial resolution of electrochemical activity. After a proof-of-concept is displayed for the technique, we present several applications of the technology including catalyst screening, spatial-temporal catalyst measurements, catalyst-defect and ageing detection, and the impact of exothermic CO₂-hydroxide interactions during CO₂ electrolysis. Combined, we find that a spatial-thermal potentiostat has numerous uses for both novel and established electrochemical reactions towards the production of renewable fuels and feedstocks.

Interfaces play an essential role in nearly all aspects of life and are critical for electrochemistry. Electrochemical systems ranging from high-temperature solid oxide fuel cells (SOFC) to batteries to capacitors have a wide range of important interfaces between solids, liquids, and gases which play a pivotal role in how energy is stored, transferred, and converted. This talk will focus on using ambient pressure XPS (APXPS) to directly probe the solid/liquid electrochemical interface of ion exchange membranes and composite water electrolysis electrodes. APXPS is a photon-in/electron-out process that can provide both atomic concentrations, chemical-specific, and local electric potential information at pressures greater than 20 Torr. Using synchrotron X-rays at Lawrence Berkeley Nation Laboratory, the Advanced Light Source has several beamlines dedicated to APXPS endstations to probe a wide range of interfaces ranging from solid/gas to solid/liquid and liquid/liquid. To enable our approach to investigating electrochemical solid/liquid interfaces, we leverage tender x-rays to support investigations at pressures greater than 20 Torr. Electrical leads support applying electrical potentials to enable the ability to collect XPS data of actual electrochemical devices while operating near ambient pressures. This talk will introduce APXPS and share how we can use this technique to directly probe the Donnan potential of an ion exchange membrane at the solid/liquid interface for the first time. In addition, I will share the chemical information we can reveal when probing a composite water electrolysis electrode undergoing electrochemical water splitting, gaining new insight to guide the design and control of future electrochemical interfaces.

Energy storage through the electrocatalytic generation of chemical fuels such as hydrogen is an attractive pathway for storing intermittent renewable energies, and perovskite oxides are among the most attractive candidate materials to catalyze the kinetically limiting half reaction, the oxygen evolution reaction (OER).  OER activity is typically correlated to electronic and atomic structure parameters. But the catalyst surface – i.e. where the reaction happens – changes during the reaction. To design next-generation electrocatalysts, a detailed operando understanding of the relationships between catalytic activity, stability and atomic-level surface properties during the reaction is required.

In my talk I will address two essential ingredients to achieve this next-level understanding: controllable atomic-level surface properties and the development of surface-sensitive operando characterization routes.  

Epitaxial thin films are a direct route for single crystalline model electrocatalysts that can be fabricated with atomically-tailored surface composition. These offer the ideal platform to derive structure-property-function relationships, track the evolution of the surface properties with applied potential and enable direct comparison to the surfaces investigated in density functional theory. I will summarize our recent findings for the role of surface termination, surface contamination, point defects and transformation pathways during the reaction and the role of solid/solid interfaces in proximity of the solid/liquid interface.

Next, I will summarize recent advances in surface-sensitive operando characterization in a liquid medium and with specific attention to studies with atomically-defined thin film model electrode surfaces. A focus will be the current development of meniscus photoelectron spectroscopy, a discussion about utmost surface- or interface-sensitivity using a standing-wave approach^1,2 and a pathway to extract surface-sensitive information from nominally bulk-sensitive techniques such as UV-Vis spectroscopy.^1,3

The key example throughout the talk will be LaNiO₃ thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination. Using a suite of ex situ, in situ and operando spectroscopy tools, we found that the Ni-rich surface undergoes a surface transformation towards a catalytically active Ni hydroxide-type surface.¹ If LaNiO₃ surfaces are exposed to the ambient, however, surface carbonate groups form, which prohibit the formation of the active phase, leading to an activity decrease compared to the clean surfaces.⁴

Our work thus demonstrates tunability of surface transformation pathways by modifying a single atomic layer at the surface and it shows that active surface phases only develop for select as-synthesized surface terminations, highlighting the instructional value of epitaxial model electrocatalysts. It also highlights the need of and summarizes pathways for the exploration of the three-step relationship between as-prepared surface, transformation under applied potential, and electrocatalytic activity.

The performance of an electrocatalyst (its activity, selectivity, and stability) is strongly dependent on the electrode structure and composition, particularly in the near surface region. A successful approach in trying to understand the impact of structure is the use of well-defined model electrodes such as single crystals, to isolate how various changes in structure and composition impact upon the catalyst behavior. Surface x-ray diffraction gives the average surface structure of an isolated facet, whereas real catalysts often contain multiple facets and edge sites. This contribution will discuss two recent advances in the determination of electrocatalyst structure:

a) The application of high energy surface x-ray scattering to quickly map out large volumes of 3D reciprocal space and then extract crystal truncation rods. These truncation rods can then be used to determine atomic coordinates of surface atoms, in operando. I will present examples during methanol oxidation on both Pt(111) and Au(111) surfaces.

b) I will then discuss how we can complement single crystal studies by using coherent Bragg diffraction imaging (and some of the challenges) to look at single nanoparticle electrocatalysts and ceate strain maps to identify important facets and sites.

The search for commercially viable catalysts for the oxygen evolution reaction (OER), which is typically the bottleneck for electrochemical water splitting, is a key challenge in the worldwide transition to renewables-based electricity generation. Transition-metal oxides in general and cobalt (hydro-)oxides in particular are of great interest in this regard, as they show promising catalytic properties, are stable in alkaline and neutral electrolytes under ambient conditions, and can be easily prepared from earth-abundant materials [1]. A wide variety of different catalysts, based on oxides of Co, Fe, and Ni, have been synthesized and studied, exhibiting rather diverse nanoscale morphologies and usually an unknown or poorly defined surface structure. This makes it difficult to compare their reactivity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determin­ation of the OER reaction mechanism.

We here present studies of structurally well-defined Co₃O₄ films of 10-30 nm thickness, prepared by electrodeposition or molecular beam epitaxy (MBE) on Au(111), Au(100), and Ir(100) substrates. In all cases a well ordered epitaxial Co₃O₄(111) arrangement results, but the film morphology differs substantially, as shown by AFM and X-ray diffraction measurements. The oxide structure under reaction conditions was studied by detailed operando surface X-ray diffraction [2,3] and in situ X-ray absorption measurements. These techniques allow characterization of the surface and bulk oxide structure over a wide potential range, including the OER regime, and can be performed simultaneously with electro­chemical measurements, enabling direct correlations of structure and reactivity.

We find that the films prepared by molecular beam epitaxy are perfectly stable whereas all electrodeposited Co₃O₄ films exhibit reversible changes in the oxide structure and oxidation state, in agreement with a previous study of polycrystalline Co₃O₄ catalysts [3]. Specifically, we observe the formation of an ultrathin X-ray-amorphous CoO_x(OH)_y skin layer above 1 V vs. RHE and the gradual buildup of tensile lattice strain with increasing potential. These structural changes occur for all electrodeposited samples, albeit to a different extent, depending on the film morphology. They were found at pH values between 7 and 13 and in the presence of different cations. Potential step experiments show that the structural transformation proceeds within seconds and is highly reversible. Despite the similar structure, differences in the OER overpotential of up to 150 mV are observed for these samples, with the structurally stable MBE-prepared films having a larger overpotential than the electrodeposited films. This indicates a beneficial effect of the CoO_x(OH)_y skin layer on the OER activity of Co oxide catalysts.

[1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347.

[2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811.

[3] T. Wiegmann, et al., ACS Catalysis, 2022, 12, 256.

[4] A. Bergmann, et al., Nature Communications, 2015, 6, 8625.

Platinum dissolution and a resulting decrease in the electrocatalyst active surface area is the main cause of the efficiency decrease of the proton exchange membrane fuel cell (PEMFC) with time. It has been extensively investigated on both the device and simplified model catalyst levels for many years, as summarized in our review [1]. In order of complexity increase, the systems can be ordered as follows: Pt low indexes single crystals; polycrystalline Pt; Pt nanoparticles; supported Pt nanoparticles in thin and thick catalyst layers, where the last one closely reminds the catalyst layer of PEMFC.

Over the last years, all these systems were investigated in our group to understand the complex mechanism of Pt dissolution. In most of these studies, an inductively coupled plasma mass spectrometer (ICP-MS) directly connected to an electrochemical cell was used for time- and potential-resolved Pt dissolution analysis. Scanning flow cell (SFC)- and gas diffusion electrode (GDE)-based setups were developed and successfully applied to study different Pt/electrolyte interfaces [2-4].

In this talk, I will present the most recent results of this research. Two systems: Pt/aqueous electrolyte and Pt/ionomer electrocatalytic interfaces, will be compared and contrasted [4, 5]. The main focus will be the understanding of the influence of system complexity and experimental parameters, simulating cell operational conditions, on Pt dissolution, with a final goal to clarify how the results on Pt dissolution from model catalysts can be extrapolated to PEMFCs. Moreover, mitigating approaches to stabilize platinum electrocatalytic interfaces will be discussed [6].

The quest to design active and stable (electro)catalysts hinges on the ability to identify the reaction mechanism under realistic operating conditions. The past few decades of research have focused on developing a range of spectroscopic and imagining tools to enable the study of the chemical and structural changes at the solid/liquid interface.

This talk will bring the most recent understanding of the electrochemical interface during the energy conversion and storage reactions. The molecular-level understanding of the interfacial electrochemical processes on metal surfaces by operando surface-enhanced infrared absorption spectroscopy (SEIRAS) will be particularly discussed. We will also show that integrating those experimental approaches and density functional theory (DFT) calculation further strengthens the understanding of the reaction mechanism. It will also be demonstrated that the holistic information about the nature of the adsorbed species can provide additional knobs to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for energy conversion and storage reactions.

A sustainable future requires highly efficient energy conversion processes. I will discuss two unconventional strategies to reach significant boosts in the electrocatalytic activity of Earth-abundant materials using spin control:

-At the interfacial level,  magnetic fields affect the mass transfer of charged reactants (to) and products (from) the electrolyte. This effect is universal on both magnetic  and non-magnetic electrodes. I will demonstrate this phenomenon operando using Pt and transition metal oxides as catalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER).

-At the local level,  we use chiral molecules to induce spin selective pathways on the catalyst’s surface. As a case study, I will show the use of functionalized helicenes to boost the oxygen evolution reaction (OER) by ca. 131 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D catalysts via a spin-polarization mechanism) [1].

By decoupling the effects involved in interfacial vs local spin control, we provide a versatile strategy that can be easily implemented to boost electrocatalytic reactions on different materials. 

Surface and interface sensitive operando characterization methodologies are required to unlock electrochemical and electrocatalytic processes. Liquid phase electrochemical transmission electron microscopy (TEM) can provide the sensitivity and selectivity at single particle level. Herein, we investigate oxygen-evolving cobalt-based oxides in alkaline solution and we retrieve morphological, structural, chemical information during cyclic voltammetry measurement. We compare the highly active Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) to the spinel Co₃O₄ and rocksalt CoO. The potential-dependent variation of the local contrast is associated to the modification of the wettability from hydrophobic to hydrophilic character at the oxide surfaces through interfacial capacitance phenomena. Molecular oxygen is probed in real-time during its evolution which results to further dewetting of the surfaces. Our work on catalysts exemplifies the crucial role that surface sensitive and electrochemical microcell TEM techniques can play in detailing the mechanism of electrocatalytic processes and can provide a new characterization framework aiding development of novel design routes for targeted catalyst preparation.

For over 50 years, Raman spectroscopy has remained a key tool in the arsenal of the electrochemists investigating structural and ionic (both solid-state and liquid electrolyte) dynamics in battery and electrocatalytic systems. However, imaging of evolving Raman signals has remained challenging due to the small signals (and hence slow acquisition speeds) of spontaneous Raman. Here, we overcome these limitations and develop and apply high-resolution (sub-300 nm) computational Raman imaging for real-time, chemically specific imaging of operating electrocatalysts and batteries.

Firstly, we track the OER from IrO₂ and Li-IrO_x electrocatalysts, demonstrating at low overpotentials that oxygen is evolved by the combination of an electrochemical-chemical mechanism and classical electrocatalytic adsorbate mechanism, whereas at high overpotentials only the latter occurs. We then examine electrochemical degradation in LCO and NMC811  electrodes operating at high voltages (above 4.2 V with LP30 electrolyte). For the LCO system, we observe a build-up of degraded electrolyte through the P-F bond intensity and oxidised Co along the edge of the particles.

Our results demonstrate the power of high-resolution Raman microspectroscopy for non-invasive operando, tracking of electrochemical dynamics and reveal new insights into the operation of state-art-of-the-art battery materials and electrocatalysts.

The field of electrocatalysis experienced an intensely increased interest in the last years due to the urgency to move from a fossil fuel-based industry to one based on green energy. While new electrocatalyst materials are urgently needed, their rational design is still largely hindered by the lack of knowledge about the nature and structure of active centers in differently synthesized catalyst materials and the understanding of operational parameters that influence the catalysis.

Recently, high entropy alloys (HEA) or complex solid solutions (CSS) emerged as new promising materials in the field of electrocatalysis. A minimum of 5 different elements are mixed together, leading to a HEA material stabilized by entropy. Theoretical studies indicate that such materials, which possess many surface atom arrangements with different binding energies, could generate surfaces with an increased density of optimal or close to the optimal binding energy.[1] This may lead to increased activity compared with the already established catalyst and this was recently shown to be the case for the oxygen reduction reaction.[2]

One of the big challenges in the CSS research field arises from the multitude of possible surface atoms arrangement which could result from mixing five different elements. Exploring millions of possible combinations requires the use of high-throughput methods as well as new strategies to explore the space. Besides, the experimental probing and the identification of the surface atom arrangements with increased activity is impossible using classical electrochemical techniques.

Here we will show the potential of two high-throughput techniques: scanning droplet cell (SDC) [3,4] and scanning electrochemical cell microscopy (SECCM) to evaluate the multitude of active sites present in a complex solid solution (CSS).[5] By combining the two techniques and using a zooming-in approach from macro to the nanoscale, we can see an increase in the distribution of active sites in a HEA material.

Sustainability in batteries needs to be considered at a holistic level, from materials source, manufacturing, lifetime and if there is value in the materials what happens to the batteries at end of life. In this work we discuss three different aspects of sustainable considerations within a sodium-ion technology development. Materials developments, cell lifetime and optimisations, and design for recycling.

The cell is comprised of a nickel based layered oxide material cathode with hard carbon anode. To maximise energy density and life-time degradation of the bulk and surface of the O3-type oxides require stabilisation.¹ We discuss a facile method for manufacturing a stabilised O3- type layered oxide via simultaneous doping and surface coating. A higher average voltage is obtained, which stabilised the high voltage phase transition to higher voltages, and results in longer-cycle life.²

To improve the materials life-cycle lifetime of the battery is also considered, and methods to improve the 1^st lifetime and use discussed. The formation of the interface layers is key, and in addition to the stabilisation of the cathode surface stabilisation of the negative electrode is required. This is initially achieved with formation processes, and can be modified with different electrolyte additives.^3,4 It is hypothesised that water causes significant damage, and removing these small molecules, for example using ZSM-5, in both an electrode and an electrolyte results in significant improvement in lifetime of the cell and the electrodes with less SEI build up over time.

Finally design for remanufacture is also discussed, utilising water-based binder systems we show differences in delamination and material recovery. Utilising different recycling schemes through shredding and physical processing through to disassembly processes improvements in recovery rates are required. Particularly for low value materials, where the techno-economics of the processes do not currently work. Direct recycling of materials reclaimed from the batteries, wherever possible allows value within the material design to be maintained, rather than relying directly upon elemental value. Aspects of direct loop recycling and short loop recycling are discussed with respect to current lithium-ion technologies, and the ability to directly translate this to sodium-ion.

In conclusion three aspects for sustainability considerations are discussed with respect to the materials life-cycle; active material optimisation, longevity and life-time when in use and electrode developments for recycling.

1.           Song, T. & Kendrick, E. Recent Progress on Strategies to Improve the High-Voltage Stability of Layered-Oxide Cathode Materials for Sodium-ion Batteries. J. Phys. Mater. 4, 32004 (2021).

2.           Song, T. et al. High-Voltage Stabilization of O3-Type Layered Oxide for Sodium-Ion Batteries by Simultaneous Tin Dual Modification. Chem. Mater. 34, 4153–4165 (2022).

3.           Chen, L., Kishore, B., Walker, M., Dancer, C. E. J. & Kendrick, E. Nanozeolite ZSM-5 electrolyte additive for long life sodium-ion batteries. Chem. Commun. 56, 11609–11612 (2020).

4.           Kishore, B., Chen, L., Dancer, C. E. J. & Kendrick, E. Electrochemical formation protocols for maximising the life-time of a sodium ion battery. Chem. Commun. 56, 12925–12928 (2020).

One of the major hindrances to mass commercialization of low-temperature proton-exchange-membrane fuel cells (PEMFCs) is the considerable amount of expensive Pt required, especially at the cathode side, where the oxygen reduction reaction (ORR) takes place.1, 2

In the last decade, great efforts have been made to develop efficient Platinum-Group-Metals (PGM) free catalysts for the ORR, especially metal-nitrogen-doped carbons (M-N-C, with M = Fe, Co), due to the abundance and inexpensiveness of their constituent elements and their atomic dispersion. The activity gap towards Pt has successfully been narrowed, now reaching the activity requirements for practical applications.3-5 Due to the metastability of the ORR active M-N4 sites at the temperature of their pyrolytic formation, the final transition metal loading is currently limited and significant amounts of inorganic by-products are formed. Although synthesis protocols have been successfully optimized, multiple processing steps are required, making the preparation time-consuming.

In our previous work, we showed that via an active-site imprinting strategy followed by a transmetalation reaction, Mg-N-C and Zn-N-C containing Mg-N4 and Zn-N4 sites respectively, can be transformed into active Fe-N-C electrocatalysts, avoiding the formation of elemental iron, or iron carbide side phases.6-8

In this work, we present how Zn−N4 sites in tailor-made Zn−N−C materials are utilized as an active-site imprint for the preparation of the corresponding Fe−N−C catalysts with a high loading of atomically dispersed Fe. The current state of this class of materials is discussed in terms of synthetic methods, activity and stability, including both rotating disk electrode (RDE) and single-cell PEMFC testing.

A. Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 2016, 7, 1127-1137.

S. T. Thompson and D. Papageorgopoulos, Nature Catalysis, 2019, 2, 558-561.

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R. Bashyam and P. Zelenay, Nature, 2006, 443, 63.

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A. Mehmood, J. Pampel, G. Ali, H. Y. Ha, F. Ruiz-Zepeda and T.-P. Fellinger, Advanced Energy Materials, 2018, 8, 1701771.

D. Menga, F. Ruiz-Zepeda, L. Moriau, M. Šala, F. Wagner, B. Koyutürk, M. Bele, U. Petek, N. Hodnik, M. Gaberšček and T.-P. Fellinger, Advanced Energy Materials, 2019, 9, 1902412.

8. D. Menga, J. L. Low, Y.-S. Li, I. Arčon, B. Koyutürk, F. Wagner, F. Ruiz-Zepeda, M. Gaberšček, B. Paulus and T.-P. Fellinger, Journal of the American Chemical Society, 2021, 143, 18010-18019.

2D halide-perovskites as multifunctional photobattery materials – Fundamental investigations regarding stability, lithium intercalation and light induced processes

Jan Büttner, Taisiia Berestok, Stephan Burger, Michael Daub, Harald Hillebrecht, Ingo Krossing, Anna Fischer

Autonomous photo-rechargeable energy storage systems have received growing attention over the past years. ^[1–7] Among these highly integrated photobatteries are especially promising systems: they offer grid independent storage of energy to power small devices required for industry 4.0 or IOT applications. To achieve the highest possible level of integration in such photobatteries, multifunctional materials that can both convert sunlight and store energy at the same time and place, are required.

2-(1-cyclohexenyl)ethyl ammonium lead iodide (CHPI), a 2D organic-inorganic lead halide perovskite belonging to the Ruddlesden-Popper (RP) phases, has been reported to be such a type of multifunctional material, i.e. combining photo absorber properties with the ability to intercalate and especially photo-deintercalate lithium ions in combination with a carbonate based polar liquid electrolyte. ^[8]

In the present work, we investigated CHPI for its stability against dissolution, its Li⁺-intercalation and photo-assisted deintercalation properties and its behaviour under illumination in combination with carbonate based electrolytes and a newly developed electrolyte based on ortho-difluorobenzene (o-DFB), that has much lower polarity.

We show that (i) CHPI dissolves in carbonate-based electrolytes and that (ii) Li⁺ does not intercalate from non dissolving low polarity electrolytes and entirely capacitive behavior is displayed. As such, we could not confirm any photo-assisted Li⁺-deintercalation taking place in these materials.

In addition, we could reveal that under illumination, while being in contact with the non dissolving electrolyte, oxidative photo-corrosion and dissolution of the perovskite occurs. These results are in line with the absence of a reversible redox system in CHPI otherwise found in intercalation battery materials. Indeed only a hypothetical Pb⁺/Pb²⁺ system would actually be able to receive electron density, underlining the inability of CHPI to intercalate lithium ions in a significant amount, i.e. higher than just doping.

The chemical instability against polar solvent of any perovskite that can be synthesized by dissolving its educts in a polar solvent for e.g. spin coating prevents their application with standard polar lithium ion battery electrolytes in battery systems. Further electrochemical and photochemical instabilities add to the limitations of this material class towards their useage as truly multifunctional materials for higly integrated photobatteries. Finally their inability to store large amounts electron density prevents any relevant Li⁺-intercalation.

[1]       H. Wei, D. Cui, J. Ma, L. Chu, X. Zhao, H. Song, H. Liu, T. Liu, N. Wang, Z. Guo, J Mater Chem A 2017, 5, 1873–94, 10.1039/C6TA09726J.

[2]       A. Gurung, Q. Qiao, Joule 2018, 2, 1217–30, 10.1016/j.joule.2018.04.006.

