Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen [1]. Both the efficiency and the scalability of water-splitting systems are essential factors for practical utilization of renewable solar hydrogen. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because particulate photocatalyst systems can be spread over wide areas by inexpensive processes potentially.

The author has studied various oxide, (oxy)nitride, and (oxy)chalcogenide photocatalysts [2]. The water splitting activity of SrTiO₃ can be improved by two orders of magnitude by doping Al. The quantum efficiency of photocatalytic overall water splitting has reached almost unity in the near UV region [3]. This is the highest reported to date and confirms that particulate photocatalysts can drive the uphill overall water splitting reaction as efficiently as the photon-to-chemical conversion process in photosynthesis. 

The author has also been developing panel reactors for large-scale applications. A prototype panel reactor containing Al-doped SrTiO₃ photocatalyst sheets splits water and releases product H₂ and O₂ gas bubbles at a rate expected at a solar-to-hydrogen energy conversion efficiency (STH) of 10% under intense UV illumination [4]. A 1-m²-sized photocatalyst panel reactor splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A larger size (100 m²) solar hydrogen production system was constructed and its performance and system characteristics are currently under investigation.

To realize a sufficient STH, it is essential to develop photocatalysts active under visible light irradiation. Ta₃N₅ [5] and Y₂Ti₂O₅S₂ [6] show activity in overall water splitting via one-step excitation under visible light irradiation. Photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ and Mo-doped BiVO₄ split water into H₂ and O₂ via two-step excitation, referred to as Z-scheme, and exhibit STH exceeding 1.0% [7,8]. Some other (oxy)chalcogenides and (oxy)nitrides with longer absorption edge wavelengths are also applicable to Z-schematic photocatalyst sheets.

In my talk, the latest progress in photocatalytic materials and reactors will be presented.

[1] Hisatomi et al. Nat. Catal. 2019, 2, 387.

[2] Chen et al. Nat. Rev. Mater. 2017, 2, 17050.

[3] Takata et al. Nature, 2020, 581, 411.

[4] Goto et al. Joule 2018, 2, 509.

[5] Wang et al. Nat. Catal. 2018, 1, 756.

[6] Wang et al. Nat. Mater. 2019, 18, 827.

[7] Wang et al. Nat. Mater. 2016, 15, 611.

[8] Wang et al. J. Am. Chem. Soc. 2017, 139, 1675.

Electrosynthesis of ammonia via the electrocatalytic nitrogen reduction reaction (NRR) has recently become a topic of stirring research. The process attracts attention from both industry and academia as a possible future alternative to the classical catalytic technology and is expected to enable the renewable-powered, distributed production of “green” NH₃ – both for the chemical/fertiliser industry and as an energy carrier.

Notwithstanding a truly significant investigative effort invested worldwide, the recent progress in advancing the NRR technology remains questionable to the extent that the majority of the reports on the successful electrochemical conversion of N₂ to NH₃, in the first place those using aqueous electrolyte media, cannot be considered fully reliable. The introductory part of the talk will critically assess the state-of-the-art in the NRR field, will identify the most common missteps and will aim to provide a simple guide for undertaking reliable N₂ reduction experiments.

The second part of the talk will focus on the solutions to the fundamental problems of the NRR in aqueous media, viz. suppression of the competing dihydrogen evolution and promoting the availability of N₂, through the application of the aprotic electrolyte media. Our progress towards the direct electrocatalytic reduction of dinitrogen and the redox-mediated conversion of N₂ to ammonia under these conditions will be presented, with a specific focus on the challenges and possible pathways towards further improvements in the NH₃ electrosynthesis technology.

Rational design and comparative studies of catalysts rely on detailed information about the mechanism of catalysis that in most cases is not available. We have used time-resolved UV/VIS and mid-IR spectroscopy to resolve highly reactive catalyst intermediates on the nano- to millisecond time scale, elucidate their structures and measure their reactivity in proton- and electron transfer reactions of the mechanistic cycle [1].

The azadithiolate bridge of FeFe-hydrogenase active sites has been taken as an example design for proton relays in the second coordination sphere, facilitating protonation and deprotonation of the metal centre. We have shown that for the corresponding model complexes, the aza-group has no role as proton shuttle in the Fe^IFe⁰ state, as had been proposed [2]. Instead, the effect of aza protonation is to shift the catalyst reduction potential, surprisingly with no or only little reduction in the rate of subsequent metal protonation by external acid.

