Recently molecular photovoltaics, such as dye sensitized cells (DSCs) and perovskite solar cells (PSCs) have emerged as credible contenders to conventional p-n junction photovoltaics. Their certified power conversion efficiency currently attains 25.5 %, exceeding that of the market leader polycrystalline silicon. This lecture covers the genesis and recent evolution of DSCs and PSCs, describing their operational principles and current performance. DSCs have meanwhile found commercial applications for ambient light harvesting and glazing producing electric power from sunlight. The scale up and pilot production of PSCs are progressing rapidly but there remain challenges that still need to be met to implement PSCs on a large commercial scale. PSCs can produce high photovoltages rendering them attractive for applications in tandem cells, e.g. with silicon and as power source for the generation of fuels from sunlight. Examples to be presented are the solar generation of hydrogen from water and the conversion of CO2 to chemical feedstocks such as ethylene, mimicking natural photosynthesis.

The electrochemical CO₂ reduction (CO₂RR) driven by electrical energy from renewable sources has attracted attention as an environmentally friendly path to convert the undesired greenhouse gas into feedstock chemicals and fuels. Among the metal catalysts used for CO₂RR, copper-based catalysts are of particular interest due to their unique capability to transform CO₂ into various hydrocarbons and alcohols with high energy density. However, controlling the selectivity toward a specific product remains a challenge in this field. Interestingly, some of the critical parameters affecting the catalyst’s properties, including its structure and oxidation state, as well as the local pH near the active sites, may be tuned by applying potential pulses during CO₂RR.

In this talk, I will discuss the impact of applying potential pulses to well-defined Cu-based materials (e.g. Cu(100), Cu₂O nanocubes). In particular, I will establish structure/composition-selectivity correlations based on in depth in situ/operando microscopic (EC-AFM, EC-TEM), spectroscopic (XAS, XPS) and diffraction (XRD) characterization under realistic reaction conditions, including high current densities in flow cell electrolyzers. Thus, this work is expected to set up the ground for steering catalyst selectivity through dynamically-controlled structural and chemical transformations.

Beyond surface reaction energetics, the structure and composition of the electric double layer exerts an influence on the activity and selectivity of electrochemical reactions. These phenomena often manifest themselves as so-called “pH effects”, i.e. deviations in the dependence of activity on a potential vs. RHE scale. In this talk, I first discuss how adsorbate dipole-field interactions give rise to higher activity for CO2R in alkaline conditions, and how these interactions are tuned by cations and in supported single site catalysts. I then discuss the impact of solution phase reactions and mass transport on the activity and selectivity towards acetate on nanostructured Cu at various pH. I discuss the implications of the mechanistic insights on catalyst design.

A very important parameter for the electrocatalytic performance of a material is its potential of zero free charge (pzfc). For Cu(111) at pH 13 it was identified at −0.73 V_SHE in the apparent double layer region.^1,2 It shifts by (88 ± 4) mV to more positive potentials per decreasing pH unit.² At the pzfc, Cu(111) starts to restructure. At higher potentials, full reconstruction and electric field dependent OH adsorption occur, causing a remarkable decrease in the atomic density of the first Cu layer.²

It is this restructuring that leads to the ability of Cu(111) to efficiently oxidize CO and to reduce water. Therefore, knowledge of the surface structure and the position of the pzfc is of paramount importance for the understanding of copper’s catalytic properties and for the rational design of electrocatalysts, in general.

CO is a key intermediate in the electro-oxidation of energy carrying fuels and known to act as a catalyst poison. Single-crystal Cu(111) model catalysts can efficiently electro-oxidize CO in alkaline media,³ where strong surface structural changes are observed under reaction conditions with electrochemical scanning tunneling microscopy (EC-STM). Supported by first-principles microkinetic modelling, we have shown that the concomitant presence of high-energy undercoordinated Cu structures at the surface is a prerequisite for the high activity. In water electrolyzers, it is possible to produce H₂ in the course of the hydrogen evolution reaction (HER), which was studied with Ni(OH)₂ modified Cu(111) electrodes in alkaline media.⁴ Strong morphological changes upon adatom modification lead to a significant HER rate enhancement. Intriguingly, this is induced through a decrease of the electric field strength negative of the potential of zero charge. This implies an easier reorganization of the interfacial water molecules facilitating charge transfer through the double layer, and thus enhancing the efficiency of electrocatalytic reactions.