[3]       V. Vega-Garita, L. Ramirez-Elizondo, N. Narayan, P. Bauer, Prog Photovoltaics Res Appl 2018, 27, 346–70, 10.1002/pip.3093.

[4]       D. Lau, N. Song, C. Hall, Y. Jiang, S. Lim, I. Perez-Wurfl, Z. Ouyang, A. Lennon, Mater Today Energy 2019, 13, 22–44, 10.1016/j.mtener.2019.04.003.

[5]       Z. Fang, X. Hu, D. Yu, Chempluschem 2020, 85, 600–12, 10.1002/cplu.201900608.

[6]       Q. Zeng, Y. Lai, L. Jiang, F. Liu, X. Hao, L. Wang, M.A. Green, Adv Energy Mater 2020, 10, 1–30, 10.1002/aenm.201903930.

[7]       A. Hauch, A. Georg, U.O. Krašovec, B. Orel, J Electrochem Soc 2002, 149, A1208, 10.1149/1.1500346.

[8]       S. Ahmad, C. George, D.J. Beesley, J.J. Baumberg, M. De Volder, Nano Lett 2018, 18, 1856–62, 10.1021/acs.nanolett.7b05153.

Organic-inorganic metal halides (Perovskites/non-Perovskites phases) are emerging as a next generation optoelectronic material in both nanoscale and bulk form. Besides its novel applications in energy, light emitting diodes etc. these materials are also showing fascinating results in other appealing fields like battery research and sensors. [1]

Meanwhile Ionic liquids (ILs) have gained tremendous attention in various research fields over the last decades. Within melting points generally below 100 °C (much lower than conventional inorganic salts), ILs have been used in several applications, like as an electrolyte in fuel cells or in photovoltaic modules for the ion transport etc. Due to their low vapor pressure and low flammability ILs are quite safe and easy to handle. Even by altering the functional groups ILs or replacing the anions or cations, both the chemical and physical properties can be drastically changed. [2,3]

In this oral presentation, I will talk about organic-inorganic transition metal-halide structures, where ionic liquids act as organic cations. Besides its synthetic and structural details, we have discussed general optical, thermal and conductivity properties. These organic-inorganic metal-halides form 1D structures, which are confirmed by scanning electron microscopy and single crystal analysis. Like other ionic liquid mediated metal-halide structures, these compounds also have a low melting point, which are confirmed through DSC measurements. Most importantly, the here presented compounds show a high conductivity of 10^-4 Scm^-1 at room temperature. With elevation of temperature the conductivity increases and it reaches to 10^-2 Scm^-1 at 70 ⁰C. In general, this is the very first report of superior ion conductivity of an organic-inorganic metal-halide structure that could be a very important milestone for future solid-state battery research. [4]

The energy transition to Zero carbon emissions started, and hydrogen is playing a very important role as replacement of the conventional fossil fuels, together with the large research and development in portable and stationary electrochemical energy storage devices as Li and Redox Flow batteries (RFB). An important aspect for these technologies is the materials used; RFBs store the energy in the electrolyte, requiring the use of porous electrodes with high surface are to maximise the electrolyte utilisation. For the Li Batteries, we will focus on the emerging Lithium Sulfur (Li-S), a carbon porous material is required in the cathode, which can accommodate the sulfur expansion during the charge and discharge, and as well avoid the polysulfides shuttle effect. An interesting material that can tackle this issue are carbon aerogels (CAGs), which are hierarchical carbon materials with tuneable porosity. The porosity and structure of these materials can be very easily tuned, changing the synthetic conditions, for each application.

In RFBs, to improve the performance and utilisation of the electrolyte can be done by reducing the thickness of the electrodes. However, some challenges need to be addressed, as mechanical properties, a good porosity and surface area. The CAGs, can be easily growth over different supports as carbon papers and carbon cloths, being in both cases, around 10 times thinner, compared with the standard graphite felts used in RFB. We had studied a series of these thinner electrodes, using carbon paper as support in all-Vanadium RFB, having a very interesting results in the behaviour of the different electrodes prepared [1].

The flexibility in the synthesis of these materials, give us a chance to use them as cathodes for Li-S batteries. In this case, we growth the CAGs in thinner electrodes successfully. In addition, we modify the CAGs doping them with Nitrogen and as well with Fe, trying to obtain a supported high porous single atom catalyst. We studied the electrodes in coin cell batteries to understand the performance and cyclability. In parallel, we started to analyse the CAGs powders and single atom catalyst with different metals in RDE and RRDE [2], trying to get a protocol to benchmark catalyst and materials for the sulfur reactions and understanding his kinetics in the oxidation and reduction.  

Other important actor for the zero-carbon energy system pointed out at the beginning is the H₂. A very important aspect is to develop materials for an efficient hydrogen production using renewable energies (Green H₂), which requires the use of electrolysers to split the water. The major problem for the electrolysers is within the anode, as they use Iridium (Ir), being not very abundant, restricting the large-scale production. However, there is other alternative, the alkaline electrolysers, in which the Ir can be replaced by other metals, more abundant on Earth. A very interesting materials to explore for the alkaline electrolysers are the transition metal phosphides, as they have a very interesting activity for the oxygen reactions with a good corrosion resistance characteristic [3].

Using H₂ for energy conversion, we need to mention Fuel Cells, focusing on the low temperature PEMFC, they are electrochemical devices that convert H₂ and O₂ in electricity and water. This system cannot be reversible due to the slow and complicated kinetics in the O₂ reactions. But replacing it with a fast and reversible species used in RFB, give us a very interesting and flexible electrochemical energy storage system, known as hybrid redox flow batteries. For these devices the cathode side it is quite flexible, having the possibility to use an organic molecule [4], vanadium [5] or manganese [6].

Finding new materials for different electrocatalytic reactions for renewable energy conversion is essential to build a sustainable future. High entropy alloys (HEAs), in which 5 or more elements are mixed forming a new crystallographic structure, have emerged as a new class of materials with interesting properties for electrocatalysis¹. There are innumerable possibilities of combining the metals, which allows tailoring the electrocatalytic properties by tuning electronic and geometric effects in the HEA. In addition, investigating these multifunctional electrocatalysts can be used as a platform for the discovery of novel sustainable materials. However, there is a lack of information regarding how the elements distribute on the surface and their stability and structure under different applied potential conditions, which is crucial for the rationalization of energy conversion reactions. Thus, potential-controlled experiments and benchmarking with model well-known monometallic electrode surfaces are necessary to assess catalytic and interfacial properties of HEAs². In this work, we investigate the oxidation of carbon monoxide on an extended high entropy alloy electrocatalyst over a wide range of pH³. We combine electrochemical characterization with surface-sensitive techniques including x-ray photoelectron spectroscopy and ion scattering spectroscopy to gain an understanding on the structure-reactivity relations.

Low temperature proton exchange membrane fuel cells powered by green hydrogen provide a means to sustainable energy production for stationary and transport applications, such as back-up power and fuel cell vehicles, respectively. Their widespread commercialisation is limited by the cost of the platinum catalyst at the cathode, where oxygen reduction occurs. Atomic FeN_x sites within carbon offer a cheap and sustainable alternative, exhibiting the most promising non-precious metal activity for oxygen reduction. However, atomic Fe loading typically cannot exceed >2 wt.% without unstable and inactive FeC and Fe⁰ formation due to high pyrolysis temperatures (700-1000^oC) required during synthesis of the conductive catalyst support. Recent progress has identified the successful use of a decoupled two-step procedure whereby Fe is incorporated at low temperature, following the high temperature pyrolysis, which has enabled >2 wt.% Fe.^[1–3]  In our work, we adapt a Zn active site imprinting and subsequent low temperature (170^oC) Fe (trans-)metalation process,^[1] instead using a zeolitic imidazolate framework-8 precursor to yield Fe >5 wt.% (ICP-MS) with oxygen reduction active FeN_x sites. The ex-situ atomic nature of the FeN_x active site is elucidated by aberration corrected high-angle annular dark field scanning transmission electron microscopy, x-ray absorption spectroscopy, and electron paramagnetic resonance. The high Fe loading also enables novel characterisation by time-of-flight secondary ion mass spectrometry.

The electrochemical reduction of CO₂ (eCO₂RR) is a promising eco-friendly alternative to fossil-fuel-based processes used for the production of chemicals. Currently, the conversion of CO₂ to CO is a more industrially appealing process than the eCO₂RR to multicarbon products, as it requires lower energy consumption and presents high product selectivity.[1] Also, the concomitant hydrogen evolution reaction (HER) occurring at the same potential window is not a serious setback in this case since the mixture of H₂ and CO (known as syngas) is a valuable product, which can be directly utilized in established industrial processes such as the Fischer-Tropsch to produce energy-dense chemicals. [2] Silver- and gold-based materials are hitherto considered the state-of-the-art of CO-forming catalysts but their cost and scarcity impede their industrial application. In this regard, composite materials based on transition metal oxide nanoparticles supported on metal-nitrogen-doped carbons (MOx/M-N-C) have been recently explored as inexpensive and efficient electrocatalysts for the eCO₂RR-to-syngas process. Several works in literature have reported the synergistic effect between MOx nanoparticles and M-N-Cs, which results in an enhanced eCO₂RR electrocatalysis with respect to each one of the materials separately. [3,4] The research of MOx/M-N-Cs for eCO₂RR is still in its infancy and, therefore, further development is needed to reach the application of these materials in industrial CO₂ electrolyzers.

In this contribution, we introduce several approaches for the preparation of effective MOx/M-N-C-based GDEs. On the one hand, we demonstrate the potential of polybenzoxazine (pBO) resins to synthesize active MOx/M-N-Cs materials, most likely due to the unique properties of the resins. In doing so, we report that gas diffusion electrodes (GDEs) prepared with pBO-derived MOx/M-N-Cs (M= Ni, Fe) exhibited higher activity and CO selectivity in 1 M KOH than the pBO-free ones. On the other hand, we explore the optimization of GDEs in terms of eCO₂RR electrocatalysis by the addition of binders into the catalyst layer (CL). The addition of PTFE to the catalyst layer was proved to favor the production of CO over HER, disclosing the essential role of hydrophobicity in the CL for eCO₂RR. The GDE fabricated with pBO-derived NiOx/Ni-N-C and PTFE displayed suitable syngas production at industrial current densities, with an H₂/CO ratio of ~0.9 at -200 mA cm^-2 and ~-0.6 V vs RHE. Finally, we also conclude that the combination of PTFE and Sustainion in the CL has a synergistic effect that confers stability to the pBO-derived NiOx/Ni-N-C-based GDE while keeping intact the hydrophobicity, activity, and CO selectivity, with respect to the GDE with only PTFE. The results of this work provide relevant insight into both the development of MOx/M-N-C materials as syngas-forming electrocatalysts and the engineering of eCO₂RR-to-CO GDEs aiming for the industrial implementation.

Metal-Nitrogen-Carbon catalysts are being developed and studied as alternative catalysts to platinum for catalyzing the O₂ reduction reaction (ORR), in both acidic and alkaline media.^{1, 2} The beginning-of-life activity and performance of optimized M-N-C materials is now sufficiently high to raise interest at the application level in fuel cells (proton-exchange membrane fuel cells, PEMFC, and anion exchange membrane fuel cells, AEMFC), but this class of materials typically suffers from poor stability in acidic medium while the stability in operating environment in alkaline medium is still under-investigated.

This presentation will discuss the most recent understanding on the deactivation/degradation of M-N-C catalysts, highlighting in particular the negative effect of combined electrochemical potential and O₂ observed in acidic medium, leading to a loss of active sites but also an oxidation of the carbon matrix. The different effects that Fenton reactions have in different pH conditions will also be shown, and how this explains very different stability trends during operation of such catalysts. Tracking of the fate of the single-atom Fe-N₄ sites after operation in fuel cells will be shown to be possible with ⁵⁷Fe Mössbauer spectroscopy acquired at low temperature, distinguishing clearly nano-Fe-oxides from Fe-N₄ sites.³ Finally, an overview of the recent approaches taken towards improving the stability of M-N-C materials will be given.

References

(1)       Osmieri, L.; Park, J.; Cullen, D.A.; Zelenay, P.; Myers, D.J.; Neyerlin, K.C.; Current Opinion in Electrochemistry 2021, 25, 100627.

(2)        Firouzjaie, H.A.; Mustain, W.E.; ACS Catalysis 2020, 10, 225-334.

(3)        Li, J. et al, Nature Catalysis 2021, 4, 10-19

The development of noble metal-free electrocatalysts for their application in alternative energy conversion and storage technologies, including fuel cells, water electrolysers and batteries is an important step towards a sustainable economy based on renewable energies. Recent studies in electrocatalysts containing abundant metals such iron (II) phtahlocyanine (FePc) and NiFe layered double hydroxide have proved the remarkable potential of these materials as electrocatalysts for the oxygen reduction and evolution reactions, respectively  [1-3], key processes governing the working principle of fuel cells, electrolysers and metal-air batteries. In order to implement these electrocatalysts in energy devices, efficient deposition in a conducting matrix is key. Here we present the use of electrospinning to produce freestanding carbon-based non-woven fibres with excellent conductivity that also enable a homogeneous deposition of electrocatalyst. Moreover, lignin, a by-product of the paper industry, was used as carbon source. The resulting materials have exhibited remarkable performance in alkaline environment. The synergetic effect between matrix and electrocatalysts will also be explored.

Platinum group metal free (PGM-free) electrocatalysts represent an attractive low-cost alternative to PGM-based catalysts for several reactions of fundamental importance for electrochemical energy conversion and storage. Of various proposed PGM-free catalysts, the atomically dispersed transition metal-nitrogen-carbon (M-N-C, M = Fe, Co, Ni, or Cu, etc.) materials have been found to be especially promising for oxygen reduction reaction (ORR), as potential replacement for Pt-based cathode catalysts in low-temperature polymer electrolyte fuel cells (PEFCs), and more recently as catalysts for electrochemical reduction of carbon dioxide (CO₂RR). In this presentation, we will summarize recent progress in the development of M-N-C catalysts for these two important reactions.

The research effort in the development of M-N-C ORR electrocatalysts at Los Alamos dates back to early 2000s and has since yielded catalysts with substantially enhanced ORR activity and, lately, also with much improved durability [1-3]. Using advanced Fe‑N-C catalysts, we have achieved an activity of 38 mA/cm² at 0.90 V (iR-free) in an H₂-air PEFC, approaching the DOE 2025 current density target of 44 mA/cm². We have also demonstrated Fe-N-C catalysts with excellent durability, with no more than 30% loss of their initial activity at 0.70 V in a 600-hour PEFC test. Our Fe-N-C catalysts have also shown promise for anion exchange membrane fuel cells (AEMFCs), achieving 48 mA/cm² at 0.90 V (iR-free) and peak power density of 0.83 W/cm².

The development of sustainable carbon-neutral energy technologies to mitigate greenhouse gas emissions has become imperative and urgent. Of special interest and importance in this context is the use of electricity from intermittent renewable energy sources for electrochemical conversion of CO₂ to easily storable and transportable value-added products. Recently, M-N-C materials have emerged as promising CO₂RR catalysts thanks to their structure and high selectivity for certain reduction products, e.g., CO and HCOOH [4,5], though so far at lower current densities achieved with well-established Cu nanoparticle catalysts [6,7]. In this presentation, we will summarize our study of the activity, selectivity, and stability of M-N-C catalysts for CO₂RR in both an H-cell and at a rotating disk electrode (RDE). The CO₂RR activities have been improved by increasing metal loading, optimizing local M-N coordination environment, and enhancing metal-carbon substrate interactions. We will also demonstrate the impact of mass transport on performance of M-N-C catalysts in the H-cell and review methods of combatting the formation of hydrogen bubbles on stationary CO₂RR cathodes. Finally, we will discuss the use of M‑N-C catalysts in a tandem configuration to generate high energy-density CO₂RR products such as C₂H₄ and C₂H₃OH.

The rising demand of rechargeable batteries for electric vehicles and grid storage applications sparks a lot of interest on alternatives to “standard Li-ion battery technology”. The size of these markets is so large that great efforts are currently undertaken towards using more cost-effective materials that will not run into supply and/or resource constraints. Here, sodium-ion batteries are one important option that primarily aim at realizing high energy batteries based on sodium and other abundant elements such as carbon, iron or manganese.^[1] On the other hand, solid-state batteries (SSBs) are considered as promising option for electric vehicles. In these types of batteries, a solid electrolyte replaces the flammable organic liquid electrolyte, which improves safety. At the same time, SSBs might enable energy densities exceeding conventional lithium-ion technology.

This talk gives an overview on materials aspects on sodium-ion and solid-state batteries and how they compare to lithium-ion batteries. Specific examples on inorganic materials will be discussed, including high capacity metal/carbon negative electrodes^[2], layered oxides of the type Na[Mn_xFe_yTM_z]O₂^[3], solvent co-intercalation reactions (graphite)^[4] and metal sulfides (CuS, Cu₃PS₄, NaTi_xTM_yS₂)^[5] (TM = transition metal).

[1]          aI. Hasa, S. Mariyappan, D. Saurel, P. Adelhelm, A. Y. Koposov, C. Masquelier, L. Croguennec, M. Casas-Cabanas, Journal of Power Sources 2021, 482; bY. Li, Y. Lu, P. Adelhelm, M. M. Titirici, Y. S. Hu, Chemical Society Reviews 2019, 48, 4655-4687; cP. K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angewandte Chemie - International Edition 2018, 57, 102-120.

[2]          aT. Palaniselvam, C. Mukundan, I. Hasa, A. L. Santhosha, M. Goktas, H. Moon, M. Ruttert, R. Schmuch, K. Pollok, F. Langenhorst, M. Winter, S. Passerini, P. Adelhelm, Advanced Functional Materials 2020, 30; bT. Palaniselvam, M. Goktas, B. Anothumakkool, Y. N. Sun, R. Schmuch, L. Zhao, B. H. Han, M. Winter, P. Adelhelm, Advanced Functional Materials 2019, 29.

[3]          L. Yang, J. M. L. del Amo, Z. Shadike, S. M. Bak, F. Bonilla, M. Galceran, P. K. Nayak, J. R. Buchheim, X. Q. Yang, T. Rojo, P. Adelhelm, Advanced Functional Materials 2020, 30.

[4]          aI. Escher, Y. Kravets, G. A. Ferrero, M. Goktas, P. Adelhelm, Energy Technology 2021, 9; bM. Goktas, C. Bolli, E. J. Berg, P. Novák, K. Pollok, F. Langenhorst, M. V. Roeder, O. Lenchuk, D. Mollenhauer, P. Adelhelm, Advanced Energy Materials 2018, 8; cB. Jache, P. Adelhelm, Angew. Chem. Int. Ed.  2014, 53, 10169-10173.

[5]          aA. L. Santhosha, N. Nazer, R. Koerver, S. Randau, F. H. Richter, D. A. Weber, J. Kulisch, T. Adermann, J. Janek, P. Adelhelm, Advanced Energy Materials 2020, 10; bW. Brehm, A. L. Santhosha, Z. Zhang, C. Neumann, A. Turchanin, A. Martin, N. Pinna, M. Seyring, M. Rettenmayr, J. R. Buchheim, P. Adelhelm, Advanced Functional Materials 2020, 30; cF. Klein, B. Jache, A. Bhide, P. Adelhelm, Physical Chemistry Chemical Physics 2013, 38, 15876-15887.

Electrocatalyst materials that drive fuels cells and other fossil fuel-free energy technologies have shown to play an important role on the production of clean and sustainable energy. However, those electrocatalysts containing precious metals, such as Pt, are currently hindered by their short-term durability. Recyclability and re-use of highly active nanocatalysts is still an outstanding global challenge of increasing importance in energy conversion and heterogeneous catalysis[1]. As these precious elements are rapidly diminishing, the research community is forced to urgently address this major issue until more abundant efficient electrocatalysts are put forward. In this respect, hollow carbon nanostructures can provide an excellent mean for the fabrication of highly durable electrocatalyst materials through nanocatalyst confinement [2-4], allowing their sustainable use in a whole variety of electrochemical processes. Thus, these hybrid nanocatalysts have shown performance comparable to commercial electrocatalysts (Pt/carbon black), but most importantly they exhibit outstanding durability, retaining most of the electrocatalytic activity even after 50,000 and 30,000 cycles of the ORR and HER, and thus significantly outperforming all existing electrocatalytic systems under these conditions. The observed behaviours are directly linked to interactions between the metal nanoparticles and the graphitic step-edges within the carbon nanoelectrodes. These surprising and remarkable properties of the reported hybrid electrocatalyst materials have opened up a new strategy for the sustainable use of precious metals in electrocatalysis and other technological applications that require stabilization of metal nanoparticles under harsh conditions (Figure 1).