We have also been able to follow the charge transfer dynamics via mid-IR signals of the catalyst in catalyst-semiconductor systems, with both band-gap excitation and dye-sensitization. Ultra-fast (<300 fs) catalyst reduction of molecular catalysts for CO₂ or proton reduction is observed on CuInS₂ quantum dots ([3] and manuscript in progress).

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic semiconductors are currently gaining substantial momentum for application as photocatalysts - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerizing selected monomers in defined ratios. This tunability has sparked intense research interest in organic photocatalysts,[1] however, the fundamental understanding of photoinduced processes in these systems and the characterisation of their catalytically active sites have stayed behind the rapid development of new materials.

In this presentation, I will demonstrate how transient and operando optical spectroscopic techniques can be used to track the evolution of photogenerated reaction intermediates in polymer photocatalysts on timescales of femtoseconds to seconds after light absorption. To this end, short laser pulses are used to study these photocatalysts under transient conditions whereas long LED pulses are employed to establish operando catalytic conditions, and photogenerated reaction intermediates are then probed optically. Firstly, these techniques reveal insights into the yield of photogenerated charges, which enables an understanding of differences in hydrogen evolution activity between different materials.[2] Secondly, these techniques allow to monitor the transfer of photogenerated electrons to catalytically active sites as well as their accumulation under operando photocatalytic conditions, where differences in electron transfer time translate into different kinetic bottlenecks of the hydrogen evolution reaction for different polymers.[3] To illustrate these points, I will draw direct comparisons between nanoparticle photocatalysts made from the polymers F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer, P10, which is one of the most performant polymer photocatalysts reported to date.[2]

Artificial photosynthesis by photoelectrocatalysis is one of the most promising ways to store solar energy in the form of fuels, thus constituting a sustainable alternative to fossil fuels [1,2]. Conjugated polymers (CPs) are used as part of some photoelectrodes due to their good conductivity and the possibility to tailor their optoelectronic properties at the molecular level. Some of the most used CPs, such as PEDOT, have a linear structure; which makes them easy to process as thin films, but also unstable under UV illumination if they are in contact with water [3]. Conjugated Porous Polymers (CPP) [3-5] show higher stability due to their 3D structure. However, it is difficult to produce thin films with them by conventional methods such as drop casting or spin coating because of their morphology.

Thanks to the electropolymerization process, we are able to prepare homogeneous, transparent and light-absorbing CPP films both on conducting glass substrates and on inorganic semiconductors. One of these CPPs, IEP-19 (Imdea Energy Polymer-19), has been synthesized for the first time and it shows promising photocurrents, which are significantly higher than those of a previously known CPP with a similar structure: CPP-3TB. Moreover, hybrid photoanodes where the CPP is electropolymerized on top of the inorganic semiconductor present higher photocurrents than the semiconductors alone, showing a synergistic effect between the organic and inorganic semiconductors. These results will be explained according to the optical, photoelectrochemical and morphological properties of the photoanodes.

Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. In this context, we have investigated charge accumulation in two model multi-redox molecular catalysts for CO₂ and proton reduction attached onto mesoporous TiO₂.[1,2] Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO₂ to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. Under an applied electrochemical bias and at high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO₂ to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO₂ is observed to be faster than microseconds, while back electron transfer from multi-reduced species to the conduction band occurs in the millisecond timescale. Finally, under steady state irradiation conditions, we show how the redox state of the catalyst is regulated as a function of the applied bias and the excitation light intensity.[3]

Currently, water electrolysers (WE) based on acidic electrolytes, viz. proton-exchange membranes (PEM), are becoming a preferred technology for the electrolytic generation of green hydrogen fuel. One key limitation of PEM-WE is the low stability of anode catalysts, which are nowadays exclusively based on one of the rarest and most expensive metals – iridium. The less expensive transition metal oxide catalysts are most commonly less active and even more unstable. The instability problem can be overcome via integration of the catalytically active component, transition metal oxide, into a highly-conductive matrix that is thermodynamically stable under the conditions of the operating PEM-WE anode, i.e. at low pH and high temperature. The present report demonstrates the effectiveness of this approach with the antimony oxide matrix that is used to stabilise non-noble third row transition metal catalysts and ruthenium. Significantly improved stability as compared to that reported in the literature is demonstrated, in particular on a week timescale at industrially relevant temperature of 80^oC at pH 0.3. Factors affecting the activity and stability as well as possible strategies for further improvements will be also presented and discussed.

