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.

M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science, 2009, 324, 71-74.

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

H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Appl. Catal., B, 2005, 56, 9-3

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].