Fe single atoms in nitrogen doped carbon materials (Fe-NC) have attracted plenty of attention during the last decades in the field of electrocatalysis for oxygen reduction and carbon dioxide conversion amongst others. In the cathode of proton exchange membrane fuel cells Fe-NC are the most promising solution to scarce and expensive Platinum-group-metal catalysts;^[1] In CO₂ reduction, they have achieved similar performance to that of nanostructured Au and Ag. However, their controlled synthesis and stability for practical applications remains challenging. Approaches to enhance their catalytic performance include increasing the loading of Fe single atoms, for example by decoupling high temperature pyrolysis and Fe coordination atoms, or enhancing the intrinsic activity of the FeN_x sites through engineering of the coordination environment or by creation of dual atom catalysts.^[2,3]

Currently, the utilization of Fe within these materials remains very low owing to the lack of C-N scaffolds that combine adequate micro- and mesoporosity. In this work we employ inexpensive 2,4,6-Triaminopyrimidine (TAP) with MgCl₂.6H₂O as porogen to prepare a highly porous N-doped carbon material.^[4] The hydrogen bonding between nitrogen moieties of TAP and the water molecules of the Mg salt allows an optimal interaction during pyrolysis that leads to remarkable porosity in the nitrogen-doped material (~3300 m² g^-1) and very available N sites for Fe coordination. The subsequent low temperature Fe coordination results in a highly active O₂ reduction to electrocatalyst with a mass activity 4.0 A g^-1 at 0.8 V_RHE in acid electrolyte. Additionally, the material shows near 100% Faradaic Efficiency for the CO₂ reduction to CO (FE_co = 93.5% at -0.55 V vs RHE) with one of the highest TOF reported up to date (4.5 s^-1)

Aberration corrected high-angle annular dark field scanning transmission electron microscopy with energy dispersive x-ray spectroscopy of the catalyst confirms the solely atomic Fe and N distribution pre- and post-accelerated degradation tests. In-situ nitrite stripping reveals a high active site density of 2.54×10¹⁹ sites g^-1; the remarkable porosity of the graphitic material and hierarchal structure ensures remarkably high electrochemical active site utilisation of 42%. Ex-situ X-ray absorption extended fine structure suggests the presence of penta-coordinated FeN₅ sites, potentially enabling the stability of the active site for O₂ and CO₂ reduction.^[5]

Photoelectrochemical (PEC) solar water splitting could be a potential route to large-scale production of green hydrogen. However, systems based on earth-abundant metal oxides do not yet achieve the high efficiencies required for economically feasible applications. One obstacle to the realization of higher efficiency devices is the appropriate design of the semiconductor-electrolyte interface. In this context, we point out a common misconception that the built-in electric field at the solid-liquid interface is essential for charge separation [1]. A similar discussion, controversial at the time, regarding the role of the built-in field took place in photovoltaic (PV) research community. The purpose of this talk is to present the state-of-the-art knowledge in this field and to seek common ground for an interpretation valid for both photovoltaics and PEC. We hope to provide a more detailed understanding of the energy loss mechanisms and the driving forces that determine charge separation, transport, and recombination of electron and hole pairs in PEC devices. We believe this will be critical for selecting the most appropriate design routes. We emphasize the well-established viewpoint in the photovoltaic research community that the gradient of electrochemical potential is the main driving force for charge separation and efficient solar energy conversion [2]. Based on this insight, we argue that improved (selective) contact design in PEC devices should be one of the most important research directions in PEC device development. To address this challenge, we take a closer look at how optimized contacts have been designed to date and provide examples of potential approaches that may be implement to further improve the performance of PEC devices [3].

Keywords: Water splitting, PEC, charge selective contacts, charge separation, charge transport, interfaces, drift and diffusion, quasi fermi level gradients

The electrocatalytic reduction of CO₂ to formic acid can be performed with promising faradaic efficiencies.[1] It is one of the first reduction products of CO₂ and H₂O having a relatively high added value among the reported products.[1] Nevertheless, for practical applications, several requirements need to be met. For example, the current density needs to surpass 100 mA/cm², and the catalyst should be stable and active for thousands of hours.[1] To improve this, researchers have devoted significant efforts to understand the active phase of promising electrocatalysts such as In₂O₃ or Bi₂O₃. Several reports suggest that the presence of an oxide surface state of the catalyst is necessary for a high faradaic efficiency towards formate.[2],[3] Such studies were performed for relatively short times (<5 h) and at low current densities (<5 mA/cm²), meaning that the involved oxide phases might not be stable during prolonged use.

Here we report on the use of In₂O₃ nanoparticles in a gas diffusion electrode (GDE) configuration for CO₂ reduction under practical conditions (high current density, long reaction time). Flame spray pyrolysis (FSP) was used to synthesize In₂O₃ nanoparticles with precise size and crystal structure as shown by TEM and XRD. These particles show good faradaic efficiencies (FE > 80%) at current densities up to 200 mA/cm². Different GDE configurations were prepared and compared in CO₂ electroreduction to understand its influence on overall performance. The active phase was characterized by XPS and in situ Raman spectroscopy. The results show that, under the applied conditions, the initial indium oxide phase is readily reduced, yet remains active for CO₂ reduction to formate. By tuning the carbon support and hydrophobicity in the catalyst layer of the GDE, the electrode maintains its activity during 50h CO₂ electroreduction.

All-perovskite tandem solar cells promise high photovoltaic performance at low cost. So far however, their efficiencies cannot compete with traditional inorganic multi-junction solar cells and they generally underperform in comparison to what is expected from the isolated single junction devices. Understanding performance losses in all-perovskite tandem solar cells is a crucial aspect that will accelerate advancement. Here, we perform extensive selective characterization of the individual sub-cells to disentangle the different losses and limiting factors in these tandem devices. We find that non-radiative losses in the high-gap subcell dominate the overall recombination losses in our baseline system as well as in the majority of literature reports. We consecutively improve the high-gap perovskite subcell through a multifaceted approach, allowing us to enhance the open-circuit voltage (VOC) of the subcell by up to 120 mV. Due to the (quasi) lossless indium oxide interconnect which we employ for the first time in all-perovskite tandems, the VOC improvements achieved in the high-gap perovskites translate directly to improved all-perovskite tandem solar cells with a champion VOC of 2.00 V and a stabilized efficiency of 23.7%. The efficiency potential of our optimized all-perovskite tandems reaches 25.2% and 27.0% when determined from electro- and photo-luminescence respectively, indicating significant transport losses as well as imperfect energy-alignment between the perovskite and the transport layers in the experimental devices. Further improvements to 28.4% are possible considering the bulk quality of both absorbers measured using photo-luminescence on isolated perovskite layers. Our insights therefore not only show an optimization example but a generalizable evidence-based strategy for optimization utilizing optical sub-cell characterization.

Porous carbons with tuneable functionalities and morphologies have extensively been employed as electrode materials in a variety of electrochemical energy conversion and storage systems for instance in fuel cells and electrolysers as active catalysts and catalyst supports, and in secondary batteries as anode materials. Amorphous carbons with well-developed pore structures are of particular interest due to their superior mass-transport characteristics and remarkable charge storage capacities. The salt-templating method with its advantage of combined soft and hard templating effects provides a sustainable way to synthesize nano- and mesoporous carbons with tailored porosities via in-situ ionothermal template transformation [1]. In this work, we utilized a MgCl₂-based salt melt to prepare nitrogen doped carbons (N-C) with different morphologies and porosities, which were evaluated as anode materials in sodium ion batteries. Simultaneously, use of MgCl₂ salt leads to the formation of Mg-N₄ moieties in those carbons by means of a pyrolytic template-ion effect (active site imprinting) [2]. Porous carbon frameworks with imprinted Mg-N₄ sites are interesting particularly for electrocatalysis applications as they offer an ideal platform to prepare M-N-C catalysts (where M= Co, Fe, Ni etc.) by ion-exchange reactions at low temperatures. The resultant M-N-C catalysts consist purely of M-N₄ active sites and high porosity of carbon framework facilitates efficient mass-transport of reacting species.

We utilized Mg-N₄ imprinted carbons to synthesize morphologically equivalent Ni-N-Cs and Co-N-Cs, containing phase pure Ni-N₄ and Co-N₄ sites, for electrochemical reduction of carbon dioxide (CO₂RR). In electrochemical tests, Ni-N-Cs exhibited an excellent CO₂ reduction activity with considerably higher CO selectivity and mass activity as compared to Co-N-C. The faradic efficiency value of Ni-N-C for CO formation was 95% at U= -0.5 to -0.8 V_RHE (vs reversible hydrogen electrode) and a mass activity of 23 A g^-1. The performance stability test carried out at -0.65 V_RHE demonstrated above 90 % retention of the current density and CO selectivity after 100 h of continuous operation, reflecting the structural robustness of the Ni-N-C catalyst.

Finally, these ionothermal carbons with two different morphologies (but without any Ni or Co incorporation) were employed as the anode materials in sodium-ion batteries to evaluate the effects of carbon morphology and functionalization on sodium storage capacities. Compared to the reference carbon material, substantially higher reversible sodium storage capacities were reached with these high porosity carbons that were in the range of 300-500 mAh g^-1 [3]. Although the reversible capacity was obtained only after extensive SEI formation, our results reveal the potential for much higher reversible capacities than usually observed using carbons with a tailored porosity in sodium-ion batteries. The talk will include greater details of the structural analysis and sodium storage and CO₂ reduction results of these ionothermal carbons.

Iron-based catalysts represent one of the most promising alternatives to platinum for oxygen reduction. However, the mechanism of oxygen reduction on Fe-based catalysts is still debated and the challenge for pyrolyzed catalysts is to unambiguously identifying the active sites.  

Iron macrocycles can assist in understanding the mechanism of oxygen reduction by providing well-defined and reproducible active sites. A first indication of the difference in ORR kinetics among different macrocycles can be obtained by observing the cyclic voltammetry (CV) peaks. Most Fe macrocycles present two peaks in the cyclic voltammetry curve. The conventional wisdom is that the peak at more positive potentials is due to the desorption of *OH and consequent reduction of iron from Fe^III to Fe^II, and the peak at more negative potentials is due to the further reduction of iron to Fe^I.¹ The position of the peak due to *OH adsorption provides a measure of the binding energy of this species; which, in turn, is controlled by the electron-withdrawing or donating nature of the ring.² This causes a previously reported correlation between the position of the OH adsorption peak and the ORR activity.^3,4 However, other macrocycles only present one potential peak in the CV and the activity is inconsistent with its position

In this work we provide a systematic study of oxygen reduction on selected Fe-macrocycles, combining DFT calculations, electrochemical measurements, and in-situ characterization, including operando UV-Vis spectroscopy and x-ray absorption spectroscopy. Our investigations shows that the OH adsorption peak is not accompanied by a change in Fe oxidation state, while the peak at more negative potentials results from the reduction of iron from Fe^III to Fe^II. Furthermore, our analysis indicates the reason why some macrocycles do not show a second voltametric peak in the experimental CV. The insight gained by studying these model molecules will allow the interpretation of more commercially relevant pyrolyzed catalysts and to understand the nature or redox activity in molecular catalysts. 

Carbon is a key electrode material in energy-related applications.  A large number of studies have revealed the importance of designing carbon electrodes in terms of their porous morphology.  In most cases, porous carbons in a powdery form are produced, which are fixed on an electrode substrate by mixing with binders and conductive agents, resulting in so-called composite electrodes. These additives make it difficult to correlate physicochemical characteristics of an electrode material with electrochemical properties of a composite electrode thereof. In addition, it is impossible to control the porous morphology of a composite electrode because of the uncontrollable pore properties of interparticle gaps. In this context, free-standing and binder-free monolithic electrodes have advantages over the composite electrodes in the controllability of electrodes.

Sol–gel process is a powerful tool to tailor various porous structures at different length scales in a monolithic gel.  Several strategies such as supramolecular self-assembly, hard templating, and phase separation have been developed to date.  Among them, the sol–gel process accompanied by phase separation offers three-dimensionally interconnected macroporous structures with narrow pore size distribution [1], which contribute to efficient mass transport in a monolith.  In addition, the combination of the phase separation method with other techniques allows us to introduce smaller pores in macropore frameworks resulting in a hierarchically porous structure [2,3].

In our research group, we have developed porous resorcinol-formaldehyde (RF) gels based on the phase separation method.  Carbon monoliths with controlled pore properties can be obtained by carbonization of the porous RF gels, which are available as free-standing and binder-free electrodes for energy storage devices, such as supercapacitors [3] and batteries [4,5].  The details about the syntheses, pore controls and electrochemical investigations of the monolithic carbon electrodes will be presented [6].

Sodium ion batteries (SIBs) emerged as alternative energy systems to lithium ion batteries (LIB) owing to the large abundance and availability of sodium resources, which might afford a long term and low cost solution to satisfy continuous growing demands. Hard carbon (HC) materials become the most appealing anodes due to their large interlayer distance and disordered structure facilitating Na-ion insertion-extraction. Many precursors were explored, leading to the design of hard carbons with different features and electrochemical performances. The aim of this work is to give a general overview on how the synthesis conditions (precursors, temperature, process) impact the hard carbon characteristics (porosity, surface chemistry, morphology and structure) [1-2]. Further, the relationship between the HC properties and the performance (initial coulombic efficiency, capacity and cycle stability) will be emphasized [3,4]. Lastly, insights on the sodium storage mechanism in hard carbon will be provided [3, 5] along with some challenges that need to be still addressed [6].

The advent of solid-state batteries has spawned a recent increase in interest in lithium conducting solid electrolytes. However, many open questions remain when trying to optimize electrolytes and understand solid state battery chemistries.

In this presentation, we will explore the current focus of halide-based ionic conductors in solid state batteries and discuss stability limitations in solid state battery cells at the anode as well as the cathode composites.

In a second part, we show the influence of Si type anode materials on the effective transport and behavior of solid-state batteries

Finally, we will discuss that it is not only important to find fast ionic conductors but that for an effective thermal battery management the thermal transport properties of solid ionic conductors need to be explored and understood. Here we will show the diffusive thermal transport nature of solid electrolytes and their different scaling relations that put in question the assumption of Bruggeman transport in solid state batteries.

Solid polymer electrolytes based on poly(ethylene oxide) (PEO) are of considerable interest in lithium metal batteries. Electrolytes with high modulus are essential for enabling high specific energy batteries with lithium metal anodes [1]. One approach for obtaining high modulus electrolytes is through the use of microphase separated block copolymers comprising a structural block to provide mechanical rigidity (block A) and a soft conducting block to provide avenues for ion transport (block B) [2]. Block copolymers self-assembly into different morphologies such as lamellae, bicontinuous gyroid networks, cylinders arranged on a hexagonal lattice, and spheres arranged on a body-centered-cubic lattice. The ionic conductivity of a block copolymer electrolyte should increase with the volume fraction of the conducting phase. Herein, we explore the impact of several polymers (PS, PMA-POSS, etc.), lithium salts (LiTFSI, POSS-LiTFSI, etc.) and volume fractions of the conducting phase on morphology, mechanical and electrochemical properties of nanostructured block copolymer electrolytes [3,4].

References

1 Soo, P.P; Huang, B.; Jang, Y.; Chiang, Y.M.; Sadoway, D.R.; Mayes. A.M. Rubbery block copolymer electrolytes for solid‐state rechargeable lithium batteries. J. Electrochem. Soc. 1999, 146, 32.

2 Hallinan, D.T.; Villaluenga I.; Balsara N.P. Polymer and Composite Electrolytes. MRS Bulletin, 2018, 43, 759.

3 Sethi, G.; Jiang, X.; Chakraborty, R.D.; Villaluenga, I.; Balsara, N.P. Anomalous Self-Assembly and Ion Transport in Organic-Inorganic Block Copolymer Electrolytes. ACS Macro Letters, 2018, 7, 1056.

4 Villaluenga, I.; Inceoglu, S.; Jiang, X.; Chen,X.C.; Chintapalli, M.; Wang, D.R.; Devaux, D.; Balsara, N.P. Nanostructured Single-Ion-Conducting Hybrid Electrolytes based on Salty Nanoparticles and Block Copolymers. Macromolecules, 2017, 50, 1998.

The possibility of developing practical solid-state batteries (SSBs) has gained momentum in recent years thanks to striking advances at the material level, such as the discovery of new highly-conducting solid-state electrolytes. However, the development of SSBs still faces formidable challenges, such as the integration of the various components, the functionality at full cell level, and the scalability of the fabrication processes.[1] Inorganic solid electrolytes stand out for their high Li-ion conductivity but exhibit drawbacks such as low electrochemical stability (sulfides) or rigidity (oxides, phosphates). Indeed, oxide and phosphate-based electrolytes usually require temperature-assisted densification methods that, when applied to composite cathodes, can induce chemical reactivity among the different cathode components resulting in resistive interfaces and by-products.

In this talk we will discuss our recent work regarding oxide and phosphate-based ion-conductors, including the recent synthesis and characterization of a new form of crystalline LiPON,[2] processing challenges of rigid ceramic materials (in particular regarding high temperature material compatibility in composite cathodes), and low-temperature densification strategies, which in turn allow to shed light into grain-boundary contributions to the total ionic conductivity of the studied materials.

Solid-state batteries pairing a ceramic solid electrolyte with an alkali metal anode promise to improve the safety and energy density of cells. However, cycling solid-state cells at practical current densities in the mA/cm² range can lead to cell failure. While much attention has been devoted to understanding dendrite penetration when plating the anode (charging), the formation of voids when stripping the metal anode (discharging) also has an important role in failure. In this study, stripping of lithium and sodium metal anodes has been investigated as a function of current density, stack-pressure and temperature, revealing the importance of creep in the alkali metal anode on void formation. To explore this, we have used a combination of 3-electrode cells, scanning electron microscopy and X-ray tomography. We show that above a critical current density on stripping, voids will accumulate on cycling at the metal anode/solid electrolyte interface, eventually leading to failure of the cell. It is therefore necessity to strip the metal anode below this limiting current density, as discharging above this critical rate leads to high local currents that can trigger dendrite penetration into the solid electrolyte on subsequent charges. However, we demonstrate that cycling solid-state batteries under moderately elevated stack-pressures and temperatures increases the critical current for voiding by promoting creep in the metal anode, enabling stable discharge at higher rates.

Solid-state batteries with polymer electrolytes (PEs) are at the forefront of candidates to boost energy density and improve the safety of conventional Li-ion batteries. PEs require a wide electrochemical stability window to withstand high-voltage positive electrodes (i.e. LiNi_0.6Mn_0.2Co_0.2O₂, NMC622) and offer stability with the negative electrode (typically Li-metal).¹ However, solid PEs still suffer from low electrochemical stability against highly oxidative and reductive potentials simultaneously. The Double Layer Polymer Electrolyte (DLPE) approach overcomes this issue from the choice of appropriate polymers for the positive electrode and separator respectively, combining them within the same electrochemical device. In this work, polyethylene oxide (PEO), offering stability against Li metal, and polypropylene carbonate (PPC), offering stability at high potentials, were used to prepare DPLE cells with NMC622 and Li-metal electrodes. Nevertheless, the use of a conventional dual-ion conductor lithium-salt (e.g. lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) was found to play a key role on the thermodynamical compatibility between the polymer phases due to ion diffusion from PPC to the more solvating PEO, leading ultimately to cell failure. By modifying the lithium-ion conductive salt we successfully overcome those hurdles and provide guidelines on the design of improved performance cells with DLPE technology.² Ultimately, this allows to assemble cells with high loading NMC622 (1 mAh cm^–2) delivering 160 mAh g^–1 and overcoming the performance of ethylene oxide -based cells, which rapidly degrade in the presence of high cut-off voltage active materials.³ This study provides a detailed understanding on the transport properties and ageing mechanisms taking place in SSLMBs, inspiring further the rational design of new DLPE technology with robust electrochemical performance.

Many efforts in the field of Li-ion batteries are focusing on the development and implementation of solid electrolytes in order to overcome the drawback related to their liquid counterparts such as: flammability, complex encapsulation requirements, high cost and complexity manufacturing processes.  Li-conducting glass-based materials gained attention in the last years as solid state electrolytes for lithium batteries, thanks to their good ionic conductivity at room temperature (10^-3-10^-4 S cm^-1) and their wide electrochemical stability windows.    

In the present work, Li_1.5Al_0.5Ge_1.5P₃O₁₂ (LAGP) glass was 3D-printed by robocasting and stereolitography (SLA) in free-form robust self-standing structures with the main target to obtain 3D batteries with high active area (allowing high specific energy and power per unit volume). The use of 3D printing techniques allowed the fabrication of simple as well as complex architectures, with enhanced contact area with the electrodes. The inks and the printing processes were both optimized in order to reach an accuracy up to ≈100 µm. The printed structures demonstrated an excellent densification after the firing/sintering treatment, as a result of the optimization of the inks formulation, printing process and sintering conditions. Furthermore, ionic conductivity of 3D-printed LAGP electrolyte resulted to be 1.8 10^-4 S cm^-1 at room temperature, in accordance with the values measured on the same material processed with conventional methods. No detrimental reaction products or degradation phenomena were detected by chemical analyses after the sintering.

3D-printed LAGP was successfully cycled in contact with metal-Li. The interface with the anode was previously engineered by depositing 200nm of metallic Ge in order to prevent detrimental reactions between the two materials. A symmetrical cell Li/Ge coated LAGP/Li was successfully cycled for more than 100h.

The successful implementation of 3D printing techniques in LAGP processing represents an innovative approach that will push further the development of all solid state Li-ion batteries with enhanced energy density, thanks to the easy fabrication of 3D structured solid electrolytes.

Self-diffusion in polycrystalline Li1+xTi2-xAlx(PO4)3 (0.2 ≤ x ≤ 0.4) samples followed by 7Li PFG (Pulse Field Gradient) NMR spectroscopy

Isabel Sobrados, Virginia Diez-Gómez, Cristina Ruiz-Santaquiteria, Wilmer Bucheli,

Ricardo Jiménez, Jesús Sanz.

Short and long range lithium motions are discussed in powder Li1+xTi2-xAlx(PO4)3 (LTAP) NASICON compounds prepared by ceramic (x= 0.2 and 0.4) and sol-gel (x= 0.3 and 0.4) routes.

Self-diffusion coefficients were determined with the PFG (pulse field gradient) technique. In these experiments, the stimulated echo π/2-t1-π/2-t2-π/2 sequence was used, in which two field gradient pulses of δ width and g intensity were applied between the two first π/2 pulses and after the third π/2 radiofrequency pulse. From the echo-signal attenuation, induced by the increment of exponent parameters, self-diffusion coefficients (DPFG) were deduced, using the Stejskal and Tanner expression [1]

A(2t1+t2)/A0(2t1+t2)=exp[-ɣ2.g2.δ2.(Δ-δ/3)·DPFG]=exp(-b·DPFG)

where A and A0 stand for echo signal intensity at (2t1+t2) with and without field gradient pulses, ɣ is the nuclear gyro-magnetic ratio, and Δ is the diffusion time used in experiments.