Main group sulphides such as In₂S₃, SnS₂ or ZnIn₂S₄, with bandgaps in the 2.0-2.2 eV range, can use significant amounts of visible light. So we have shown in earlier studies of their photocatalytic response, evidencing that they are active in the oxidative photodegradation of aqueous HCOOH with photons having wavelengths ≤ 650 nm. In addition, they are more photoactive in the same reaction than the typical reference compound CdS, and the first two (especially SnS₂) are also more resistant to photocorrosion in oxidative environments than the same photocatalyst CdS, as shown by the analysis of sulphide components detected in solution after prolonged photocatalysis.

More recently we have undertaken the study of their combination with enzymes for photoinduced water splitting. Thus, combining In₂S₃ with a hydrogenase enzyme allowed the photocatalytic generation of H₂ using a sacrificial agent [1]; the results suggested furthermore that the process was not limited by the transfer of electrons from the semiconductor to the enzyme. Later we have verified, for the first time ever, the photoelectrochemical generation of O₂ coupling a laccase-type enzyme with these sulphides; first with In₂S₃ [2] and later with SnS₂ [3]. It could be shown that a substantial decrease in the overvoltage needed was found in the presence of light. There were some problems with the stability of these enzymes in these O₂ generation conditions; as expected, SnS₂ could be shown to be more stable than In₂S₃.

A summary of all the mentioned results will be presented here.

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],[2] 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, electrocatalytic or photoelectrocatalytic 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 are identified as excellent candidates, since these materials can fulfill most of the needed requirements, although their performance should be significantly improved for a more realistic technological assessment. Consequently, in order to boost the performance of these materials, as a preliminary step, a clear mechanistic understanding of the physical-chemical processes taking place in the bulk and at the interface with the liquid solution is essential. In the present talk, we will address different examples of the metal oxides (TiO₂, Fe₂O₃, BiVO₄...) combined with catalytic layers (Fe-Co Prussian Blue,^[3],[4] CoOx,^[5] Ag₃PO₄,^[6] NiO_x,^{[7], [8]} 2D-Sb,^[9] etc…), emphasizing the mechanistic insights leading to enhanced performance. Our studies focus on the correlation of the photoelectrochemical response of the materials with a detailed structural, optoelectronic and photoelectrochemical characterization carried out by different microscopic and spectroscopic tools.

We will present an approach to synthesizing single-phase complex metal oxides thin film photoelectrodes and discuss the challenges involved using CuBi₂O₄¹ as a model material. CuBi₂O₄ films with thickness gradients were deposited on 50x50 mm² FTO substrates by pulsed laser deposition (PLD) from a pure CuBi₂O₄ target. The relationship between the crystal structure, synthesis conditions, and properties (including photoelectrochemical performance) has been studied over a thickness range of 25 – 250 nm using combinatorial, high-throughput approaches.² A comparative study with conventional furnace annealing (FA) reveals the importance of radiative heat transfer during rapid thermal processing (RTP)³ at temperatures that exceed the normal thermal stability limit of the substrates (in this case, transparent conductive oxide films on glass). This study shows that single-phase CuBi₂O₄ is formed after RTP at 650 °C. In contrast, similar films heated up to 12 h by FA at 400 and 500 °C did not, and the films’ thicknesses had a substantial effect on the formation of phase impurities. Phase-pure CuBi₂O₄ photoelectrodes exhibit higher photocurrents, longer carrier lifetimes, higher photo-conversion efficiency, and better stability than non-pure photoelectrodes. Additionally, pure CuBi₂O₄ demonstrates typical photocurrent vs. thickness behavior of a single-photoabsorber photoelectrochemical device. However, non-pure photoelectrodes showed an unexpected photocurrent vs. thickness behavior, suggested to derive from different photoelectrochemically active impurity phases in the films.

Fe₂O₃ based photoanodes have been extensively studied in the context of solar fuels, with a variety of different strategies being proposed to mitigate their limited performance associated with short carrier lifetimes and surface recombination phenomena [1]. On the other hand, significantly less is known about other types of ferrite materials such as perovskites (general formula AFeO₃), which typically exhibit p-type conductivity. Our initial studies on phase pure LaFeO₃ and YFeO_3­ nanostructured electrodes have shown photocurrent responses towards the hydrogen evolution reaction with onset potentials (photovoltages) above 1.2 V vs RHE [2,3]. Despite the high photovoltages, the performance of these perovskites is limited by quantum yields below 1% [2-4]. In this contribution, we will examine different strategies to improve the photocurrent responses in these complex materials including surface modification and A-site substitution.

Our contribution initially focuses on pure rhombohedral LaFeO₃ featuring crystalline domains in the range of 60 nm, which are prepared by thermolysis of an ionic-liquid precursor and subsequently deposited onto FTO electrode by spin-coating [5]. Cyclic voltammetry and electrochemical impedance spectroscopy clearly show a p-type behavior with a flat band potential located at 1.44 V vs RHE. As the potential is swept into the accumulation regime, a faradaic current is observed associated with interfacial hole transfer (i.e. oxygen evolution reaction). Interestingly, a significant increase in the photocurrent responses is observed upon deposition of nanocrystalline TiO₂ layer by solution-based methods. We demonstrate that TiO₂ forms an abrupt heterojunction which act as barrier to hole-extraction, leading to photovoltages of 1.47 V vs RHE, which are among the highest reported for a single absorber. The introduction of Pt clusters further improves the quantum yield, but only in the presence of the TiO₂ overlayer. We shall also discuss the effect of introducing divalent cations such as Ca²⁺, Ba²⁺ and Sr²⁺ on the photoelectrochemical responses of LaFeO₃ [6]. These cations occupy La sites, introducing significant changes in the density of acceptor states as well as creating deep states associated which strongly accelerates the photoelectochemical oxygen reduction reaction. The complex relation between doping levels and photocurrent efficiency will be briefly discussed.

In order to obtain high solar-to-hydrogen (STH) efficiencies, a suitable semiconducting material with a band gap of 1.7 to 1.9 eV is needed as the top absorber in tandem solar water splitting devices [1]. An interesting candidate is α-SnWO₄, a ternary metal oxide with an ideal band gap of 1.9 eV. Recently it was reported that a record photocurrent density of 0.75 mA/cm² and extended stability can be achieved with NiO_x protected films prepared by pulsed laser deposition (PLD) [2]. However, the use of NiO_x protection layer limits the photovoltage as observed from the cyclic voltammetry and open circuit potential (OCP) analysis. This suggests that the interface between α-SnWO₄ and NiO_x is not ideal and understanding this interface is crucial to improve the performance further.

In this study, we present a thorough α-SnWO₄/NiO_x interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). These data are complemented with OCP analysis, density functional theory (DFT) calculations and Monte-Carlo-based photoemission peak intensity simulation. We found that the deposition of NiO_x with PLD introduces a strong upwards band bending (~400 meV) at the interface, which is favorable for charge separation. However, a significant oxidation of Sn²⁺ to Sn⁴⁺ can be simultaneously observed at the interface with increasing NiO_x layer thickness. Our photoemission spectra simulation indicates that this can be attributed to the formation of SnO₂ at the interface. A ~2 nm-thick SnO₂ layer is estimated after the deposition of 20 nm NiO_x. The implications of this SnO₂ layer to the interface junction properties and the limited photovoltage will be discussed and verified using a series of control experiments. Overall, our findings provide an important insight that future efforts need to be focused on the development of overlayers which do not oxidize the surface of α-SnWO₄, in order to further improve the performance of α-SnWO₄ photoanodes.

Artificial photosynthesis, inspired by natural photosynthesis, is considered a promising technology to store solar energy into chemical bonds (e.g., hydrogen) via (photo)electrochemical water splitting. In this process, the oxygen evolution reaction (OER) water acts as electron donor and it is considered to be the bottleneck of the process when using metal-oxide photoanodes. The efficiency of these photoanodes does not only depend on the nature of the metal oxide but also on the methodology used to synthesise them due to morphological, surface facets and doping variations, amongst others.[1] However, the mechanism of the OER on metal oxides as well as the nature of the efficiency loses remains elusive.