In PFG experiments, time scales differ considerably from those involved in 1/T2 and 1/T1 measurements, however, DPFG values deduced for short Δ times are similar to those deduced by NMR relaxometry. In all analyzed samples, diffusion coefficients measured at short Δ times, are between 5x10-12 and 1x10-11 m2 s-1. At increasing Δ, diffusion coefficients decrease due to restriction. In ceramic LTAP02-C sample, Li diffusion is less restricted than in LTAP04-C sample, where DPFG values increase and the particle size decreases. The analysis of DPFG coefficients in sol-gel LTAP03-SG and LTAP04-SG samples, show strong restriction effects that considerably reduce DPFG values when Δ increases, suggesting that Li diffusion is strongly restricted when the LTAP particles are smaller than 1µm. The restricted diffusion inside NASICON particles is compared to "free" diffusion processes.

From the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carrier concentrations were determined. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature.

To reduce restriction effects, denser samples should be prepared. In future works, the PFG technique will permit a better optimization of transport properties in fast ion conductors prepared with different conditions.

1. J.E. Tanner, E.O. Stejskal. J Chemical Physics. 49, 1768- 1777 (1968).

The development of high performance all-solid-state batteries relies on the development of solid electrolytes that have high conductivity, wide chemical, electrochemical and mechanical stability and are easy to process at low temperatures. Halide-type solid electrolytes are a relatively new class of solid electrolytes being studied owing to their compatibility with high voltage cathodes and processability at low temperatures however few structures are known that possess high ionic conductivities within the halide family. Li₂ZrCl₆ is one such material that demonstrates moderate ionic conductivity of 10^-6 S/cm while Cu₂ZrCl₆ exhibits conductivities several orders of magnitude higher. Understanding the factors affecting lithium mobility in A₂ZrCl₆ can aid the design of novel solid-halide electrolytes with tailored frameworks for high ionic conductivity.

In this work, we investigate the composition A₂ZrCl₆ where A is a monovalent cation and the role the A site element has on the crystal structure, ionic conductivity and the associated migration pathways. We demonstrate via long timescale dynamics i.e. kinetic Monte Carlo (KMC) and molecular dynamics (MD) that the A site element does affect the A⁺ ionic conductivity of the material. In general, the more covalent character the A site element possesses the higher ionic conductivity the material exhibits. These results provide a basis to understanding the origin of fast ion transport in solid halide materials.

The solid-state electrolyte field knows a hard competition to provide an ideal composition which needs to fulfill numerous criteria such as, high Li⁺ ionic conductivity, low electronic conductivity, and a high level of mechanical, chemical, electrochemical and air stability as well as interface stability against active electrode materials. Several compositions and chemistries have been investigated, including: oxides, sulfides, halides, polymers; each of them getting their own strengths and weaknesses.^[1]

Dense thin-film solid–state electrolyte separator are interesting to offer (a) a model system to investigate interfaces, and more importantly, (b) a promising pathway to increase the energy density of solid-state cells. The reference of thin-film solid-state electrolyte, for a while, is the lithium phosphate oxynitride LiPON, due to its easy way of deposition/synthesis via reactive RF-Magnetron sputtering. However, LiPON has a ceiling ionic conductivity of ~10^-6 S.cm^-1.^[2]

For a decade, the new family of Lithium Rich Antiperovskite (LiRAP) Li_3-xOH_xHal (Hal = Br,Cl) has been investigated and exhibits low raw material costs, low temperature of synthesis and a bulk ionic conductivity of ~10^-3 S.cm^-1.^[3][4] In addition, previous studies report the possibility to use PVD method to obtain thin layers of the Li₃OCl composition with an acceptable Li+, and electronic conductivity.^[5][6]

Here, we explore different ways to synthesize thin layers of the Li₃OCl solid-state electrolyte by using RF-Magnetron Sputtering or e-Beam with a post annealing treatment. We will presents the structural, chemical and physical characterization, for LiRAP films prepared on glass and platinum coated sapphire substrates, via grazing incident x-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, impedance spectroscopy and chronoamperometry. These findings might accelerate the development of thin film electrolyte as a potential pathway to further increase the energy density of solid-state cells.

In this lecture I discuss a digital twin devoted to the accelerated optimization of the manufacturing process of Lithium Ion Batteries (LIBs). This digital tool is developed by us within the context of the ERC-funded ARTISTIC project.¹ It is supported on a hybrid approach encompassing experimental characterizations, a physics-based multiscale modeling workflow and machine learning models.² Different steps along the LIB cells manufacturing process are simulated, such as the electrode slurry, coating, drying, calendering and electrolyte infiltration. The physics-based modeling workflow encompasses a sequential coupling between Coarse Grained Molecular Dynamics, Discrete Element Method and Lattice Boltzmann Method. It allows predicting the impact of the process parameters on the final electrode microstructure in three dimensions. The predicted electrode microstructures are injected in a performance simulator capturing the influence of the pore networks and spatial location of carbon-binder within the electrodes on the electrochemical response. Machine learning models are used to accelerate the physical models’ parameterization, to mimic their working principles and to unravel manufacturing parameters interdependencies from the physical models’ predictions and experimental data, and for inverse design. The predictive and optimization capabilities of this digital twin, coupling physical models with machine learning models, are illustrated with results for different electrode formulations in the context of LIBs. I also demonstrate the applicability of our approach to Sodium Ion and Solid State Batteries. Finally, the free online battery manufacturing simulation services offered by the project³ to optimize battery electrodes are illustrated through several examples.

Solid-state electrolytes with fast lithium conduction are the core of the all-solid-state Li-battery technology. By substituting the organic electrolyte with a piece of non-flammable ceramic material, we can achieve better safety, higher specific capacity, and a higher energy density. To date, the major bottleneck for this technology is the slow Li diffusion in the solid-state electrolyte and the interfacial incompatibility between the electrolyte and electrodes. To resolve these issues, several families of fast ionic conductors have been developed. Understanding Li diffusion in these materials is essential to the development of novel family fast ionic conductors. To this end, atomistic modeling provides us with a unique tool to obtain comprehensive information on atom motion, which is difficult to access with experimental techniques. In this talk, we showcase our group’s atomistic simulations regarding a novel family of superionic conductors, Li-rich antiperovskites (LiRAPs)

LiRAPs are a promising family of solid electrolytes, which exhibit ionic conductivities above 1 mS/cm at room temperature, among the highest reported values to date. Here, we report on the defect chemistry and the associated lithium transport in Li₃OCl, a prototypical LiRAP, using DFT calculations and classical MD simulations [1]. We studied these materials’ phase, interfacial, and voltage stability [2,3] with DFT, showing good agreement with experiments, further proposing low-dimensional superionic antiperovskites [3]. In addition, the interfacial properties were studied for both protonated and fluorinated materials [4]. Analogous simulations were also carried out for Na-rich antiperovskites [5].

Recent proof-of-concept of electrolytes enabling successful plating and stripping of calcium metal catalyzed research exploring the feasibility of a rechargeable battery technology based on calcium.[1] Estimates of prospective figures of merit at the cell level using open energy-cost models[2] indicate that performances comparable to state-of-the art LIB could be achieved with positive electrodes exhibiting moderate potential (2.5V) and a capacity of 200 mAh/g, and that volumetric energy densities above 1000 Wh/l would be possible considering the same capacity but operation at 3V.

In spite of such nice expectations, the investigation of suitable positive electrode materials has been plagued with a number of hurdles.  On one hand, the lack of standard electrolytes and protocols may result in side reactions inducing misinterpretation of the electrochemical response achieved, which makes real bulk activity difficult to elucidate unless complementary characterization techniques are used. On the other, testing of materials similar to those used in the Li-ion battery field which would enable reversible intercalation/de-intercalation of calcium has to date led to the identification of few active compounds with diverse success.[3] Electrochemical extraction of calcium in some ternary transition metal ions is feasible but the reversibility of the process is more difficult to achieve, which is likely related to strong solvation of calcium ions, with reactions sometimes involving solvent co-intercalation and high cell overpotential.

Overall, there is a long and winding road to follow before reliable proof-of-concept can be achieved and technological prospects evaluated. Development of reliable experimental setups, including reference and counter electrodes, coupled to complementary characterization techniques, as well as computational tools, is mandatory if steady progress is to be achieved.

Solid-state batteries (SSBs) offer the opportunity to leverage energy dense metallic anodes and high voltage cathodes to achieve safe, durable, and affordable secondary energy storage systems^1-3. However, the intrinsic thermodynamic, electrochemical and mechanical instabilities at the buried interfaces limits their performance. Unfavorable electro-chemo-mechanical dynamics can contribute to non-optimal material utilization, mechanical degradation, and poor ion transport. These mechanisms occur at various time- and length- scales and are not fully understood. Lithium metal anodes undergo a lithium deposition reaction which involves the reduction reaction of Li/Li⁺, and a lithium stripping reaction which involves the oxidation of surface Li atoms.  Deposition processes are surface driven mechanisms and have been widely studied in order to understand dendrite formation.  Stripping processes are sub-surface driven mechanisms and involve charge transfer at the interface between lithium and the solid electrolyte (Li|SE). Li stripping can alter the morphology and structure of the lithium metal interface and affect the subsequent deposition process. Non-uniform stripping can lead to sub-surface/interfacial void formation and decrease interfacial contact area between the Li anode and solid electrolyte. Smaller interfacial surface area can lead to high local current densities and decrease the cells threshold for dendrite formation (cell failure). While there are numerous theoretical studies that provide mechanistic hypotheses for pore formation, directly probing lithium stripping is challenging because the phenomena of interest occurs at buried interfaces. Multi-scale characterization of these materials and their electro-chemo-mechanical responses are pivotal towards engineering solid electrolytes for high power and energy applications. Herein, we combine bench top experiments with synchrotron techniques to resolve chemo-mechanics across multiple length scales which impact electrodeposition and dissolution mechanisms (grain, pore, interface).  

Lithium and sodium (Na) mixed polyanion solid electrolytes for all-solid-state batteries display some of the highest ionic conductivities reported to date. However, the effect of polyanion mixing on ion transport properties is still debated. Here, we focus on Na_{1+ x}Zr₂Si_xP_{3− x}O₁₂ (0≤ x≤ 3) NASICON electrolyte to elucidate the role of polyanion mixing on Na-transport properties. Although there is a large body of data available on this NASICON system, transport properties extracted from experiments or theory vary by orders of magnitude, signifying the need to bridge the gap between different studies. Here, more than 2,000 distinct ab initio-based kinetic Monte Carlo simulations have been used to map the statistically vast compositional space of NASICON over an unprecedented time range and spatial resolution and across a range of temperatures. We performed impedance spectroscopy of samples with varying Na compositions revealing that the highest ionic conductivity (~ 0.1 S cm^–1) is achieved in Na_3.4Zr₂Si_2.4P_0.6O₁₂ , in line with our predictions (~ 0.2 S cm^–1). Our predictions indicate that suitably doped NASICON compositions, especially with high silicon content, can achieve high Na⁺ mobilities. Our findings are relevant for the optimization of mixed polyanion solid electrolytes and electrodes, including sulfide-based polyanion frameworks, which are known for their superior ionic conductivities.

Developing high energy density and fast charge/discharge batteries requires new coatings strategies to overcome interfacial impedances between battery cell components. Silicon is a promising anode material for next-generation batteries due to its high volumetric capacity of 2190 mAhl^-1 and low discharge potential of ~ 0.3 V vs Li/Li⁺. In this paper, we report the fabrication, characterisation and electrochemical performance of nano Si electrodes coated with titanicone (TiGL) using a combination of Atomic Layer Deposition (ALD) and Molecular Layer Deposition (MLD). The optimised coated electrode delivers a capacity of 1200 mAhg⁻¹ at 1C for 350 cycles with a capacity retention of 93 % and a high C-rate performance of 150 mAhg⁻¹ at 20C. The improved discharge capacity, efficiencies, rate capability and electrochemical stability are related to enhanced lithium kinetics, stable SEI formation, and better structural integrity of the TiGL coated electrode compared to bare silicon. The use of titanicone as an interface modifier is discussed in the frame of high capacity electrode materials and solid state batteries.

All-solid-state thin-film batteries (TFBs) combine several features of solid-state batteries, such as long lifetime, high charge/discharge rates, and safety with the potential of large-scale manufacturing using physical vapor deposition (PVD) methods. The talk will review several research highlights from our group on how to build an efficient TFB. These include flash-lamp annealing of layered oxide cathodes that enable thin and flexible aluminum foils to be used as conductive substrates [1], engineering a cathode/electrolyte interface with Nb₂O₅ interlayers for a fast ionic transport [2], exploiting ultra-thin amorphous LLZO electrolytes to block lithium dendrites [3], and inserting amorphous carbon interlayers at the Cu/LIiPON interface to facilitate homogeneous Li plating and stripping in an anode-free configuration [4]. At last, we will present the concept of monolithic multi-cell (so-called bipolar) TFBs fabricated with PVD methods, which could resolve the principal handicap of the low specific and areal capacity in TFBs and lead to a new generation of high-power solid-state batteries.

Solid-state batteries offer great potential for large improvements in safety and lifetime, as well as higher energy and power densities. However, the interfacial composition and structure between solid electrolytes and electrode materials often present major deviations from those of the bulk materials. Elucidating the nature of the involved interfaces is required to establish a rational approach towards the successful combination of materials in a new generation of solid-state cells. Controlled interfaces between a solid-state electrolyte, cathode and anode have been realized in 2D-planar and 3D-vertical thin film geometries by applying pulsed laser deposition. The thin film epitaxial model geometries enable full control over the material combination, crystal orientation and elemental termination at all interfaces to explore their evolution during battery operation. Epitaxial engineering is used to realize crystalline thin film model systems, which enables an unique insight into the relation between electrochemistry and structural ordering of the layers and interfaces, not obtainable in conventional polycrystalline battery architectures. Here, I will show the latest results on several different thin film battery architectures, 2D as well as 3D, involving various different cathodes (LiMn2O4, LiCoO2, NMC111, NMC622, NMC811), anodes (Li4Ti5O12, Nb18W16O93) and solid electrolytes (Li0.33La0.57TiO3, Li7La3Zr2O12, Li3PO4).

All-solid-state batteries are seen as next generation energy storage systems and heavily researched worldwide. Within the three major clases of solid electrolytes, oxide-ceramic Li-ion conductors attract significant attention due to their intrinsic safety.  In case of the garnet type Li7La3Zr2O12 (LLZO), the possibility to directly use Li-metal anodes paves the way for high energy densities on cell and battery level. However, the widespread application of such an oxide-based all-solid-state cells depends on the development of scalable synthesis and manufacturing processes while increasing the cell capacity to competitive levels.
While oxides are the most stable solid electrolytes when compared to poylmers or sulfides, there is still some sensitivity to air and protic solvents due to the Li+/H+-exchange. However, with careful tailoring of the manufacturing process due to mechanistic understanding allowed for the development of a new, tailored tape-casting process for LLZO components. 70 µm thick, free-standing LLZO separators can now be fabricated via water based tape casting and all-ceramic free-standing LiCoO2/LLZO mixed cathodes show high active materials utilization. Microstructural engineering by introduction concentration gradients for the active material and the electrolyte allowed to boost discharge capacities to 2.8 mAh cm-2 utilizing 99% of the theoretical capacity. Additionally, the obtained free-standing cathodes have sufficient mechanical stability to allow the application of hybrid and ultra-thin separators to further increase the energy density on the full cell level to industrially relevant levels.

Solid-state lithium metal battery has a huge potential to be a more energy-dense architecture, safety and fast charging capability relative to a current lithium-ion battery. In particular, the cell interface made of oxide-based electrolyte and cathode is believed to withstand more tightly than any other ‘soft’ solid electrolytes (polymer, sulfide electrolytes) once optimized. Sintering at high temperature is generally required to produce dense interfaces for oxide-based electrochemical devices; however, the poor chemical compatibility of the oxide electrolyte with common cathode active materials limits a wide range of material choice, processing and microstructure-performance relationship. Here, potential processing strategies to fabricate oxide-based cathode composite are presented based on an infiltration of LiCoO₂ active material and Li₇La₃Zr₂O₁₂ catholyte through the porous scaffolds or a co-firing of both. Discussion is aimed toward opportunities to meet important target parameters including low interfacial resistance, high-loading capability and specific capacity for high energy and power Li-metal battery.

Solid-state lithium-ion batteries are a promising energy storage solution which promise greater safety with energy and power density comparable to current Li-ion technology. Although many advances have been made recently on the materials level, there still remains a need to understand how the properties of individual parts of the cell (e.g. electrolyte, active material, interfaces) combine to determine device performance on the cell level [1]. Research has focussed on improving solid-state battery performance by tailoring individual properties, such as the electrolyte’s ionic conductivity κ, the cathode active-material conductivity σ, and interface resistances or capacitances. When several materials are brought together to form a porous electrode, the cell performance does not always reflect individual properties in straightforward ways. Optimization of cell designs to achieve satisfactory performance requires an understanding of how the properties of various cell constituents interact. We believe that at this early stage of solid-state battery development, broad and simple design principles are preferable to finely resolved mechanistic models.

To that end, in this work we present a simple model of a planar solid-state battery cell based on the Newman–Tobias theory [2], which illustrates the roles of cathode porosity, interfacial contact, electrolyte ionic conductivity, and active-material electronic conductivity. We show that the current distribution in the porous cathode is governed by a few key dimensionless parameters, including the fraction of the conductivities of the two phases s=σ/(σ+κ) , and the ratio of the kinetic resistance to the bulk resistances. The results show – somewhat counterintuitively – that balancing the bulk conductivities of the interpenetrating cathode phases can lead to improved cell performance, as well as a higher electrode utilization. Insights provided by this approach provide useful guidelines for optimizing solid-state battery design.

LiNi0.8Mn0.1Co0.1O2 (NMC811) is among the cathode materials most discussed for high-performance Li-ion batteries thanks to its capacity of approximately 200 mAh g-1 and its low Co content. Currently, NMC811 is considered for implementation in next-generation electric vehicles, but the high Ni content (> 60 %) still poses challenges, such as oxygen loss at voltages above 4.3 V, impedance rise, transition metal dissolution, and finally, a loss of active lithium in the electrode. These challenges can be mitigated by utilizing appropriate coating and doping strategies and limiting the voltage to standard cycling conditions (2.8 - 3.0 V to 4.3 - 4.5 V). NMC811 can be overlithiated to form Li2NMC811,  in analogy to pure LiNiO2, which can form Li2NiO2 [1]. Such a high lithiation degree increases the initial discharge capacity but leads to a fast voltage loss, especially during the first cycle [2]

In this study, we address two intriguing research questions: (i) Can we control the interface of Ni-rich cathode material with the electrolyte and prevent the formation of detrimental solid electrolyte interphase (ii) Is it possible to take advantage of multi-electron cycling by introducing additional lithium into the host NMC811 structure? To address these questions, Li-rich NMC811 thin-film cathodes were prepared by sputtering from a Li₂O-NMC811 target. Li-rich NMC811 cathodes were tested with LiPON as a solid electrolyte in a thin-film architecture, which offers a simplified 2D model with direct access to the cathode-electrolyte interface. The solid-state electrolyte helped to stabilize the interface and prevented capacity fading, voltage decay, and interface resistance growth. In combination with LiPON, cycling at extended voltage ranges of 1.5-4.7 V was possible without increasing interfacial resistance. It was possible to reversible cycle Li₂NMC811 by lowering the applied potential to 1.5 V. resulting in a discharge capacity of up to 350 mAh g^-1 due to multi-electron cycling. Overall, the all-solid-state cells with a lithium metal anode can cycle in the range of 1.5 V to 4.3 V for 1000 cycles with an average coulombic efficiency of 98.79 %. Our results demonstrate how solid electrolytes that are stable against NMC811 cathodes can unlock the full potential of this Ni-rich cathode class.

Glassy-type solid state electrolytes such as state-of-the-art LiPON avoid grain boundaries and prevent Li-dendrite propagation leading to extremely good cyclability and the commercialization of planar thin-film LiPON-based microbatteries. However, its reduced ionic conductivity (~1 μS/cm) limits its performance at high/discharge rates and the severe degradation of its electrochemical and structural properties upon their exposure to ambient conditions and high temperatures (> 300 ºC) hinders scalability and manufacturability. Among the next generation solid-state electrolytes, NASICON superionic solid electrolyte Li_(1+x)Al_xTi_(2-x)(PO₄)₃ (LATP) with 0.3 ≤ x ≤ 0.5 remains one of the most promising solid electrolytes thanks to its good ionic conductivity (~0.5-1 mS·cm^-1) and outstanding stability in ambient air. Despite the intensive research for bulk systems, there are only very few studies of LATP in thin film form. In particular, the only successful reports of relatively high ionic conduction (~10^-5 S·cm^-1) have been achieved through amorphous sputtered films.

In this talk, we will explore the properties of high performance LATP thin films fabricated by Large-Area Pulsed Laser Deposition. The as-deposited thin films exhibit an ionic conductivity around 0.5 μS·cm^-1 at room temperature (comparable to the state-of-the-art of LiPON) which increases to remarkably high values of 0.1 mS cm^-1 after an additional annealing at 800 ºC. We will discuss the formation of a glassy, intergranular phase connecting highly conducting LATP grains as a possible cause for the significant enhancement in ionic conductivity by two orders of magnitude. The performance of both as-deposited and annealed LATP films makes them suitable as solid electrolytes, which opens the path to a new family of stable and highly performing thin solid-electrolytes. We will discuss its integration with other battery components, with special attention to interfacial coatings matching the electrochemical window at low potentials.