In this talk, I will present recent advances on the understanding of the kinetics of OER on different metal-oxide photoelectrocatalysts, focusing particularly on hematite synthesised by different deposition methods. A detailed mechanistic analysis of the OER on hematite photoanodes will be presented by the study of the water oxidation rate law under different physicochemical conditions.[2] I will also show evidence of the OER exhibiting equivalent kinetics despite the exhibited different overall performance and the correlation of lower photoanode performance with the presence of intragap states.[3]

This talk will describe our group’s recent efforts in the design and mechanistic evaluation of new molecular, macrocyclic Co(II) catalysts for aqueous H₂ generation. These catalysts were designed to incorporate redox-active bipyridine groups which are linked by nitrogen groups, both components which can participate in electron and proton transfer steps in the catalytic cycle. In comparing two molecular catalysts that differ by only one linking nitrogen, single crystal analysis reveals a profound impact on the molecular geometry, which in turn influences their relative catalytic activity. Photocatalysis experiments show that both catalysts are highly active for aqueous proton reduction at moderate pH levels, with the closed macrocycle reaching almost 2 x 10⁴ turnovers of H₂ when photo-driven by [Ru(2,2’-bipyridine)₃]²⁺ using ascorbate as an electron relay and a phosphine compound as the terminal electron donor.  Measurements of the electrocatalytic activity were used to investigate key steps in the mechanism of proton reduction, and from a detailed analysis of these experiments we propose a mechanism for catalytic proton reduction to H₂ that involves both intramolecular proton and electron transfer steps between the macrocycle ligand to the cobalt center. We will also describe how substituting pyrazine groups for the pyridyl groups in the macrocycle influence the redox properties of the macrocycle ligand, and in turn the photocatalytic activity. This work demonstrates the vital role of the second coordination sphere in the catalytic cycle, and places these relatively simple complexes on the pathway toward molecular catalysts that mimic the valuable features of enzymatic catalysis.

Carbon neutral energy sources that are scalable, deployable, and cost effective will be required at an unprecedented scale to halt irreversible climate change. To positively affect the status quo, polycrystalline, yet defective and heterogeneous, semiconductor materials are excellent candidates for targeting high efficiency, as well as low production cost, and long lifetimes of the device. However, understanding and controlling how defects, chemical heterogeneity, and microenvironments affect the efficiency and durability of integrated systems for real applications is still challenging. Yet is a necessary task to address mankind’s energy needs. This seminar will focus on the opportunities offered by the utilization of sunlight for solar fuel production. We will discuss the synthesis and the advanced characterization of integrated semiconductors and catalysts for (photo)electrocatalytic systems as they can be used under realistic operating conditions for solar fuel production.

Photoelectrochemical (PEC) water splitting is one technology to produce clean hydrogen fuel from abundant sunlight and water. To fabricate an efficient and stable photoelectrode for PEC water splitting, a multilayer structure, consisting of a protection layer (e.g., TiO₂) and a catalyst layer (e.g., Pt), is generally required. However, despite the importance of understanding the photo-physics underlying these multilayer photoelectrodes, the detailed analysis of them under operando condition is challenging due to the complexity of the device structures. In this talk, we demonstrate the versatility of the electrochemical impedance spectroscopy (EIS) method for investigating multi-layered photocathodes for PEC water splitting. By carefully analyzing the EIS data of various photocathodes with different classes of light absorbers, such as metal chalcogenide (Sb₂Se₃), metal oxide (Cu₂O), and crystalline Si, we were able to obtain information about the constituent semiconductors such as carrier lifetimes, doping densities, flat band potentials, and charge transfer rate constant under operando conditions. The EIS analysis presented in this study has made significant progress in establishing proper EIS models describing the realistic photo-physical behavior in complex multi-layered photocathodes.

Semiconductor/electrolyte interfaces attract intense interest to convert solar energy tochemicalfuels. Althoughmanyanalyticalmodelsdescribingthephotocurrent-voltage response of these devices exist, they have difficulty to reproduce full numerical simulations under small anodic bias. 

We recently derived an analytic model of a weakly absorbing n-type semiconductor/electrolyte interface with a slow rate of water oxidation reaction, fast direct recombination rate and under small anodic bias [1]. Excellent overlap of our model was demonstrated with full numerical simulations. Our model enabled us to simplify calculation of the impedance of the semiconductor/electrolyte interface derived in the work of Bertoluzzi et al[2]. The comparison of analytic and measured impedance allows to extract the reaction rate for redox reaction from the dark impedance and the direct bulk recombination constant of the semiconductor from the impedance under illumination.

This work provides easy-to-use recipe for the quantitative comparison of the recombination and reaction properties of the different semiconductor photoelectrodes by impedance spectroscopy and a starting point for development of even more general analytical models covering other recombination pathways and larger bias range in the future.

Hydrogen and other solar fuels have been highlighted as one of the future energy vectors. Having natural photosynthesis as inspiration, we can develop a device capable to split water using sunlight, obtaining oxygen and hydrogen. [1], [2] Different strategies can be used to achieve this: from separated light harvesting and catalytic systems to all integrated devices able to transform directly sunlight into fuels. These systems can be built with a variety of (photo)electrodes such as organic based material, chalcogenides or metal oxides. 

Although rapid progress is being made in the field, the efficiency of artificial systems still remains modest. Understanding of the limiting factors of these materials has allowed remarkable improvements in their performance. However, compared to of molecular systems, whose reaction mechanisms are better understood, there is still a lack of knowledge in the metal oxides mechanism of action.

In this talk I will focus on the use of the combined electrochemical and optical technique to probe the catalytic function of metal oxides for solar-to-fuel synthesis.[3-4] This technique opens a new possibility of studying multielectron reaction mechanisms on non-ideal metal oxides. I will show how using this combined technique we can elucidate the rate law of current flow in these systems and how our analysis can shed light into the reaction mechanism of photoelectrochemical solar fuels production. [5] From these studies I will discuss the key role of the catalyst through different examples. [6-8]

All this acquired knowledge will help to depict the mechanism of action of non-ideal metaloxides and so to systematically improve the next generation of photoelectrodes for water splitting.

Photoelectrochemical water splitting offers a great potential to store the intermittent solar energy as chemical fuels, i.e., hydrogen.To this end, tremendous efforts have been devoted to develop efficient light-absorbing semiconductors and electrocatalysts, which have resulted in various demonstrator devices with appreciable solar-to-hydrogen (STH) efficiencies.[1] Although it is not always discussed in these devices, efficient and safe separation of products from anode and cathode is also an essential requirement. Commercial electrolyzers employ separators, such as ion exchange membranes and diaphragms, between electrodes to avoid forming explosive product mixtures. However, implementing these separators in large scale photoelectrochemical devices may add extra complexity and potential loss. Alternatively, a hydrodynamic control can be introduced without having separators, in which products are collected from the outlet through the continuous electrolyte flow before they get mixed. The feasibility of such approach has been confirmed in previous numerical simulation reports.[2][3] However, these studies only considered dissolved gases, and gas bubbles were ignored. Also, the orientation of the device, i.e., whether the device has to be tilted from horizontal orientation for efficient solar absorption, was not considered. In tilted devices, product gas bubbles may largely influence the crossover due to the buoyancy force on the bubbles.

In this study, two-dimensional Euler-Euler multiphase fluid dynamic simulations, which calculate the volume fractions of liquid and gas phases, were introduced to investigate the crossover in a solar-driven membrane-less water splitting device. Our simulations revealed that, although overlooked in previous reports, gas bubbles contribute more to the crossover rather than dissolved gases. We also performed an extensive evaluation of various important parameters that affect the overall product crossover, e.g., the device tilt angle, bubble diameter, bubble formation efficiency, etc. For example, we found that smaller gas bubbles suppress the crossover due to the stronger momentum exchange between the liquid and gas phases. This suggests the potential benefit of using surfactants, which are known to decrease the bubble diameter, for efficient product separation in membrane-less devices. Based on our two-dimensional model, we also performed a dimensionless analysis in order to obtain a more universal picture of the membrane-less device, and we were able to present an operational design space for efficient product separation (< 1% crossover). Overall, our study highlights the critical importance in understanding and controlling the bubble formation under operating conditions to design efficient membrane-less water splitting devices.

Small perturbation techniques have been widely employed as powerful tools in the understanding of the mechanisms behind the relevant chemical reactions in (photo)electrochemical (PEC) systems, as the Oxygen Evolution Reaction (OER), the Hydrogen Evolution Reaction (HER) and the CO2 Reduction Reaction (CO2 RR). This contribution is focused on the relevant mechanistic insights that can be extracted from the analysis of relevant materials for the OER, as BiVO4, TiO2 and Ni-based electrocatalysts, with Electrochemical Impedance Spectroscopy (EIS) and Intensity Modulated Photocurrent Spectroscopy (IMPS). By analyzing a specific system, an equivalent circuit model can be designed and then, different parameters, as resistance and capacitances, can be extracted, giving relevant information from the different physical processes which take place during the studied reaction, as charge extraction, injection, or recombination. The combination of small perturbation techniques with conventional electrochemical methods is crucial for decoupling the different physical processes involved during the studied reactions.  

The development of a sustainable energy economy based on renewable, carbon-neutral energy is a necessary and urgent task. Photo-electrochemical (PEC) approaches to solar fuels and materials are interesting, provided they can be efficiently, stably, scalably, and sustainably implemented.