The development of energy storage and conversion devices demanded to generate clean and sustainable energy relies on the design, synthesis, and characterisation of novel materials with suitable properties in terms of ionic or electronic conductivity, ionic insertion capabilities, redox activity, catalytic properties, etc. The structure of the materials used in energy may condition these properties and their overall good performance, and its knowledge is essential for proper understanding and optimising of these materials. To define the crystalline structure, Neutron powder diffraction (NPD) is a powerful tool that, combined with the Rietveld refinement method, is able to provide detailed structural information. This way, properties closely related to the structure and its evolution with external factors (temperature, pressure, cycling in in-operando systems, etc.) can be assessed with greater certainty. Well-known applications of neutron diffraction are the location of oxygen atoms and oxygen vacancies in oxide ion conductors, Li ions in solid electrolytes or Li-ion battery cathodes, protons in fast H⁺ conductors, etc. The ionic conductivity of these diverse materials is partly conditioned by the structure, composition, atomic radii, and formal charges of each atom, among others. An inexpensive and relatively efficient way to exploit these data in order to determine properties such as ionic percolation activation energy is the use of Bond‑Valence Energy Landscape (BVEL), which relies on structural data that can be determined by NPD. In this way, NPD and BVEL form a synergistic tandem that is useful in the assessment of ionic conductors. In this talk, we will present some recent results obtained for different types of energy-related materials, including Li-ion battery electrodes, electrolytes in Na batteries, and H⁺ conductors.

As Li cathode example, an N-doped LiFePO₄-type material, where nitrogen partially replaces an oxygen atom of the olivine tetrahedra lattice at 4c Wyckoff site in the Pnma space group. The structure was correctly determined by the Rietveld method from NPD data, and a subsequent calculation by BVEL confirms the decrease in activation energy (Ea) by about 6 % with respect to the pristine structure upon N-doping. As a solid electrolyte example, collaborative work is presented, in which the structures of Mg-doped NZSP NASICON-type electrolytes were determined at different temperatures from neutron data. A decrease in the activation energy concerning the undoped sample and its temperature dependency is assessed by BVEL. Finally, an example of a proton conductor used as a catalyst is presented, the so-called antimonic acid, where the presence and location of two different H⁺ are verified and identified by means of Fourier density difference maps. In this talk, a brief review of these different materials will be made, with emphasis on the specific structural features determined by the neutrons that explain the desired properties of the materials applied to energy.

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Severe challenges LHP NCs form by sub-second fast and hence hard-to-control ionic metathesis reactions, which severely limits the access to size-uniform and shape-regular NCs in the sub-10 nm range. We show that a synthesis path comprising an intricate equilibrium between the precursor (TOPO-PbBr₂ complex) and the [PbBr₃^-] solute for the NC nucleation may circumvent this challenge [1]. This results in a scalable, room-temperature synthesis of monodisperse and isolable CsPbBr₃ NCs, size-tunable in the 3-13 nm range. The kinetics of both nucleation and therefrom temporally separated growth are drastically slowed, resulting in total reaction times of up to 30 minutes. The methodology is then extended to FAPbBr₃ (FA = formamidinium) and MAPbBr₃ (MA = methylammonium), allowing for thorough experimental comparison and modeling of their physical properties under intermediate quantum confinement. In particular, NCs of all these compositions exhibit up to four excitonic transitions in their linear absorption spectra, and we demonstrate that the size-dependent confinement energy for all transitions is independent of the A-site cation.

   We then show that this synthesis – relying on the labile ligand capping with TOPO-phosphinic acid mixture – makes for a convenient platform for the subsequent surface functionalization with diverse capping ligands [2]. Robust surface functionalization of highly ionic surfaces, as is the case of LHP NCs, has remained a formidable challenge due to the inherently non-covalent weak surface bonding. Leveraging the vast and facile molecular engineering of phospholipids, we present their efficacy as surface capping ligands for LHP NCs. Molecular dynamics simulations and solid-state NMR confirm that the surface affinity of these zwitterionic molecules is primarily governed by the geometric fitness of their anionic and cationic moieties. Judicious selection of the ligands yielded colloidally robust FAPbBr₃ and MAPbBr₃ NCs and enabled colloids in a variety of solvents, from n-hexane to acetone. Robustness of the surface capping is also reflected in optical properties: NCs exhibit PL quantum yield (QY) above 96% after numerous purifications. NCs are essentially blinking-free at a single particle level.

1. Q. Akkerman et al. submitted

2. V. Morad et al. submitted

The flexibility of lead-halide perovskites plays a crucial role in their functional response, but the exact structural distortions and their effects on physical properties are still unclear [1]. Here, we provide for the first time the description of the photoinduced and thermally-activated structural distortions in CsPbBr₃ perovskite nanocrystals with atomic-level precision. We combine time-resolved X-ray absorption spectroscopy (TR-XAS) and ab initio simulations to show that the photoexcitation of CsPbBr₃ nanocrystals (NCs) leads to well-defined polaronic lattice changes, rather than photoinduced structural phase transitions. Additionally, our analysis rules out thermal effects in the photoactivation of the system [2]. We also investigated the purely thermal response of CsPbBr₃ with temperature-dependent XAS and first principles molecular dynamics (MD) simulations across its phase diagram. We show that the thermally-activated lattice cannot be reduced to a cubic average structure, because of the presence of dynamically distorted local configurations and lattice anharmonicity [3].

Our comprehensive investigation demonstrates that the structural changes of CsPbBr₃ NCs induced by light and thermal functional triggers have fundamentally different physical origins. Even though both effects are related to the flexibility of the perovskite lattice, the photoexcitation is selectively driven by electron-phonon coupling, while the thermal activation drives lattice phonon anharmonicity. The latter leads to significant distortions from the CsPbBr₃ space- and time- average lattice symmetry, and it is well rationalized by the soft-mode model in the framework of displacive thermal phase transitions.

These finding clarify the underlying mechanisms of the lattice response under functional activation and offers strategies to control the perovskite nuclear degrees of freedom with different external stimuli. Understanding the thermal processes acting at the atomic-level represents the first step toward a rational design of perovskite-based devices with improved stability.

Traditionally defects have been considered detrimental for the performance of photoactive devices.  For example, in photocatalysts point defects introduce trap states that act as recombination centres lowering quantum yields. However, recent advances in spectroscopy, microscopy and modelling, have opened the door to ground-breaking studies which are challenging our views of disorder. New evidence suggests that defects could play a much more active role than anticipated by controlling parameters as important as the reaction selectivity. The role of disorder is particularly critical in photocatalysis where the act of photoexcitation can, in itself, generate dynamical defects in the active phase.  In this talk I will discuss the many different shapes and forms in which disorder can emerge in photo-electrocatalysis. In particular, I will focus on the formation of electronic defects and structural reconstructions during catalytic operation.  I will present examples of how disorder-associated phenomena can be monitored in situ and operando and how it can be correlated with catalytic function. Moreover, by drawing parallelisms with the biological world I will discuss if disorder is unavoidable to achieve high performance.  Finally, I will contrast the role of disorder in photocatalysis with observations made in other photoactive materials in order to highlight potential routes to use disorder in our favor.

Photocatalytic solar fuel production provides a potential alternative to sustainable energy production. Whilst attention to date has focused on inorganic photocatalysts, carbon-based materials and organic semiconductors have emerged as potential low cost and efficient photocatalyst for hydrogen evolution, mainly due to the tunability of their properties through synthetic control.^[1,2] This allow the design of families of materials with tuned opto-electronic properties by incorporating different building blocks.^[1]  The best performing systems are bulk heterojunctions nanoparticles prepared from a blend of conjugated polymer donor and non-fullerene small molecules acceptor, particularly due to their improved light absorption in the visible range.^[2,3] Despite the efficient performance of the donor/acceptor bulk heterojunction photocatalysts for hydrogen evolution, the fundamental understanding of the photophysical processes that determine their performance remain limited. In this presentation, I will discuss the charge carrier dynamics of donor/acceptor heterojunction nanoparticles photocatalysts with different hydrogen evolution activity.[2,3] Transient and operando photoinduced absorption spectroscopies, on timescales of femtoseconds to seconds after light absorption, were employed to monitor the kinetics of photogenerated charges and their correlation with photocatalytic performance.[3] Differences between the function of Donor/Acceptor bulk heterojunction photocatalysts and single conjugated polymers photocatalyst will be discussed. These results can provide design guidelines towards efficient organic semiconductors photocatalyst.

Semiconductor nanoparticles (NPs) have attracted a great deal of interest in the last decades due to the increasingly range of applications. However, NPs were produced usually in batch processes and the production on a larger scale is still limited. Among n-type semiconductors, Bismuth vanadate (BiVO₄) has emerged as one of the most promising metal oxides as photoanode for water oxidation due to its moderate band gap (2.4 eV), adequate band positions, low-cost, good chemical stability in aqueous medium and environmental inertness.^[1] In this regard, our group designed and studied different methodologies for the preparation of this type of photoelectrodes.^[2] However, the large-scale synthesis of this material is still limited to the use of conventional batch processes, and it is necessary to achieve high-production yields due to the increasing demand in the field of energy materials. Since the automatization of this process has significant advantages with respect to cost, safety and more precise control of the physical and chemical properties, the emergence of continuous flow technology offers an alternative for preparing inorganic nanoparticles (NPs).^[3]

In this communication we show the preparation of BiVO₄ NPs by using a simple continuous-flow method based on two liquid peristaltic pumps. The proposed system allows to up-scale the synthesis through a microreactor and contributes to the possible production of large-scale photoelectrodes.

Two-dimensional (2D) VA group materials, including black phosphorene, bismuthene, and antimonene, attract considerable attention in energy devices because of their unique properties, such as tunable bandgap and superhigh carrier mobility. However, their application in photoelectrochemical devices is rarely reported. In this presentation, we present the successful design of semiconductor /2D VA material/ co-catalyst composite photoanodes, such as BiVO₄/bismuthene/NiFeOOH, BiVO₄/bismuthene/CoPi, and BiVO₄/Sb/NiFeOOH. Specifically, the BiVO₄/bismuthene/NiFeOOH photoanode that achieves 3.7 mA cm^-2 at 0.8 V_RHE are deeply investigated. Comprehensive (photo)electrochemical techniques and surface photovoltage were comprehensively employed to identify the roles of bismuthene and NiFeOOH on BiVO₄. We found that bismuthene increases the density of V_O of BiVO₄ that are beneficial for the oxygen evolution reaction via the formation of oxy/hydroxyl-based water oxidation intermediates. Moreover, bismuthene increases interfacial band bending and fills the V_O-related electron traps with charges, leading to more efficient charge extraction. The NiFeOOH significantly increase the surface hole concentration by passivating the r-ss. The use of 2D VA group materials and co-catalysts as functional layers opens new avenues to tune the surface properties of photoanodes for water oxidation.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

Photoelectrochemical energy conversion from sunlight directly into chemical bonds is a promising path to fabricate alternative solar fuels. But a single semiconductor is not capable to provide enough voltage for the chosen electrochemical reaction and/or to do so with significant productivities. Tandem PEC/PV monolithic devices can generate the necessary voltage meanwhile increasing overall productivity.

BiVO₄, one of the most promising large bandgap (~2.5 eV) photoanode candidates, has been fabricated in tandem with non-fullerene semi-transparent organic (OPV) based thin-film third generation photovoltaics. Although OPVs allow for fine-selection of absorber composition and thus, absorption spectra, the balance is not optimal. Nanophotonic structures are necessary to balance the photon influx to maximize the overall series current in the device.

In this talk we will explain how the many-layers device has been computationally modelled and the nanophotonic structures incorporated, and how the full device has been fabricated and tested at ICFO. The light path for the blue part of the solar spectra has been maximized in the BiVO₄, and the PV part has been adequately selected so that the device provides the necessary voltage for either water splitting or CO₂ reduction. Several parameters have been taking into account in this model, such as the photovoltage loss of the solar cells under reduced photon influx due to the light management.

This many-layers tandem PEC/PV structure can provide >2.5 V of photovoltage and the 1D light management structures have demonstrated 2x increase of the bias-free photocurrent.

In recent years, bismuth-based photocatalysts have been receiving increasing attention in photocatalysis, due to their appropriate bandgap and tunable surface structure, which make them suitable also for the photocatalytic reduction of CO₂¹. Their performances, however, are still limited by the fast charge carriers recombination. Recently, the exploitation of piezo/ferro-electric potentials in photo-active semiconductors has been adopted as an effective strategy to modulate the charge transfer properties both in the bulk phase and at the surface of semiconductors (i.e. piezo-phototronic effect)². In this perspective Bismuth-based Aurivillius compounds, owing to their usually strong spontaneous ferroelectric polarization represent a promising option as cell photo-electrodes, allowing the increase of cell efficiency by electrically polarizing the materials. In addition, thanks to their unique layered structure, this peculiar class of perovskites allows the migration of photo-generated holes and electrons within different areas of the materials, thus intrinsically facilitating charge separation.

In this work, the utilization of different Aurivillius compounds (i.e. Bi₄Ti₃O₁₂ – BiTO, and Bi₂MoO₆ - BiMO) as photo-electrode materials for solar conversion has been studied in detail. The use of BiTO and BiMO for the photocatalytic reduction of CO₂ has recently been reported, however, the main strategies to develop optimized ferroelectric-enhanced photo-electrodes of these materials and their utilization for the ferroelectric potential-assisted CO₂ reduction has never been fully investigated. BiTO and BiMO photo-electrodes were fabricated and accurately optimized to obtain both hierarchically oriented nanostructures with different morphologies (i.e. nanosheet/nanorod arrays), and thin-film layers via in situ hydrothermal deposition and through a sol-gel/spin coating coupled process respectively. The effect of the ferroelectric potential on the photo-electrochemical performances of the optimized photo-electrodes with different architectures was therefore accurately studied. Density current increments and enhanced charge transfer ability were registered under the optimal ferroelectric polarization which was directly reflected in the CO₂ photo-electrochemical reduction performances. This work, therefore, demonstrates the possibility to adopt ferroelectric polarization coupled with an external bias to effectively control the migration of photo-generated charges in bismuth-based Aurivillius semiconductors for the CO₂ photo-electrochemical reduction.

Photoelectrochemical cells (PECs) have been developed as environmentally friendly systems that can directly utilize photogenerated electron-hole pairs for water splitting, fuel production, conversion of carbon dioxide, and pollutant degradation. Most reports on the photocatalytic or PEC hydrogen (H₂) evolution via water splitting have focused on the H2 reduction half-reaction by generating on the photoanode oxygen or using sacrificial agents to consume the generated h+. Lately, much effort has been invested into synthesizing valuable chemicals on the photoanode while retaining the production of H₂ on the cathode.

Over the past few years, polymeric carbon nitrides (CN) have attracted widespread attention due to their outstanding electronic properties, which have been exploited in various applications, including photo- and electro-catalysis, heterogeneous catalysis, CO₂ reduction, water splitting, light-emitting diodes, and PV cells. CN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, few reports regarded CN utilization in PECs due to the difficulty in acquiring a homogenous CN layer on a conductive substrate and our lack of a basic understanding of the intrinsic layer properties of CN.

This talk will introduce new approaches to growing CN layers with altered properties on conductive substrates for photoelectrochemical applications. The growth mechanism and their chemical, photophysical, electronic, and charge transfer properties will be discussed. I will show the utilization of PEC with a CN-based photoanode as a stable and efficient platform for the oxidation of water or organic molecules to oxygen or added-value chemicals, respectively, with hydrogen co-production.

Many metal oxide semiconductors, such as TiO₂, require UV excitation in order to drive photocatalytic reactions. However, UV photons account for only 3-5% of the solar spectrum, and thus strategies to extend the range of harvested photons into the visible wavelength range are required. One potential way is to use triplet-triplet annihilation photon up-conversion (TTA-UC). TTA-UC systems generate higher energy photons from lower energy, low intensity, and non-coherent excitation. In this work, we synthesized several 9,10-diphenylanthracene derivatives and attached them to different wide-bandgap metal oxide nanoparticles, TiO₂, ZrO₂, and CeO₂, which were then dispersed in solutions containing triplet sensitizers. Triplet energy transfer leading to triplet-triplet annihilation photon up-conversion and charge generation were probed through steady-state and transient spectroscopy techniques such as photo-induced absorption (PIA), time-resolved photoluminescence (trPL), and transient absorption (TA) spectroscopy. We characterized these TTA-UC systems as films and suspensions and applied them in different photocatalytic processes as proof of concept.

Transition metal-oxides are promising photoelectrode materials for photoelectrochemical water splitting and solar fuel conversion. Several materials with empty or filled d-shell configurations (e.g. TiO₂, BiVO₄) have shown high internal quantum efficiencies, but their band gaps are too large for practical purposes. Metal-oxides with suitable bandgaps generally have open d-shell configurations and suffer from poor photoconversion efficiencies. We have recently introduced spatial collection efficiency analysis, based on optical and external quantum efficiency measurements, as a non-destructive method to probe losses in thin film semiconductor photoelectrodes under operando conditions. [1,2,3] Using hematite as a case study for strongly correlated electron materials, we show that in addition to the well-known recombination losses arising from poor charge transport properties, some of the photon absorption leads to localized excited states rather than to generation of mobile electrons and holes that contribute to the photocurrent. We extract the wavelength-dependent mobile charge carrier photogeneration yield spectrum of hematite and show that it is less than unity across the entire absorption range, fundamentally limiting the attainable photocurrent to roughly half that predicted based on above bandgap absorption.  I will discuss our recent advancements on this front, and the implications of our findings on photoconversion efficiency in open d-shell transition metal-oxides.   

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission. In the present talk, we will describe our efforts towards the application of these materials for solar-driven processes spanning from photocatalysis, environmental remediation,[1] H₂ production and organic transformations.[2] We will discuss on the rational design of these fascinating materials towards photocatalytic and photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics,[3] the influence of defect states and to define adequate defect passivation strategies to maximize the performance and stability of these materials.[4] Moreover, we emphasise the need for proper interrogation tools to validate their photoelectrocatalytic activity and selectivity. Our results also highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

Since their discovery cadmium chalcogenide nanoplatelets (NPLs) attracted immense attention due to the fact that they exhibit optoelectronic properties, which are superior to the properties of counterpart nanocrystals of other dimensionalities with the same composition. In addition, due to their shape, they can be arranged into oriented assemblies further extending control over the optical and electronic features. One of the particular features of such NPLs is atomic smoothness of their surface and strong one-dimensional quantum confinement in the thickness direction, which can be controlled down to one monolayer. On the other hand, the ability to change thickness only in integer steps becomes a severe disadvantage preventing precise tuning of spectral position of absorption and photoluminescence bands.

To some extent, spectral tunability was achieved by varying the lateral size of NPLs, and their surface modification via ligand exchange or shell growth. However, these approaches are considerably limited and still not nearly as flexible as the simple size-tuning of quantum dots. In addition, they generally result in a red-shift of optical bands of NPLs, thus highlighting another longstanding challenge for NPLs – the lack of bright emitters in the UV-blue region.

Another more powerful approach consists in the modification of nanocrystal composition through the synthesis of alloyed nanoparticles. Although some advances in this direction have been already achieved by modifying existing protocols for the synthesis of CdSe and CdS NPLs, the synthesized nanoparticles either suffered from poor photoluminescence quantum yields of < 5 % or a limited range of available compositions.

In this work, we demonstrate that both of these issues can be overcome by employing highly reactive stearoychalcogenides, which rapidly react with cadmium carboxylate yielding small polydisperse nanocrystals. At later reaction stages, these nanocrystals serve as a monomer source for the expansion of NPLs in the lateral direction in a slow Ostwald-like ripening process guided by acetate ions. The separation of these stages essentially decouples the precursor conversion step and lateral growth of NPLs thus affording much better control over the lateral dimensions of NPLs, which in turn is a significant factor for achieving brightly emitting nanocrystals. Conveniently, unlike in previously reported procedures, the Se/(Se+S) ratio in the synthesized NPLs is essentially the same as in the starting reaction mixture which enables straightforward control over the nanocrystal composition. Moreover, we demonstrate that by using essentially the same procedure and changing the reaction temperature it is possible to control the thickness of NPLs thus further expanding the spectral range of the synthesized nanocrystals.

Optical spectroscopy measurements show that the increase of selenium content in the alloy results in a steady red shift of absorption and photoluminescence bands consistent with the shrinkage of the material bandgap. Due to their relatively small lateral size enabled by controlling the lateral growth, obtained alloyed CdSe_xS_1-x NPLs exhibit bright band-edge emission covering the blue-green region from 380 to 520 nm with quantum yields of around 30–50 %, which are ca. 10 and 2 times higher than previously reported for respectively 3.5 and 4.5-monolayer thick NPLs with similar composition.

Halide perovskite materials have attracted great scientific interest due to their solution-processability and excellent optoelectronic properties. Applying these perovskites photoactive layers for direct solar water splitting has the potential to provide efficient and inexpensive photoelectrodes for solar hydrogen generation. In this presentation all-inorganic CsPbBr₃ perovskite (2.3 eV bandgap) photoelectrodes will be presented, where the photoactive layer is protected from direct contact with the aqueous electrolyte by a graphite sheet and a low-temperature printed carbon layer. The critical role of controlling 2-dimensional (CsPb₂Br₅) and 0-dimensional (Cs₄PbBr₆) perovskite phase formation and their effect on stability and efficiency will be discussed. The low annealing temperature carbon layer enables the deposition of an organic hole transport layer, while the graphite sheet allows for electrodeposition of nanoscale NiFeOOH co-catalyst on its surface. It will be shown that all these developments lead to all-inorganic halide perovskite photoanodes with remarkable days-long stability and photocurrent generation (~10 mA cm^-2) close to the theoretical efficiency limit of CsPbBr₃. Finally, the potential for scaling up these devices will be demonstrated by presenting photoanodes with an area above 1 cm².

The efficient transport of energy carriers plays an essential role in all of optoelectronic technologies. Energy transport characteristics have traditionally been derived from time-resolved spectroscopy or device-level techniques. While such techniques work well for the characterization of the spatially homogeneous lattices of crystalline semiconductors, they fail to capture the spatially varying complexity of nanostructured semiconductors. To tackle this issue, a range of time-resolved microscopy techniques have emerged, capable of directly visualising energy transport with sub-nanosecond and few-nanometer resolution.[1]

In this talk, I will give an overview of the most recent work of my group on improving our understanding of the various forms of anomalous transport phenomena that can be encountered in semiconductors nanomaterials.[2-5] In the first part of the talk, I will discuss the extensive models that we have developed using Brownian dynamics in which we make use of the detailed spatiotemporal information obtained from time-resolved microscopy. In a second part of the talk, I will present recent work in which we use Mn-doped layered perovskites as highly versatile test-bed to understand the role of carrier trapping in energy transport, with Mn sites acting as deep traps for diffusing excitons. By controlling the doping level, we can controllably modify the trap landscape and relate these changes to the observed spatiotemporal dynamics.