While significant progress on the development of earth abundant materials and on high-efficiency demonstrations has been achieved, there is relatively little known about the behavior of materials and devices under more realistic, varying conditions and on the implication of degradation on the performance and lifetime.

Here, I will first discuss multi-scale numerical modeling approaches to investigate and quantify the effect of photocorrosion on the performance evolution. I will show how important the local reaction environment is for the stability of a component, and how heterogeneity in the operating variables (current density, species concentration, temperature, etc.) affect degradation. Furthermore, I will comment on the importance of device design on device stability.

Second, I will then transition towards discussing material, device and system performance as a function of varying, non-design-point operation. I will show how these dynamics can affect the performance, show how multi-physical transport can help in controlling and smoothing some of the observed variations, and generally highlight synergistic effects. I will end with providing general guidelines on operating materials, devices and systems under more realistic conditions.

In my talk I will focus on the underlying charge carrier dynamics which determine the efficiency of solar driven water splitting in metal oxide based photoelectrodes and photocatalyst  suspensions and sheets. Experimentally my talk will be based upon a range of optical absorption spectroscopies, including transient absorption and operando spectroelectrochemical analyses. I will start by considering metal oxide photoelectrodes, addressing the impact of defect / dopant sites such as oxygen vacancies in determining photoelectrode performance. I will go on to consider the kinetics of water oxidation catalysis on metal oxide photoanodes, and the potential to apply rate law analysis of these kinetics using a charge carrier density based model as an alternative to Butler-Volmer based analyses. Finally I will discuss the role of charge carrier dynamics in determining the performance of metal oxide photocatalysts, and in particular the remarkable ability of La,Rh co-doped SrTiO₃ to drive proton reduction even under positive applied potentials.

Semiconductor-based solar fuel synthesis represents an important method for direct conversion and storage of solar energy in the form of chemical energy.  The performance of such a system is highly sensitive to the nature of the semiconductor surfaces.  For instance, the photovoltage of the system is determined by the difference between the electrochemical potential of surface chemistry and the Fermi level of the semiconductor.  Due to the complexities at the solid/liquid interface, however, it is often difficult to understand the detailed surface behaviors.  Consider water oxidation as an example.  While it is known that the nominal overall 4-e, 4-H⁺ reaction features an electrochemical potential of 1.23 V vs. reversible hydrogen electrode, the process typically proceeds through multiple steps. Despite extensive studies, it remains unknown which step is the rate determining step for many heterogeneous systems.  The knowledge is particularly weak for semiconductor-based systems.  As a result, the actual electrochemical potential of the process is often difficult to define.  At the root of this challenge is the lack of knowledge on the detailed water oxidation mechanisms at the molecular level on solid-state surfaces.  In this presentation, we discuss how to address this issue using molecular catalysts.  We show that the homogeneous catalysts can work well when immobilized on the surface.  Importantly, the reaction mechanisms of these catalysts are well defined.  Facile treatments of these catalysts can readily convert them into heterogeneous catalytic centers with well-defined atomic structures.  These new catalysts provide a new platform to study photoelectrochemical water oxidation with unprecedented details.  The knowledge is expected to accelerate development of solar fuel synthesis materials.

There has been a vast improvement in photovoltaic (PV) technology in the last couple of decades; however, solar PV technology has a lot of drawbacks, including that it is intermittent, broadly dispersed, and non-transportable. Hence, cost-effective solar energy storage is critical for the widespread implementation of solar energy as the primary energy source. A novel slurry reactor design, for performing unassisted water-splitting to generate low-cost hydrogen with high solar-to-hydrogen (STH) efficiency, will be presented. The design includes employing two semiconductors in tandem, to achieve sufficient photovoltage to overcome the thermodynamic minimum of 1.23 V for water splitting at room temperature and the overpotential associated with hydrogen evolution reaction (~0.05V) and oxygen evolution reaction(~0.3V). As a proof-of-concept, tandem TiO₂/FTO/p⁺n-Si microwire photoelectrodes were fabricated demonstrating its ability to perform unassisted water splitting. These microwires were then detached from the substrate into a slurry. Ni was photoelectrodeposited at the Si base to prevent silicon oxidation, function as a hydrogen evolution catalyst, and be used as a ferromagnetic handle for magnetic alignment. Ongoing efforts to orient these microwires for minimizing spectral-mismatch and characterizing its impact on transmittance and hydrogen quantification will also be discussed.