Our measurements and the resulting models provide valuable information for the development of optimized nanostructured semiconductors for optoelectronics, including light harvesting and light emitting devices.

The solution−liquid−solid (SLS) mechanism is a well-established method for forming one-dimensional (1D) nanostructures in a solution. Herein, an SLS mechanism is explored for the formation of metal oxides for the first time. Two key synthetic achievements allow this synthesis: (i) the design of a tailored catalyst with a low melting point and high stability and (ii) control over the reactivity and the oxidation of the precursors. Once these conditions are achieved, the SLS growth of indium and tin oxides ensues. Structural characterization of the products at various stages of the growth confirms the formation of 1D In₂O₃ and SnO₂ nanoscale heterostructures using AuIn₂ and Au₇Sn₃ as catalysts. Both metal oxides showed photocatalytic abilities by the photodegradation of 2-mercaptobenzothiazole (MBT). Furthermore, SLS growth was easily adopted to insert SnO₂ rods selectively between two domains of an Au/ZnO rod heterodimer, demonstrating the potential of achieving highly complex multi-component metal oxide nanostructures.

Perovskite solar cells currently demonstrate more than 25% efficiency, however many fundamental processes still remain unclear. In particular, charge carrier mobility and diffusivity are still poorly characterized and understood. The reported mobility values differ by many orders of magnitude depending on the materials preparation, device architecture and measurement techniques. Therefore, carrier motion in real perovskite solar cells still remains far from clear and the lack of suitable carrier mobility investigation techniques is one of the major problems.  Moreover, evaluation of the actual electric field strength in perovskite layer of solar cells is also not a trivial task. Therefore, the charge carrier transport in real operating perovskite solar cells still remains a controversial, heavily disputed   question. Carrier motion is expected to be particularly complex in case of archetypical perovskite solar cell architecture, where majority of the perovskite is embedded into mesoporous TiO₂ (m-TiO₂) layer.

We investigated charge carrier motion and extraction from archetypical methylammonium lead iodide (MAPI) perovskite solar cell. We used an ultrafast electric field-modulated transient absorption technique enabling to directly visualize the carrier motion with subpicosecond time resolution by evaluating electric field dynamics from the time-resolved electroabsorption spectra. We demonstrate that photogenerated holes drift across the mesoporous TiO₂/perovskite layer during hundreds of picoseconds, however their extraction to the Spiro-OMeTAD hole transporting layer takes place during tens of nanoseconds suggesting that the hole extraction is limited by the perovskite/Spiro-OMeTAD interface rather than by the hole transport through the perovskite layer.  Additionally, we use the ultrafast time-resolved fluorescence technique which reveals fluorescence decay during tens of ps, which we attribute to the spatial electron and hole separation.  

Hybrid halide perovskites show heterogeneity at multiple length scales.[1] Cathodoluminescence (CL) in a scanning electron microscope (SEM) can investigate semiconductor materials by scanning an electron beam over a semiconductor. By collecting the optical properties of the material during scanning, CL-SEM can elucidate structure-property relations of halide perovskites at sub-micrometer spatial resolution.

While most CL studies on perovskites have focused on the inorganic compositions, the characterization of hybrid (organic-inorganic) halide perovskites - including the highest-performing compositions - is limited due to their lower stability under the electron beam. To reduce beam damage, we used a pulsed electron-beam (PM) to measure hyperspectral maps of hybrid perovskite compositions.[2] When PM was used, the CL spectra strongly resembled that of pristine perovskite emission, suggesting the use of PM for more robust high spatial resolution spectral mapping of beam-sensitive hybrid halide perovskite materials.

In this presentation, a series of examples will be presented whereby PM and low-current CL aided in the study of luminescence of several different perovskite compositions. For example, the use of CL was crucial to understand the effect that focused ion beam milling has on the perovskite emission, generally considered to be a rough processing step for TEM specimen preparation. Using CL-SEM in PM, the perovskite layer in the lamellae was found to remain optically active, yet with a small blue-shift due to surface amorphization.[3]

Going beyond conventional high-current CL setups, used for traditional semiconductors, enables CL to start to play a role in resolving the complex heterogeneity of hybrid halide perovskites, and it could also enable the elucidation of many other novel beam sensitive “soft” semiconductors.

Simply prepared chalcogenide materials have been investigated as light absorbers for photoelectrochemical water splitting. I will begin the talk with our work on antimony selenide, a promising quasi-1D material whose efficiency has rapidly increased in the past ten years. Thin films of antimony selenide are easily prepared by selenization of antimony metal deposited by electrodeposition or sputtering. The effect of a low temperature sulfurization treatment was investigated with time resolved microwave conductivity, photoluminescence, and low frequency Raman spectroscopy, and the effect on both bulk and surface recombination will be discussed.  The water splitting photocathodes were characterized under operando conditions using electrochemical impedance spectroscopy. We will then explore the related compound antimony sulfide and its band alignment with atomic layer deposited titanium dioxide, which serves both as an electron extracting contact and corrosion protection layer. Finally, a simple solution processing method for phase pure cuprous sulfide will be presented, where the nanostructure can be controlled with the appropriate solvent mixture of the precursor solution.

The interconversion of spin, charge and light are being exploited in perovskite materials leading to efficient, new, and unique device concepts. In this talk, we present the design rules which control and explain the mechanism behind chiral transfer from chiral molecules to the HOIS framework in nanostructured embodiments. Chirality can be imparted by chiral ligands attached during synthesis or in post-synthetic ligand exchanges where the chirality can be studied as a function of the nanocrystal size and surface to volume ratio. We show chirality induced properties from halide perovskite nanocrystals of different sizes and in films and then construct multiple types of prototype devices using heterostructure concepts that harness and display these fascinating properties. Some examples include heterostructured LEDs which emit spin-controlled circularly polarized light at room temperature without external magnets, detectors which couple perovskite nanocrystals and carbon nanotubes to detect and discern circularly polarized photons, neuromorphic devices which exhibit persistent photoconduction lasting thousands of seconds beyond a weak light pulse and can also display read/write capabilities.

Halide perovskite nanocrystals (NCs) have emerged as an intriguing material for optoelectronic applications, most notably for light-emitting diodes (LEDs), lasers, and solar cells. Their emission wavelength depends not only on material composition but also on size and dimensionality, as in the case of two-dimensional (2D) nanoplatelets (NPLs). These colloidal quantum wells have additional appeal for light emission, as the one-dimensional quantum confinement enhances their radiative rates and enables directional outcoupling. On top of this, due to a monolayer-precise control over their thickness, they constitute an intriguing system for spectroscopic studies on their fundamental optical, phononic, and energetic properties.

In this talk, I will explore our recent results on halide perovskite NPLs, including their synthesis and stabilization. I will focus on their interesting excitonic properties, such as the energetic fine structure^[1] with a strong thickness-dependent bright-dark exciton splitting and exciton diffusion in nanocrystal films.^[2]  Further, I will highlight their advantages and disadvantages for integration into optoelectronic applications.

As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. We have recently discovered a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors in new expanded perovskite analogs. Compressing the metal-halide framework drives up the valence band relative to the acceptor orbitals of the organic molecules. Thus, the material’s electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm–1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by reduction of the redox-active molecules. This work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

Redox-active organic cations can act as charge reservoirs in the expanded perovskite analog (dmpz)[Sn2I6]. Material compression increases electronic conductivity by five orders of magnitude. This conductivity rise is attributed to an increased hole density in the valence band, caused by electron transfer to the redox-active molecules

Carbon capture, conversion and utilization (CCU) is gaining attention from the worldwide community for its ability to minimize CO2 accumulation in the atmosphere. Electroreduction of CO2 (eR-CO2) in combination with renewable electricity sources can be one of routes to achieve the target. Economic viability of eR-CO2 relies on improved performance accompanied with scalable system design. Membranes are commonly used for the separation of reduction and oxidation products as well as to provide a suitable micro-environment for CO2R. Commercial membranes often address only one of the key challenges in CO2R: either they offer suitable micro-environment for CO2R (e.g., anion exchange membrane) or suppress carbonate cross-over (e.g., cation exchange membrane and bipolar membrane). Here, we present a cation-infused ultrathin (~3 µm) solid polymer electrolyte (CISPE) that concomitantly addresses both of these challenges via bidirectional ion transport mechanism and suppressed anolyte diffusion. The CISPE was directly deposited into the copper cathode catalyst, eliminating the requirement of a standalone membrane. Our demonstrated system offers high selectivity towards CO2 electrolysis (~90%) and a single-pass CO2 utilization of ~18% at 200 mA/cm2 with the primary product being ethylene (faradaic efficiency of 65%). Our directly-deposited CISPE enabled record low energy consumption of 294 GJ/ton C2H4 along with ~110 hours of stable operation with C2H4 as the primary product. The present work offers a versatile design paradigm for functional polymer electrolyte, opening doors towards stable, and efficient electrolysis for high-value feedstock chemicals and fuels production using low-cost catalysts.

Rising atmospheric carbon dioxide (CO₂) levels have motivated the development of alternative chemical production technologies to reduce emissions. The electrochemical CO₂ reduction reaction (CO2RR) is one of several promising strategies that have been proposed for the utilization of CO₂. Through CO2RR, CO₂ can be converted into renewable fuels and valuable chemical feedstocks using clean electricity as an input. Of the possible CO2RR productions, multicarbon hydrocarbons and oxygenates, such as ethylene and ethanol, are promising commercial targets due to their large market sizes and high potential impact for emissions reduction.

CERT Systems Inc. is a carbontech start-up focused on the development of CO2RR to multicarbon product technology. Here we present an overview of CERT’s scale up journey through the NRG COSIA Carbon XPRIZE, a $20M global competition to develop and scale CO₂ utilization technologies. We will summarize the scale up process from the laboratory to CERT’s pilot demonstration during the finals of the competition. The key learnings and challenges of scaling up CO2RR to multicarbon products will be discussed. Finally, we will identify next steps for the technology and outlook for electrochemical CO₂ utilization.

Electrochemical power-to-gas and power-to-liquid technologies are promising chemical energy conversion approaches to be coupled with intermittent renewable energy sources to balance the grid while utilizing cheap excess energy at peak times. Production of CO from CO₂ reduction seems to be feasible even at pilot-scale, but the selective production of high-value multi-carbon products is challenging. CO reduction is a possible second step in the cascade electrochemical valorization of CO₂. Gas-phase electrolyzers offer a suitable platform for this process, but the low solubility of CO still poses notable challenges. This can be tackled by tailoring the microenvironment of the catalyst particles.

Here we demonstrate that a commercially available polymeric pore sealer can be used as catalyst binder, ensuring a high rate and selective CO reduction. Under optimal conditions, we achieved above 70% Faradaic efficiency for C2+ products formation at j = 500 mA cm⁻² current density. As no specific interaction between the polymer and the CO reactant was found, we attribute the stable and selective operation of the electrolyzer cell to the controlled wetting of the catalyst layer, due to the homogeneous polymer coating on the catalyst particles’ surface. These findings point out that polymers that form thin films on the catalyst surface (instead of forming nanoparticles that are incorporated among the catalyst particles, e.g., PTFE) might be suitable binders for CORR. In addition, sophistically designed surface modifiers are not necessarily required for CO electrolysis, hence the capital costs can be significantly decreased.

At the end of my talk I will discuss some general differences and similarities of electrochemical CO2 and CO conversion processes, mostly focusing on the role of the reaction environment, provided by the cell components and the cell architecture.

Energy- and cost-efficient CO₂ capture is of crucial importance for its subsequent viable utilization. Ionic liquids (ILs) have emerged as a promising material for reducing carbon dioxide emissions from large-scale sources due to their properties and versatility. ILs are considered green solvents with high thermal stability, tunable structure, low volatility, and a strong affinity for CO₂ molecules. These properties of ILs make them a potential alternative to amine-based CO₂ capture processes. The challenges of using ILs come with their high viscosity, which causes unfavorable transport barriers. Encapsulated ionic liquid has been proposed to overcome mass transfer limitations, increasing the contact between gas and liquid, and improving the mass transfer rate. This study showed the development of new materials, combining efficiency, safety, and sustainable processes. Water-based nanocapsules of Emim[BF4], using poly(ionic liquid) Poly(diallyldimethylammonium tetrafluoroborate) as shell, were successfully obtained by Nanospray dryer B-90. Capsules were characterized and have the potential for CO₂ capture evaluated. The combination of these promising materials showed great potential for CO₂ capture and CO₂/N₂ separation (53.4 mg CO₂/g; CO₂/N₂ selectivity: 4.58). Thermal stability and the regeneration of this material after successive sorption–desorption cycles were also successfully demonstrated, emphasizing this as a potential alternative.

Progressive replacement of fossil fuels by sustainable and renewable energy sources is one of the most actual contemporary challenges. In this context, the solar-based electroconversion of water has become a valuable concept for production of an alternative fuel based on “green” hydrogen generation. Transition metal nanoparticles, such as nickel, are the benchmark catalysts for hydrogen evolution reaction, since they are highly active, stable and earth-abundant.

One of the emerging fields of solar energy harvesting is enhancing the catalytic reaction through the local surface plasmon resonance (LSPR) effect of the catalyst. Most studied mechanisms of the energy transfer from plasmonic nanoparticles are based on hot electron injection and plasmon-induced resonant energy transfer. However, the photothermal effect of the plasmon in transition metal nanoparticles is still often considered detrimental and is severely underestimated in the literature.

In this work, we have synthetized Ni nanoparticles with an elongated average shape by wetness impregnation of the hard silica template. The high distribution of sizes results in a broad plasmonic absorption in the visible range, which leads to a significant photothermal effect under solar illumination, reaching 93°C after 5 min under incident power of 0.5 W cm^-2.

Irradiation of the Ni-NPs cathode with concentrated solar light under HER cathodic conditions leads to the increase of the hydrogen production up to 27%. On the other side, by applying galvanostatic conditions to the electrode we were able to decrease the overpotential needed for the hydrogen formation by 185 mV with negligibly small overall heating of the cell.

Pulsed illumination has resulted a three-staged electrochemical response to the illumination, reflecting i) breaking of the nickel/electrolyte equilibrium towards adsorption predominance, ii) blocking of the active sites and iii) establishing of the new balance between the electronic and ionic subsystems. The transient study revealed two main processes in the mechanism of light-induced reaction enhancement at low current densities: i) enhanced electron transfer due to strong plasmonic heating and ii) thermal energy dissipation from the electrode to the electrolyte.

Based on the obtained results, we have shown that the illumination of the nanostructured nickel catalyst with solar light can lead to the enhanced HER rate, and, as a result, to improved hydrogen production. We expect the LSPR photothermal approach to pave way to a more efficient photoelectrochemical water splitting process, taking advantage of visible and near-IR illumination.

The direct conversion of greenhouse gases into green fuels and valuable chemicals is of paramount interest to achieve a decarbonized future [1]. The use of carbon dioxide and methane are of special interest as they are the main greenhouse gases emitted from anthropogenic sources, commonly considered industrial waste. In terms of fundamental research, the electrochemical carbon dioxide reduction to valuable chemicals has achieved great progress in the last few decades. Notwithstanding, few advances have been achieved in the electrochemical oxidation of methane [2].

Direct electrochemical methane activation strategies focus on tuning the electrode surface structure and composition [2]. The materials selected for the partial oxidation of methane need to activate methane and stabilize the intermediates involved to avoid the formation of carbon dioxide. Theoretical studies have shown that metal oxides are promising catalysts for the direct electrochemical activation of methane through the water discharge mechanism [3]. According to theoretical calculations, iridium oxide (IrO_x) catalysts present a low overpotential for the oxygen evolution reaction (OER) and lower free energies of methane activation and *O formation compared to *OOH formation in the OER. Thus, these calculations suggest IrO_x should be an interesting catalyst for the electrochemical conversion of methane into methanol. However, the experimental work published in this regard is very limited [4].

In this presentation, we show the results we acquired on the electrochemical methane activation using an IrO_x surface as a catalyst in HClO₄ electrolyte. For this purpose, we used an Ir polycrystalline oxidized electrochemically. The results aim to bring further understanding of the methane activation using metal oxides and discuss the purposed mechanisms reported in the literature.

The chemical instability of hybrid halide perovskites in protic solvents remains the main obstacle for their application including solar fuels generation. In our work, we successfully stabilize 2D perovskite PEA₂PbI₄ in aqueous solutions of phenylethyl ammonium iodide (PEAI). XPS and XRD spectroscopy allow us to determine the 2D perovskite changes before and after the water immersion. Additionally, inductive coupled mass spectroscopy (ICP-MS) reveals the compositional changes of the electrolyte after 2D perovskite immersion. Photoelectrochemical characterization was used in a three electrode setup to measure photocatalytic activity of this material toward the hydrogen evolution reaction. Furthermore, IMPS/IMVS and impedance spectroscopy was implemented to characterize the photoelectrochemical process involved in this reaction. This result denotes the possibility to use layered perovskite absorbers for solar fuels generation in aqueous media, providing valuable information on the general understanding of the degradation of perovskites and opening an exciting window to novel applications of perovskite absorbers.

We report the use of a silver sputtered gas diffusion electrode (GDE) for electrochemical CO₂ reduction in a membrane electrode assembly and probe the factors affecting catalyst utilization at high current densities. Catalyst utilization is significantly affected by the gas flow channel design at the cathode which distributes the reactant to the catalyst sites. The high pressure drop present in a serpentine flow channel enables a partial current density of 205 mA/cm² for CO production at a cell voltage of 2.76V. Double layer capacitance measurements reveal significant differences in catalyst wetting between the flow patterns due to changes in pressure drop, salt precipitation at the cathode and reactant distribution at the surface of the GDE. Modelling of the gas flow channel and GDE reveals inhomogeneity in reactant distribution affecting catalyst utilization significantly. These findings can be used to formulate design rules for maximizing catalyst utilization in zero gap CO­₂ electrolyzers. 

Carbon dioxide (CO₂) capture and electrolysis is a promising route to produce valuable chemicals from CO₂. Coupling the capture and electrolysis has the potential to intensify the process and reduce the overall energy cost. Taking amine scrubbing process as an example, the CO₂ electrolysis can displace the energy-intensive amine recovery of the capture and produce valuable chemicals (e.g., carbon monoxide, CO) to offset the overall carbon abatement cost. Our recent process modelling results[1] found that the integrated route could save up to 42% overall energy if the integrated electrolysers perform similarly to the state-of-the-art gas-fed electrolyser. 

However, the reported integrated electrolysers show inferior performance to the gas-fed electrolysers. Understanding the catalytically active species for CO₂ conversion in the capture media is essential to rationally design an efficient integrated electrolysis system, but remains underexplored. This talk aims to uncover the dominant active species for the CO₂ conversion to CO in a commonly studied ethanolmaine aqueous capture medium from our recent experimental findings. We will first discuss the potential CO₂-related species available in the amines. The following part will show how the chemical species can change locally under CO₂ electrochemical conditioning, and discuss their role in determining CO₂-to-CO conversion efficiency. This talk will conclude with potential research directions that can further boost the performance of the integrated electrolysis.

The electrochemical partial oxidation of methane to methanol is a promising approach to the transformation of stranded methane resources into a high-value, easy-to-transport fuel and chemical. The direct transformation of methane to methanol in remote locations requires selective catalysts capable of operating at near ambient temperatures inside a modular device. Over the last decade, transition metal oxides have been shown as potential electrocatalysts for this transformation. However, a comprehensive and systematic study of the dependence of methane activation rates and methanol selectivity on catalyst morphology and experimental operating parameters during testing has not been realized.

In this contribution, we describe an electrochemical method for the deposition of a family of thin-film transition metal (oxy)hydroxides as catalysts for the partial oxidation of methane. CoO_x, NiO_x, MnO_x and CuO_x are discovered to be active for the partial oxidation of methane to methanol in a carbonate electrolyte. TiO_x and FeO_x oxides show no selectivity for methanol at the oxidative potentials tested. Taking CoO_x as a prototypical methane partial oxidation electrocatalyst, we systematically study the dependence of activity and methanol selectivity on catalyst film thickness, overpotential, temperature and electrochemical cell hydrodynamics in a gas-tight rotating cylinder electrode cell. Optimal conditions of low catalyst film thickness, intermediate overpotentials and intermediate temperatures are identified to favor methanol accumulation in the electrochemical cell. The rate of transport of methane and methanol in and out of the electrocatalyst surface is also shown to be an important design parameter for future methane to methanol electrochemical cells. Through a combination of control experiments and DFT calculations, we show that the oxidized form of the as-deposited (oxy)hydroxide catalyst films are active for the thermal oxidation of methane to methanol even without the application of a bias potential, demonstrating that high valence transition metal oxides are intrinsically active for the activation and oxidation of methane to methanol at ambient temperatures.

The conversion of biomass platforms to value-added chemicals represents an exciting opportunity to extract value from the anodic half reaction in water/CO₂ electrolysis while also holding the potential to lower the cell voltage required to carry out the full reaction.  Despite this promise, catalysts still do not exist that can operate at sufficiently low overpotentials with high rates and selectivity and a comprehensive understanding of many of these reactions is still lacking. Against this backdrop, my lab’s work in recent years has focused on the use of operando spectroscopy (carried out as the reaction progresses) to probe reactions such as the oxidation of hydroxymethylfurfural (HMF) to furandicarboxylic acid (FDCA). Further, we translate the insights gained into the rational design of next-generation catalysts with enhanced performance, thus creating an iterative feedback cycle. This talk will focus on research that spans from our first efforts on simple gold nanoparticles all the way to recent complex materials such as metal-organic frameworks (MOFs) that operate in unique ways to electrochemically convert biomass platforms to value-added products.

One promising technology to decarbonise our economy is to use carbon dioxide (CO2) from a point source or air as a starting feedstock to produce chemicals, while leveraging the rapidly-growing renewable energy as a driving force. Such an approach relies traditionally relies on separate capture and conversion processes. Both processes are energy costly and face their own technical challenges. For example, amine-based capture media requires a typical cost of US$50-150 to capture one tonne of CO2. The recovery of capture media (e.g., amines) and CO2 cost up to 90% of the overall energy for the capture process. Meanwhile, CO2 conversion by electrochemical processes takes substantial energy for the conversion, separation and electrolyte recovery steps.

As a means to reduce energy input and reduce implementation costs, coupling the capture and electrolysis conversions steps into an integrated process has been proposed. Here the conversion process uses the CO2-saturated capture media, instead of free gaseous CO2. The new integrated CO2 electrolysis process can then theoretically displace the energy-intensive amine regeneration of the capture and produce valuable chemicals (e.g., carbon monoxide) to offset the overall carbon abatement cost. However, at present the integrated electrolysers shows inferior performance to the gas-fed electrolysers. Hence, an important question needs to be answered: will the integrated capture and conversion route fulfill its promise to be more energy-efficient than the sequential route?

This talk attempts to answer this question by discussing our recent process energy system modelling results.[1] Firstly, our results find that the electrochemical CO2 conversion is the dominant energy contributor to the overall capture and conversion, and its energy cost must be minimized. If the integrated electrolysis process is of the same energy cost to the state-of-the-art gas-fed electrolyser, the integrated route could then save up to 42% energy as compared to sequential capture and conversion steps. However, this energy advantage quickly diminishes if future gas-fed electrolyser remains more energy efficient, or if bicarbonate formation in the gas-fed system is avoided. This talk will conclude with challenges and opportunities for the development of integrated capture and electrolysis.

Glycerol is a residue generated in biodiesel production that has a low economic value due to its overproduction. Recently, different oxidation processes have been developed for glycerol oxidization towards added value products such as dihydroxyacetone or formic acid, one of them being the photoelectrochemical route that improves the process efficiency using solar light. In this work we have synthetized and studied BiVO₄ and TiO₂ semiconductor electrodes with the objective of evaluating their activity as photocatalysts using different experimental conditions (electrolyte and pH). The studied support electrolytes are sodium sulphate and sodium phosphate at pH 2 and 7. The results show that in an acidic Na₂SO₄ 0,5 M medium the BiVO₄ electrode presents its maximum photocurrent and in a neutral Na₂SO₄ 0,5 M medium TiO₂ electrode reaches its best performance. It has been assessed that faradaic efficiencies towards glycerol oxidation follows an opposite trend to water oxidation, the latter being more favoured using TiO₂ than BiVO₄ electrodes. The oxidation product analysis by HPLC and H-NMR shows that BiVO₄ presents better faradaic efficiency for glycerol oxidation than TiO₂ but with a lower selectivity towards a given product. Furthermore, in all cases, it has been proved the beneficial role of illumination with respect to dark electrolysis. It has been observed that the use of phosphate ions as supporting electrolyte is harmful for the glycerol oxidation due to the peroxide generation, besides the instability of BiVO₄ in acidic phosphate electrolyte. On the contrary, using sulphate anions as the supporting electrolyte it has been achieved faradaic efficiencies over 80 % with a constant selectivity at different reaction times.

Electrochemical CO₂ reduction reaction (CO₂RR) holds promise for the conversion of green electricity (e.g., solar electricity) into fuels and CO₂ mitigation, to address the global warming issue. Biomass, on the other hand, is an abundant renewable energy resource with a potential to provide sustainable source of fuel. Nevertheless, CO₂RR suffers from high applied potential mainly due to the inefficient oxygen evolution reaction (OER) in the counter electrode, as well as low selectivity—impeding its commercialization; and the biomass upgrading process emits significant amount of greenhouse gases—hindering its employment. Here in this work, we first design and synthesize active and selective electrocatalysts for the CO₂ reduction and biomass oxidation. Combining these two reactions within one unified electrochemical system is proposed in literature to further decrease the total applied potential and improve the energy efficiency, however often the attained current density in biomass oxidation reactions (i.e., few mA/cm²) is not comparable to the commonly used OER and subsequently not suitable for the required current densities in CO₂RR systems (i.e., > 100 mA/cm²). Thus, in this work, we demonstrate a feasible approach to attain an energy efficient combined electrochemical system that works at current density > 100 mA/cm². In this talk, I will report some of our recent achievements in catalyst design for both reactions, their performance, energy efficiency improvements, and molecular understanding of the reaction mechanism. The talk will be mostly focused on CO₂RR to formate and methane, and hydroxymethylfurfural (HMF) oxidation to furandicarboxylic acid (FDCA).

Once solar fuels are accessible economically and widely, we need a device to convert them into useful energy. In this aspect, fuel cells (FCs) are more appealing than conventional internal combustion engine in terms of energy efficiency because of no entropy energy waste. However, mass deployment of FCs demands better electrode-electrolyte interfaces to catalyze oxygen reduction reaction (ORR, especially in acidic environments), the kinetics of which depends on the energetics of surface adsorption as well as on the electrolyte environment. While the former has been well established using experimental and theoretical work, the effect of the electrolyte remains relatively unexplored. In this talk, I will first show an unanticipated effect of nonspecifically adsorbing (NSA) anions on the ORR kinetics on a Pt(111) electrode in acid. The electrolyte-related trends do not follow the usual ORR descriptor, i.e. *OH binding energy. Instead, a new voltammetry-accessible descriptor, namely the reversibility of the *O↔*OH transition on the (111) terrace was proposed to successfully track the dependence of ORR rates on electrolyte properties, including the concentration/identity of NSA anions in acidic media, cations in alkaline media, and the effect of the presence of ionomers, as observed in this study and in the literature. An enhanced model that relates the ORR rate on Pt(111) to the rate of the *O to *OH transition, in addition to the traditional “thermodynamic” *OH binding energy descriptor, was established, suggesting that there is more than one descriptor for the overall ORR rate. Our updated picture of the ORR can help in rationalizing the different trends for the ORR rate on stepped Pt surfaces in acidic vs. alkaline media. This study paves an avenue towards a more complete understanding of the various factors that determine the ORR rate on Pt surfaces.

The operational stability of organic solar cells is influenced by several factors including microstructure stability, the electrochemical stability of electrodes, and the photochemical degradation of the materials. Whilst some materials appear to perform better than others, there is as yet insufficient information on the material chemistry or device structure and degradation pathways to design better materials. We investigate the specific chemical structure – degradation relationship for a series of chemically similar materials. For the study we select isolated isomers of the bis-adduct of PCBM, given that the small differences in chemical structure between the different isomers are known exactly and the common degradation pathways for solar cells containing fullerenes are also known. We investigate degradation in the presence and absence of air and seek correlation between the chemical structure (i.e. side chain positions) and different mechanisms involving oxygen radicals and fullerene interactions. We consider more general criteria for improving the photochemical and microstructural stability.

The stability of perovskite solar cells is still an unresolved issue and must be addressed for making perovskites a valid option for industrial products, e.g. in the form of enhancing the performance of silicon solar cells in tandem cells. As an initial step for the systematic investigation of structural changes, it is therefore beneficial to enable us to track structural changes within hybrid perovskite thin films. Furthermore, we would like to have the ability to determine the dynamics of such changes as well as the responsible driving forces.

We are using in-situ time-resolved x-ray scattering to resolve structural changes that active layers of hybrid perovskite solar cells undergo during processing [1,2] and conditions necessary for operation. Systematically examining the resulting structural changes while deliberately accelerating the ageing process [3] can help on the route to discover options that help to suppress these structural changes in the end.

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy in high energy-per-weight or semi-transparent applications. Recently, the OSC field has been revolutionized through synthesis and processing advances, primarily through the development of numerous novel non-fullerene small molecular acceptors (NFA) with efficiencies now reaching >19% when paired with suitable donor polymers. The device stability and mechanical durability of these  non-fullerene OSCs have received less attention and developing devices with high performance, long-term morphological stability, and mechanical robustness remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OSC commercialization. Here, we discuss our current understanding of the phase behavior of OSC donor:acceptor mixtures and the relation of phase behavior and the underlying hetero- and homo-molecule interactions to performance, processing needs (e.g., kinetic quenches), and morphological and mechanical stability. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to x-ray scattering to determine critical morphology parameters and molecule packing and dynamic mechanical analysis (DMA) to assess specifically the hetero-interactions. The results presented and its ongoing evolution are intended to uncover fundamental molecular structure-function relationships that would allow predictive guidance on how desired properties can be targeted by specific chemical design. Comparative studies show that the molecular hetero-interactions between the donor and NFA are not always the geometric mean of the homo-interactions. This underscores the limited success often encountered when Hanson Solubility Parameters and surface energies are used to estimate molecular interactions.

Humankind is living in a climate emergency that requires immediate action. However, in the meantime, the world’s energy demand is continuously increasing; thus, making clean energy generation of paramount importance. Photovoltaics is at the forefront of this clean energy movement with organic photovoltaics emerging as a promising candidate to address this issue and meet this demand. Having said that, organic semiconductors came a long way from fullerene-based solar cells to state-of-the-art non-fullerene acceptor-based (NFAs) cells that show promise as next-generation photovoltaic devices. Organic solar cells employing such materials have already surpassed 20% in power conversion efficiencies (PCEs) after less than a decade of research, excluding initial attempts to outclass their fullerene counterparts in the 90s.[1] Much is done to realize high PCEs by the research community; nonetheless, the urgency to unravel the underlying mechanisms crippling the stability of the photovoltaic devices based on such material systems picked up only in recent years.

Even so, a thorough understanding or a guideline is still lacking to tackle this limitation once and for all. In this work, we implement a novel strategy, which we refer to as donor dilution, to improve the operational and outdoor stability of NFA-based systems in an archetypal donor:acceptor blend, namely, PTB7-Th:IEICO-4F.[2,3] Organic solar cells exploiting this material system with decreasing polymer content in the polymer:small molecule blend progressively perform superior both in operational and outdoor stability tests. Unencapsulated donor-diluted devices retain 85% of their initial PCE under continuous 1 sun equivalent metal halide lamp illumination at 40⁰C after 1000 hours. Furthermore, the in-house developed encapsulation procedure enables these devices to endure the harsh outdoor conditions (~23⁰C and ~64% relative humidity on average throughout winter) for an extended period in the Kingdom of Saudi Arabia.

These findings underpin the universal effort to improve the organic photovoltaic device stability, provide a practical strategy as a proof-of-concept to enhance the lifetime of organic photovoltaic devices, and bring this technology a step closer to commercialization. Future work consists of further examination of the universality of the donor dilution approach and is underway by testing various donor:acceptor blends with different classes of non-fullerene acceptors, such as state-of-the-art Y-series molecules.

With the advent of recent state-of-the-art blend materials, organic solar cells have become competitive to established technologies by achieving power conversion efficiencies of more than 18~\%. On the path to commercialization, device stability is the next challenge needing to be overcome. We researched the degradation of inverted devices of the high-performance material system PM6:Y6 and found two distinct degradation pathways. While the first one is marked by a reduction of short-circuit current and requires presence of both illumination and oxygen, we focused our research on the second one, which is induced thermally and shows losses of open-circuit voltage and fill factor. We find that, while bulk and interface properties remain overall stable, defect state formation is the primary cause for thermal degradation. The increased trap density reduces charge carrier mobility and leads to increased non-radiative recombination limiting the open-circuit voltage. In addition, we find an aging-induced transport resistance to be the cause for the reduced fill factor. Our findings show that device stability could be increased by suppressing trap formation.

Recently, organic photovoltaics (OPV) achieved efficiencies close to 20 % under AM1.5G. [1] In parallel OPV for harvesting artificial indoor light approaching efficiencies close to 30 % under 1000 lux warm white LED light. [2] In both cases the development of new donor and acceptor molecules as well as new interlayer materials was in the focus of interest, while the stability of OPV devices has drawn less attention. However, together with high performance the long-term stability of solar cells and modules is one of the key factors for a successful market entry. For indoor OPV the requirements are typically less harsh: much lower irradiation (UV light free), moderate and relatively constant temperature and moisture. Despite, the intrinsic stability of the materials, the stability of the full cell stack and the packaging have to be investigated.

For rigid devices a glass/glass encapsulation is probably the best choice, for flexible photovoltaics however, other solutions have to be found. Challenging are high barrier properties against moisture and oxygen while maintaining high transmission rates, UV resistance and mechanical flexibility (bendability without delamination).

We present stability data for different organic absorber materials in indium tin oxide (ITO)-based and ITO-free cell architectures on rigid and flexible substrates. [3] Our main focus is the long-term stability of devices for indoor usage, thus we performed accelerated (light) aging experiments under typical and elevated indoor light conditions.

For PV-X plus based ITO-free rigid cells with an efficiency of 17% under 500 lux, we could achieve remarkable stability with no losses observedduring more than 3000 h at 50.000 lux. This photon dose corresponds to almost 50 years at 500 lux which is much more than required for typical internet-of-things purposes. For flexible ITO-free devices promising preliminary data is obtained.

While elevated temperatures are typically not present in indoor PV, however for lamination processes temperatures of about 120 °C for shorter periods are required. We performed detailed analysis of the processes involved during thermal aging.

Organic solar cells (OSCs) have attracted great attention owing to their unique advantages such as lightweight, flexibility and solution processability for further commercial applications[1]. In recent years, the efficiency of OSC has grown rapidly with the help of burgeoning Y-series electron acceptors[2]. Benefits from structural modification, well-adjusted energy levels, absorption property, crystallinity and mobility have all attributed to the efficiency enhancement[2]. However, as another critical factor for commercialization, stability, especially photostability change generated from such structural modification remains elusive. To unveil the relationship, four Y-series acceptors with different terminal groups, internal and outer alkyl chains were selected to compare the photo-induced degradation from both device and structural perspectives. Significant increased trap-assisted recombination was observed after continuous illumination, which could be related to the structure change of the acceptors exhibited in NMR and Raman spectra. The energy barriers of happening the light-triggered reactions on acceptors are further evaluated through density functional theory (DFT) calculation. As a result, acceptors with fluorinated terminal groups and long internal alkyl chains will enhance the photostability of devices. This trend is further shown on additional two Y-series acceptors based devices. Finally, encapsulated devices with Y-series acceptors are tested under real-world conditions, showing a similar conclusion as photostability results. Our results provide an in-depth understanding of the light-soaking and outdoor stability of Y-series acceptors, and important guidelines for the designation acceptor materials to achieve stable OSCs.

With the increasing efficiency of organic solar cells, it is now imperative to focus investigation on the improvement of device stability and the understanding of the degradation mechanisms. It is known that the interface between the active layer and the charge transport materials, such as metal-oxides, can strongly influence the device stability, and many passivation strategies have been reported recently [1]. Herein, we have identified a major source of instability when using nanoparticles of SnO₂, which is determined by the presence of ionic species at the nanoparticle’s surface. The induced degradation mechanism is unveiled by means of photophysical and electrical measurements. Furthermore, we discover that with a very simple washing of the metal oxide nanoparticle layer we are able to reduce the device degradation. After the washing procedure, several non-fullerene acceptor based solar cells show a boost of the T80 device lifetime under illumination up to hundreds of hours.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. Beyond the well-established characteristics of regular impedance arcs, we address the appearance of inductor effect at high voltage in perovskite solar cell. We present a physical model in terms of delayed recombination current that explains the evolution of impedance spectra and the evolution of current-voltage curves. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response. We apply the dynamic picture based on a few neuron-like equations to the characterization of halide perovskite memristors. It has a resistance that depends on the history of the system, and the states can be switched by applied voltage. Memristors show an intense hysteresis that can be characterized in terms of the emergence of inductive components.^1-5

(1) Bisquert, J.; Guerrero, A. Dynamic Instability and Time Domain Response of a Model Halide Perovskite Memristor for Artificial Neurons, Jounal of Physical Chemistry Letters 2022, 13, 3789-3795.

(2) Bisquert, J.; Guerrero, A. Chemical Inductor, J. Am. Chem. Soc. 2022, 10.1021/jacs.1022c00777.

(3) Berruet, M.; Pérez-Martínez, J. C.; Romero, B.; Gonzales, C.; Al-Mayouf, A. M.; Guerrero, A.; Bisquert, J. Physical model for the current-voltage hysteresis and impedance of halide perovskite memristors, ACS Energy Lett. 2022, 7, 1214–1222.

(4) Bisquert, J.; Guerrero, A.; Gonzales, C. Theory of Hysteresis in Halide Perovskites by Integration of the Equivalent Circuit, ACS Phys. Chem Au 2021, 1, 25-44.

(5) Guerrero, A.; Bisquert, J.; Garcia-Belmonte, G. Impedance spectroscopy of metal halide perovskite solar cells from the perspective of equivalent circuits, Chemical Reviews 2021, 121, 14430–14484.

The use of layered, 2D perovskites can greatly improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. We investigated the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA₂PbI₄, through transient electronically contacted nanosecond-to-millisecond photoconductivity measurements, which are sensitive to long-time, long-range (micrometer length scale) transport processes. After careful analysis, accounting for both early-time recombination and the evolution of the exciton-to-free-carrier population, a long-range mobility of 8.0 +/− 0.6 cm² (V s)^–1, which is ten times greater than the long-range mobility of a comparable 3D material FA_0.9Cs_0.1PbI₃ is determined. These values are compared to ultra-fast transient time-resolved THz photoconductivity measurements, which are sensitive to early-time, shorter-range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm² (V s)^–1 in the case of the PEA₂PbI₄ and FA_0.9Cs_0.1PbI₃, respectively, are obtained. This previously unreported concurrence between the long-range and short-range mobility in a 2D material indicates that the polycrystalline thin films already have single-crystal-like qualities. Hence, their fundamental charge carrier transport properties should aid device performance, while keeping the benefits for stability as well.

Adapted from [1]

Abstract: With the emergence of high-performance nonfullerene electron acceptors (NFA), organic solar cells (OSCs) are particularly appealing as their power conversion efficiencies (PCEs) approaching 20%. Identifying and eliminating the sources of degradation in photovoltaic devices is as crucial as improving the PCEs from the manufacturing and commercialization point of view. In this talk, I will show the strategies we developed to improve the thermal stability of NFA OSCs. Further, I will share the photo degradation behavior of the state-of-the-art NFA OSCs in both indoor and outdoor conditions with multimodal characterizations and in-operando transient electro-optical spectroscopy. Their outdoor operational stability will be assessed under real-world conditions (in a hot and sunny climate in Saudi Arabia). Finally, I present a comparison of indoor and outdoor performance of Y‑series OSCs to demonstrate the importance of materials stability and device longevity for balancing the efficiency, stability, and cost potential of organic photovoltaics in real-world outdoor applications.

OPV cells have a proven efficiency of over 18 % while OPV modules have a proven record efficiency of 13.5 %. Both values are still increasing towards > 20 % for small area cells and > 15 % for large scale modules. With these performance values, OPV is reaching out to applications that are going beyond the typical niche markets. The first generation of commercially available OPV modules shows lifetimes in the order of 5 years and more under outdoor conditions. Independent of the application, the lifetime limitations of organic solar cells are not fully understood. Few publications reported operational lifetimes of over 25000 hrs. Organic solar cell materials being stable against photooxidation were as well reported, and, most recently, we demonstrated solar cells that canbe operated in water and under 1 sun for hundreds of hours – unpackaged. However, all these “best you can do” lifetime values are reported for different combinations of materials and interface systems.

This talk will discuss a degradation mechanism that has been overlooked so far but probably is decisive to understand for longlived operation. The community is well aware of the impact of the light source on the degradation mechanism. Blocking the UV part of the solar spectrum till 380 nm became an unwritten law for outdoor as well as indoor testing. However, one has to critically ask whether a 380 nm cutoff is indeed sufficient to stabilize a solar cell. We have investigated degradation as a function of the spectral distribution, and found degradation mechanisms that have not been reported yet. Most specifically, the combination of the interface material, the semiconductor and the spectrum of the light source dominate degradation kinetics. The spectral excitation spectrum for degradation does not coincide with the EQE of the solar cell, indicating interface induced mechanisms. All these findings indicate that OPV has to overcome another hurdle before further improving the operational stability.

The modest stability of perovskite solar cells (PSCs) in ambient conditions represents a major impediment to their widespread commercialization. The interface between the hole-transport material (HTM) and perovskite is known to significantly affect the stability of PSCs.[1] Recently, the idea of combining the HTM function with the passivation of the perovskite surface via ad-hoc chemical interaction between the HTM and perovskite has been successfully exploited.[2] As a result, these HTMs anchored to the perovskite surface led to compact and ordered interfaces and to PSCs with a remarkable increase in both the efficiency and stability. In this talk, I will summarize key recent examples reported by us of low-cost HTM designs that can effectively interact with the surface of a triple cation (CsFAMA) perovskite promoting the interfacial charge transfer dynamics and the stability of PSCs. These examples include halogen-bonded[3], sulfonated[4], hydrogen-bonded, and fluorene-based HTM molecular designs.

Furthermore, I will highlight the doping engineering of traditional HTMs (e.g., Spiro-OMeTAD) as an additional valid strategy when aiming at competitive long-term stability. When the widely used hygroscopic dopants (e.g., LiFTSI) of Spiro-OMeTAD are replaced with a single molecular dopant, i.e., F4-TCNQ, a remarkable stability of mesoporous PSCs in air is achieved[5]. We demonstrate that the high uniformity of F4-TCNQ doping in Spiro-OMeTAD and reduced dopant aggregation and dopant migration towards the anode are the main reasons for the increased stability of PSCs in air, beyond the well-known hydrophobic protection of the perovskite induced by the F4-TCNQ dopant.