Biomass valorization is an emerging method convert waste biomass into value-added feedstocks as an alternative to fossil-derived carbon chemicals. However, this technology has been limited by energy-intensity thermochemical reaction conditions. Alternatively, electrocatalytic biomass valorization is a sustainable and economical route that may utilize renewable electricity.

From a material design perspective, nickel and cobalt oxide derivatives have shown great potential in upgrading biomass, but the reaction still requires large overpotentials. Further, mechanistic knowledge is still lacking as the exact nature of the catalytically active sites is not fully understood. To this end, metal-organic frameworks are intriguing candidate catalyst systems as they exhibit high porosities and chemically well-defined active sites, enabling them to serve as ideal model systems for designing high efficiency catalysts. In this work, we present a MOF electrocatalyst model system featuring atomically precise Ni and Co active sites as a model system for electrochemical biomass valorization.

Synthesized via a simple solvothermal method, a MOF featuring square-planar nickel and cobalt metal ions coordinated the oxygen atoms of triphenylene li has been characterized by XRD, SEM, and TEM. Electrochemical analysis of this system reveals that the Ni and Co triphenylene MOFs surpass state-of-the-art metal oxide biomass valorization electrocatalysts in terms of onset potential and efficiency. This work opens avenues for understanding critical parameters en route to the design of next-generation biomass valorization electrocatalysts.

The interfacial charge transfer efficiency is the ratio between the charge transfer rate at the photo-electrode surface and charge carrier generation rate within the photo-electrode. It is an essential parameter not only for material performance assessment and benchmarking but also for the design and modelling of photo-electrochemical devices as it can be used to predict current densities at a given photon flux and electrode potential [1]. Many techniques for measuring interfacial charge transfer efficiencies have been proposed and further developed along the years [2-3], and the theoretical foundations have been discussed in detail for the techniques themselves and the phenomena involved.

Alas, the experimental considerations, reproducibility, accuracy, and the comparison of all these techniques have been left for consideration to each researcher, giving rise to widespread values and reported procedures that sometimes cannot be compared and reconciled with each other. The most widely reported techniques are photo-electrochemical impedance spectroscopy (PEIS), current density ratios in the presence and absence of sacrificial reagents (SR), transient photocurrents by chrono-amperometry (CA) after photon excitation, and more recently, intensity modulated photon current/voltage spectroscopy (IMPS, IMVS). From these, the charge transfer efficiency can be measured by using raw data (graphical approach), fitting to equivalent electrical circuits (EC) or by distribution of relaxation times (DRT) coupled with peak deconvolution. These techniques and data analysis approaches have their own drawbacks and impracticalities due to available hardware, quality of the data and the properties, such as nanostructure, of the photo-electrodes.

We will report the pitfalls, the accuracy and precision, and provide experimental recommendations for these techniques with a detailed comparison when employed for charge transfer efficiency determination in a stable Sn-doped hematite photo-electrode [4] for the oxygen evolution reaction. This will offer researchers in the field a pathway to select the best approach to measure this critical parameter. Findings indicate that CA offers the most reproducible values, while the use of sacrificial reagents is the least reproducible; however, these methods can only be used under specific conditions and the latter requires multiple measurements and absence of current doubling effects. IMPS coupled with EC offers reproducible and accurate values if photo-electrodes can be assessed under a wide range of light intensities and electrode potentials (rigorous approach); while PEIS coupled with DRT also offers reproducible, although underestimated, values with fewer experimental procedures and less specialised hardware.

Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. Specific reactions within this context that my research group is focused on are the reduction of CO2 and oxidation of waste biomass. Because the design of new catalytic systems is inherently linked to a precise understanding of how these reactions proceed on heterogeneous surfaces, we put considerable efforts in developing methodology for opernado probing with Raman spectroscopy CO2 reduction and biomass valorization. This talk will detail our efforts in the design, electrochemical characterization, and spectroscopic investigation of

1) Composite systems of metallic nanoparticles decorated with functional organic ligands with steer CO2 reduction reactions down a select pathway on their surface

2)  CO2 catalysis at the material-Metal Organic Framework (MOF) interface

3) Electrochemical oxidationof 5-hydroxymethylfurfural (HMF) on gold and transition metal oxide surface

In all, I show how using opernado Raman spectroscopy provides the mechanistic information on surface reaction mechanisms that enhance our understanding of functional hybrid interfaces and provides avenues for future materials design within the context of electrosynthesis of fuels and chemicals.