Quantifying the role of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) environmental stressors on the degradation of halide perovskites is key for the further development of stable optoelectronic devices. In this talk, I will present a suite of optical and electrical complementary tools that provide a detailed description of the dynamic responses within these materials, from the macro to the nanoscale. At the macroscopic scale, we unravel the influence of relative humidity on charge carrier radiative recombination in CsxFA1−xPb(IyBr1−y)3 perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Then, we apply machine learning algorithms to forecast the material optical response once exposed to distinct temperature and humidity levels. At the nanoscale, we realize state-of-the-art scanning probe microscopy methods to image and quantify ion motion within and between grains, as a function of illumination and relative humidity. Combined, the macro- and nanoscale environmental measurements performed provide an ample framework for tracking, in real-time, the relevant changes that can lead to degradation. Additionally, they exemplify trustworthy diagnosis tools that can be expanded to any perovskite chemical composition.      

Record efficiencies of perovskite solar cells (PSCs) are obtained with spiro-OMeTAD as the hole transport layer. Spiro-OMeTAD is conventionally doped by hygroscopic lithium salts with the assistance of volatile 4-tert-butylpyridine, which, however, brings a time-consuming doping process as well as poor device stability. We successfully develop an instantly efficient and clean doping strategy to replace the conventional spiro-OMeTAD doping. As demonstrated by experimental and theoretical investigations, the radical dopant leads to significant increase of the conductivity through efficient hole polarons generation. The ionic salts can further modulate the work function with negligible effects on the film conductivity, critical for reaching optimal V_OC values by a favorable energetic level alignment. Spiro-OMeTAD based on our new doping strategy enables PSCs with a high power conversion efficiency over 25% and an excellent stability against moisture, heat and illumination. Our findings pave the way for achieving PSCs with high efficiencies and excellent stability at the same time. In addition, our doping strategy goes beyond traditional organic semiconductor doping, providing new understanding of organic doping mechanisms which can inspire further optimizations of different optoelectronic devices.

Halide perovskites quickly overrun research activities in new materials for cost-effective and high-efficiency photovoltaic technologies. Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency. Recent reports demonstrated that perovskites could compete with the most efficient photovoltaic materials. At the same time, they still allow processing from solutions as a potential advantage to delivering a cost-effective solar technology.

The most stable and efficient perovskite contains lead, among the most toxic elements on earth. The lead from perovskite is more dangerous than other lead contaminants since it is absorbed rapidly from plants, and thus passed to the food chain, if dispersed in the environment. Lead-free alternatives have been reported with impressive progress in power conversion efficiency for tin-based (lead-free) perovskites. However, the stability of tin-based perovskite solar cells is still unexplored. In the present talk, we will focus on the strength of tin-based (lead-free) perovskite solar cells.

Formamidinium lead iodide (FAPbI3) is the 3D lead perovskite with the highest theoretical efficiency due to its narrower band gap in comparison with Cs or methylammonium perovskites. Unfortunately, the black phase allowing this narrow bandgap is not the most stable one at room temperature. Here, we shown as the fabrication conditions and the interaction with semiconductor quantum dots can boost significantly the stability of FAPbI3. The interaction of halide perovskite and colloidal semiconductor nanostructures (quantum dots or nanoplatelets) can produce interesting synergistic interactions. We show that the interaction of PbS quantum dots and nanoplatelets can produce the stabilization of FAPbI3and FACsPbI3 perovskite black phase and also the increase of the efficiency, stability and reproducibility of the photovoltaic devices prepared with these halide perovskites. Incorporation of PbS QDs allows the dramatic decrease of the annealing temperature for the formation of black FAPbI3 phase perovskite thin film, from the 170ºC required without QDs to 85ºC when QDs are present. In addition, stability of these systems including embedded nanostructures is extended not just for samples prepared in the glove box but fabricated in ambient conditions. In addition, fabrication of devices under air condition can further increase the stability of FAPbI3. We have also verified the synergic combination of different additives for a significant increase of device stability. These result points the interest of Perovskite-Quantum Dot Nanocomposites, for further development of advanced optoelectronic devices. Stabilization of tin based perovskite solar cells will be analyzed. Eventually, stabilization of halide perovskite nanoparticles is a necessary step for the development of high performance Halide Perovskite LEDs. Control of post synthetic washing processes allows the preparation of LEDs with enhanced performance and stability.

Perovskite solar cells (PSCs) have created much excitement in the past years and attract spotlight attention. This talk will provide an overview of the reasons for this development highlighting the historic development as well as the specific material properties that make perovskites so attractive for the research community.[1-3]

The current challenges are exemplified using a high-performance model system for PSCs (multication Rb, Cs, methylammonium (MA), formamidinium (FA) perovskites).[2,3] The triple cation (Cs, MA, FA) achieves high performances due to suppressed phase impurities. This results in more robust materials enabling breakthrough reproducibility.

Through multication engineering, usually not-considered alakali metals, such as Rb, can be studied[5] resulting in one of the highest voltages compared to the bandgap. Polymer-coated cells maintained 95% of their initial performance at elevated temperature for 500 hours under working conditions, a crucial step towards industrialisation of PSCs.

To explore the theme of multicomponent perovskites further, molecular cations were re-evaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded a high open-circuit voltage of 1.59 V. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is relevant for lasers and display technology. [6]

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years, also in areas beyond photovoltaics, for example for medical scintillation detectors.[6,7]

Perovskite semiconductors differ from most inorganic and organic semiconductors due to the presence of mobile ions in the active layer. Although the phenomenon is intensively investigated, important questions remain, such as the dominant species and overall density of the mobile ions and their exact impact on the device stability and steady-state power conversion efficiency (PCE). In this talk, we propose a simple method to estimate the efficiency loss due to mobile ions via “fast-hysteresis” measurements by preventing the perturbation of mobile ions out of their equilibrium position at fast scan speeds (1000 V/s) and choosing a suitable prebias condition.[1,2] The ”ion-free” PCE of fresh pin-type cells with low levels of apparent hysteresis at typical scan speeds (100 mV/s) is between 1.5-3% higher than the steady-state PCE. However, with device aging, e.g. under elevated temperatures or continuous light illumination, the ionic losses increase substantially. This represents the dominant degradation loss of a triple cation based pin-type perovskite cell. The enhanced ionic losses with aging are also accompanied by a significant increase of the mobile ion density. Finally, we will discuss how the stoichiometry and in particular the PbI₂ excess influence the ionic losses and how these losses vary with the charge transport properties. Finally, an outlook is presented on how these losses affect the degradation of solution processed perovskite based tandem cells. Overall, the proposed methods to quantify the ion-induced field screening and PCE losses allow for a better understanding of several key phenomena in perovskite solar cells and shed light on the complex device degradation process.

References:

1 Le Corre, V. M. et al. Sol. RRL 2100772 (2022), DOI: 10.1002/solr.202100772

2 Thiesbrummel, J. et al. Adv. Energy Mater. 11, 2101447 (2021), DOI: 10.1002/aenm.202101447

Halide perovskite semiconductors are generating enormous excitement for next-generation photovoltaics in both single junction and tandem device forms. Their performance is striking given that these absorber layers are rife with heterogeneity on the nanoscale in their structural, chemical and optoelectronic properties. Here, I will describe how this heterogeneity impacts not just performance but also operational stability of halide perovskite solar cells. Nanoscale trap clusters are present in the absorber layers that act as unwanted non-radiative recombination sites [1] and sites of instability [2] -- and thus must be removed. These trap clusters relate to phase impurities [3] and form in regions that are not stabilised by octahedral tilt [4]. Furthermore, interfaces impact local recombination and instability pathways, both on a local scale and on a macroscopic scale. I will outline how appropriate passivation and compositional control can mitigate these issues, and provide a pathway to realise highly efficient and stable perovskite solar cells.

In recent years perovskite solar cells (PSCs) have attracted attention due to their promising electro-optical properties leading to high power conversion efficiencies of 25.7% in single-junction devices [1]. However, the stability of PSCs is still limited and thus the bottleneck on their way to commercialisation. One of the main causes of the poor stability is the presence of mobile ions in the perovskite absorber, facilitated by their soft ionic crystal structure. Interestingly, the density of mobile ions reported in literature spans multiple orders of magnitude reaching from 10¹⁵ - 10¹⁹ cm^-3 [2, 3]. One technique that has been previously applied to quantify the activation energy, diffusion coefficient, and density of mobile ions is transient ion drift (TID). In TID, the modulation of the device capacitance due to redistribution of mobile ions after applying a voltage pulse is measured. However, so far, the interpretation of TID was based on an analytical model that assumed a p-doped perovskite and did only account for the drift but not for the diffusion of mobile ions [3, 4]. In this work, we provide an updated interpretation of TID, also accounting for low doping densities in the perovskite, and show what it can teach us about the activation energy, diffusion coefficient, and density of mobile ions.

Achieving long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieve this goal. Surprisingly, we find that intermediate phases that occur during the crystallization process strongly influence the long-term perovskite device stability. The commonly employed “dimethyl formamide/dimethyl sulfoxide” (DMF/DMSO) solvent system preparation method results in poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature “DMSO-free” processing method that utilizes dimethylammonium chloride (DMACl) as an additive to accurately control the perovskite intermediate precursor-phases. By precisely controlling the 2H to 3C perovskite phase crystallization sequence, we tune the grain size, texturing, orientation (corner-up vs face-up) and crystallinity of the formamidinium (FA)_yCs_1‑yPb(I_xBr_1-x)₃ perovskite system. A population of encapsulated devices showed significantly improved operational stability, with a median T₈₀ lifetime, for the steady-state PCE, of 1190 hours and a champion device showed a T₈₀ of 1410 hours, under simulated sun light at 65 °C in air, under open‑circuit conditions. Our work introduces an innovative processing method that allows higher overall perovskite device stability, by controlling the intermediate phase domains during the perovskite formation. This work highlights the importance of material quality in order to achieve long-term operational stability of perovskite optoelectronic devices.

Here we report a detailed study at the interface for dipole molecule treated inorganic perovskite CsPbI3 based solar cells. An energy level band bending at the passivated CsPbI3 perovskite and hole selective contact interface was introduced by a dipole molecule, trioctylphosphine oxide. X-ray photoelectron Spectroscopy (XPS) study shows binding energy shifts towards lower value, which indicating the acceptance of electrons from perovskite to TOPO, similar to the role as a Lewis acid. The upward shifts in work function in TOPO-treated samples, as compared to the control sample, has been revealed by Ultraviolet Photoelectron Spectroscopy (UPS) and Kelvin Probe measurements independently. Steady-state Photoluminescence (PL) and Time-resolved Photoluminescence Spectroscopy (trPL) measurements show that the dipole molecule contributes marginally to defects passivation of perovskite films. Charge dynamics at the interface was characterized by using transient Surface Photovoltage Spectroscopy (trSPV) at varied photon energies. The charge selectivity at the interface is increased by over six times for TOPO treated samples. Finally, a champion efficiency of 18.7% has been achieved for CsPbI3 perovskite solar cells with an increase of 100 mV in average in open-circuit voltage. Influence of dipole treatment on long-term stability of these devices were characterized by maximum power point tracking of over 500 hours under 1 sun intensity at room temperature with cycled illumination. It has been seen that due to better energy alignment at the interface, dipole molecule treated samples shows a better stability as compared to control CsPbI3 based devices.

A striking difference between lead-halide perovskites and conventional semiconductors (e.g., silicon, III-V’s) is the dual ionic-covalent bond nature within the anionic inorganic framework. This weaker bond nature compared to the purely covalent bond of conventional semiconductors results in the mechanically soft and dynamically disordered perovskite lattice whose alteration affects the optoelectronic properties and the stability of these solids. Thus, metal-halide perovskites are particularly sensitive to variations in composition, fabrication and external stimuli that can induce strain in the material, interesting for developing next-generation memory devices, sensors, and energy-storage technologies. Layered Dion–Jacobson (DJ) and Ruddlesden–Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications and industrial fabrication methods (<1 GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure–property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0–0.35 GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)2PbBr4 under 0.35 GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications in e.g., sensors, actuators.

Bismuth-halide-based semiconductors have gained increasing attention for optoelectronics, owing to their low toxicity, high environmental stability under ambient conditions, and easy processability by a wide range of scalable methods. These materials have also been proposed to replicate key features of the electronic structure of lead-halide perovskites that may give rise to defect tolerance, but without the toxicity limitations of the latter [1]. This talk will examine in detail the case of bismuth oxyiodide (BiOI). Through density functional theory calculations, we show that the most common point defects in BiOI are resonant within the bands, or shallow within the bandgap [2]. We develop an all-inorganic device structure, and devise a route to control the preferred orientation of the vapour-deposited BiOI films to achieve photovoltaics with external quantum efficiencies reaching up to 80% at 450 nm wavelength [2]. Further, we demonstrate the strong potential of BiOI for indoor light harvesting to power Internet of Things electronics [3], as well as for bias-free solar water and CO2 splitting [4]. We finish with a discussion of the key factors that will need to be addressed in order to further improve the performance of this material in optoelectronics, focussing especially on the role of carrier-phonon coupling on charge-carrier transport and dynamics.

The toxicity of lead, the leaching problems, and the current regulations designed to limit or eliminate substances that are dangerous to the environment and to people, impose to move towards lead-free metal halide perovskites (MHPs). In addition, while MHPs have been thoroughly investigated for photovoltaics (PV) applications, it is now evident that their superior optical properties could be advantageously exploited in several other fields ranging from photodetectors to photocatalysis. Replacing lead with tin or germanium in 3D or 2D perovskites brings well-known stability issues, and the exploration of all-inorganic perovskite-derivatives containing bismuth or antimony may lead to problems of precursors solubility in wet-chemistry film deposition. Notwithstanding these possible obstacles, the PV application of tin-based MHPs has impressively increased in terms of efficiencies in the last few years. In order to further extend and exploit more recent applications of lead-free perovskites, a thorough experimental and computational structure-property correlation investigation should be provided with the aim of designing more stable and optimized materials. In this contribution we will present some recent studies devoted to the design, synthesis, and application of lead-free MHPs with a specific focus to fields beyond PV such as photocatalysis or photodetectors. Among other examples, we will show how, through a careful materials chemistry design, we could improve the water stability of tin and germanium-based MHPs perovskites and apply these characteristics to photocatalytic applications. The combined use of experimental and computational tools allowed to gain a significant comprehension on the mechanisms underpinning such features. At the basis of the results there is the possible manipulation of the compositional space provided for example by low-dimensional perovskites which can be beneficially used to modulate not only the optoelectronic properties but also the stability of these materials. Further examples will be focused on the application of physical vapor deposition techniques to afford the preparation of challenging all-inorganic lead-free perovskites and exploit their use in photodetectors.

The outstanding opto-electronic properties of metal-halide perovskites (MHPs) have contributed several breakthroughs in photovoltaics and optoelectronics. These materials would be ideally exploited also in heterogeneous photocatalysis, but the poor MHPs stability in aqueous environment has for long inhibited their application to this field. In the last two years, however, a growing family of water-stable and photoactive MHPs have been reported, thus paving the way towards the development MHP-based photocatalysts.

Surprisingly, tin-halide perovskites (THPs), which are notoriously unstable in photovoltaics, have received a large attention because of their suitable band gap, tunable electronic energy levels, and their low toxicity coupled to a superior stability in water environment compared to their lead counterparts. Here, we present high-level ab initio calculations to unveil the key factors determining the reactivity of THPs towards photocatalytic hydrogen production at the perovskite/water interface. Our results highlight that the occurrence of electron polarons at the surface of THPs is paramount in determining the efficiency of the reaction. The stabilization of localized electrons stems from the energy of the conduction band edge and from the peculiar THP defect chemistry, largely centered on tin. Band edge tuning is governed by the interplay between the A-site cation and nature of the halogen, thus fine-tuning the THPs energy levels can be achieved by varying the chemical composition, providing a successful strategy to boost the photo-reactivity of these materials.

Halide double perovskites are a chemically diverse class of materials featuring a vast range of thermodynamically stable compounds with widely tunable optoelectronic properties [1, 2]. Dimensional reduction of 3D lattices is routinely used for band structure and exciton engineering by exploiting the effects of quantum and dielectric confinement. Excitons in quasi-2D derivatives of Pb-based halide perovskites have been explored extensively in the last years. However, the impact of dimensional reduction on the nature of excitons in halide double perovskite is not yet well-understood.

In this contribution, we discuss the optoelectronic properties of 3D and quasi-2D derivatives of Ag/Bi-based halide double perovskites based on first principles calculations using many-body perturbation theory within the GW approximation [3] and the Bethe-Salpeter equation approach [4]. In the 3D double perovskite Cs₂AgBiBr₆, non-hydrogenic and strongly localized resonant excitons arise due to the chemical heterogeneity of alternating AgBr₆ and BiBr₆ octahedra and lead to anisotropic effective masses and pronounced local field effects [5]. We show that dimensional reduction of Cs₂AgBiBr₆ to mono- and bilayer Ruddlesden-Popper and Dion-Jacobson structures has striking effects on the band structure and excitons of these quasi-2D materials. Our results are in line with the experimentally reported optical properties of these materials [6] and demonstrate that structural distortions, quantum confinement effects, and layer stacking can be used to tune exciton localization and binding energies in chemically complex quasi-2D materials.

Lead-free double halide perovskites AIMIMIIIX6 (A,M – metal cations, X – halide anions) offer unprecedented variability of structures and compositions, with all four positions variable independently resulting in many thousands of possible compounds for the same set of constituent elements. A detailed screening of the properties of these huge families of compounds requires application of a combination of high-throughput synthetic (HTP) approaches with HTP characterization and HTP testing for possible applications related to light conversion and emission.

We have developed a “green” protocol for a HTP synthesis of brightly luminescent lead-free microcrystalline Cs2AgxNa1-xBiyIn1-yCl6 (CANBIC) perovskite phosphors absorbing strongly in UV range and emitting broadband photoluminescence (PL) in the visible range with PL quantum yields reaching 98±2% for specific compositions with x = 0.35-0.40 and y = 0.01-0.02. The CANBIC perovskites revealed high stability of spectral properties during many months of ambient storage, thermal stability at open air till 200-250 oC and photochemical stability under UV illumination, making these compounds highly promising for applications in luminescent solar light concentrators and down-shifters. Our HTP approach to CANBIC perovskites includes robot-assisted automated synthesis with any desirable x and y steps combined with HTP spectral charcterization which includes absorption, PL, PL excitation, and Raman spectroscopy as well as time-resolved PL characterization, allowing many hundreds of samples to be produced and characterized in a single working session.

This HTP approach was expanded to other CANBIC-like perovskites, produced either by substituting InIII with SbIII, or by substituting BiIII with FeIII. In the first case we developed a HTP protocol for the synthesis of Cs2AgxNa1-xBiySb1-yCl6 (CANBSC) perovskites with x and y fractions varied independently from 0 to 1. The CANBSC perovskites can be converted into Cs2AgxNa1-xBiySb1-yBr6 (CANBSB) and Cs3Bi2ySb2(1-y)I9 (CBSI) perovskites by a single-step anionic exchange with NaBr and NaI, respectively. These compounds reveal a band-bowing effect, the bandgaps of mixed perovskites being typically lower than those of corresponding individual BiIII- and SbIII-based perovskites. For CANBSC and CANBSB compounds the band-bowing effect was found to increase with an increase of the silver fraction. The lowest bandgaps reached were 2.47 eV for CANBSB and 2.08 eV for CBSI both at x = y = 0.50.

We found that BiIII in CANBIC perovskite can be partially or completely substituted by FeIII resulting in the latter case in Cs2AgxNa1-xFeyIn1-yCl6 (CANFIC) perovskites showing strong absorbance in the visible spectral range and a high environmental stability. A special feature of CANFIC compounds is a structure-directing influence of In on the formation of perovskite phase. While for y = 1.00 only a multi-phase mixture can be obtained in our conditions, introduction of mere 1% InIII steers the precipitation exclusively to the formation of cubic perovskite phase with no other phases detectable. The lowest bandgap reached so far is ca. 2.0 eV for x = 1.00 and y = 0.95-0.99. Anionic exchange of CANFIC did not result in stable single-phase bromide and iodide compounds. A similar structure-directing effect was also found for BiIII and SbIII introduced instead of InIII. Our preliminary results show the feasibility of introducing simultaneously three MIII metals, for example, BiIII, InIII, and FeIII, into the chloride perovskite while preserving the single-phase solid-solution character of the final products, opening broad possibilities for the compositional design of the optical properties. The unique variability of the MIII site of these double perovskites will be addressed in future using the combined HTP approach to the synthesis and characterization developed earlier for CANBIC perovskite phosphors.

Electrical doping enables meticulous tuning of the electronic properties in novel hybrid perovskite semiconductors, which is critical for their successful impelmentation as optoelectronic applications. Nevertheless, the use of substitutional/interstitial impurities as dopants remains difficult, mainly due to dopant phase segregation and defect compensation. In contrast, molecular doping stands as a promising pathway to modulate charge carrier density via charge transfer without altering the perovskite crystal structure. However, the underlying processes facilitating molecular doping in perovskites (e.g. host-dopant interactions) remain highly underexplored. In this talk, we will present our recent work on the molecualr doping mechanism of p-type methylammonium tin-lead iodide films by using an n-type molecule, namely 4-(1,3-Dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)-N,N-dimethylbenzenamine (n-DMBI-H). By employing a combination of experimental and first principles simulation techniques, we identify preferential dative bonding between amino moieties in n-DMBI-H and Sn atoms in perovskite surfaces as a dominant host-dopant interaction that mediates charge transfer. We will discuss the dopant localization within films and deternime that n-DMBI-H is located at perovskite surfaces and grain boundaries, allowing charge carrier density tuning of nearly one order of magnitude. The talk will conclude with details on the incorporation of n-DMBI-H in p-i-n tin-lead perovskite solar cells, which results in lower charge carrier recombination and higher charge selectivity at perovskite/transport layer interfaces. This work elucidates important insights of perovskite molecular doping that are expected to inspire strategies towards next-generation perovskite optoelectronic devices.