References

[1] P. Sebastián-Pascual, F.J. Sarabia, V. Climent, J. Feliu, M. Escudero-Escribano, J. Phys. Chem. C 124 (2020) 23253.

[2] A. Auer, X. Ding, A. S. Bandarenka, J. Kunze-Liebhäuser, J. Phys. Chem. C 125 (2021) 5020.

[3] A. Auer, M. Andersen, E.M. Wernig, N.G. Hörmann, N. Buller, K. Reuter, J. Kunze-Liebhäuser, Nat. Catal. 3 (2020) 797.

[4] A. Auer, F.J. Sarabia, D. Winkler, C. Griesser, V. Climent, J.M. Feliu, J. Kunze-Liebhäuser, ACS Catal. 11 (2021) 10324.

The electroreduction reaction of CO2 (eCO2RR) into hydrocarbons over copper electrodes has been studied extensively for the past few decades.[1] However, the eCO2RR mechanism, the activation of CO2 and the exact surface structure of the copper electrode are still heavily debated.[2] Raman spectroscopy is suitable for in situ characterization of CO2RR mechanisms, but the low signal intensity and resulting poor time resolution (often up to minutes) hampers the application of conventional Raman spectroscopy for the study of the reaction dynamics, which requires sub-second time resolution. In this presentation, I will discuss the development of time-resolved Raman spectroscopy (TR-Raman) with (sub) second time resolution, which allows us to investigate the electrocatalytic activation of CO2and the dynamic chemical structure of the electrode surface.[3] By subsequently measuring different Raman regions during a cyclic voltammetry scan, we are able to collect a broad range of vibrational modes related to adsorbed species (i.e. carbonate & CO) or the active surface (i.e. copper oxide and hydroxide) with sub-second time resolution. Our TR-Raman measurements reveal that the electrode surface is dynamic in the first seven seconds of cathodic bias onset due to stripping of surface Cu oxide species, after which the surface stabilizes as copper hydroxide due to local alkaline conditions.[3] After that, dynamic coupling of surface-bound carbon monoxide (CO) intermediates into ethylene is observed at an applied potential of -0.9 V vs. RHE, whereas lower cathodic bias (−0.7 V vs. RHE) results in gaseous CO production from isolated and static CO surface species. Furthermore, we observe the potential- and time-dependent adsorption of carbonate and bicarbonate electrolyte ions, which is a measure for variations in local alkaline conditions. Our combined electrochemical and TR-Raman measurements provide detailed information about the chemical structure of and the chemical landscape at the copper surface under reaction conditions.

Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. To this end, research group develops heterogeneous catalysts (e.g. nanomaterials, MOFs) geared for the reduction of CO₂ and oxidation of biomass platforms to fuels and value-added chemicals. Further, we establish strategies in emergent areas of electrocatalysis such as electrochemically forming and cleaving C-N bonds. 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 operando probing of these systems with vibrational spectroscopy. In all, I show how using these experiments provides key mechanistic information on surface reaction mechanisms that enhance our understanding of functional interfaces and how the research provides avenues for future materials design within the context of renewable energy electrocatalysis.

A range of transition metal-based materials such as metal chalcogenides, carbides, and phosphides as well as their alloys has emerged as earth-abundant electrocatalysts alternative to Pt for HER in either acidic or alkaline in recent years.[1,2,3] Despite the high electrochemical activity obtained with these types of materials, a fundamental understanding of the “real” active sites and the reaction mechanism is usually a challenge due to their characteristic high reactivity,[4] which makes it difficult to rely on ex-situ characterisation to elucidate the species and structure responsible for the performance.

We have been recently working on series of transition metal sulphides and selenides which exhibit high HER activity. Interestingly, we often observed that these materials also require a period of activation at the beginning of the electrochemical test in order to achieve high HER activity. In this contribution, we will discuss the activation process of α-NiS electrodes during the alkaline HER process. Based on operando X-ray absorption and Raman spectroscopies on α-NiS catalyst, we elucidate a complete phase transition from α-NiS to a mixed phase of Ni₃S₂ and NiO during HER operation. The synergistic effect of Ni₃S₂/NiO boosts a highly active HER with an overpotential of 85 for 10 mA cm^-2 current density. Combined NAP-XPS and density functional theory (DFT) calculation reveal that Ni in NiO was identified as sites for OH* adsorption and S in Ni₃S₂ as sites for H* adsorption. The phase transition identified in our work highlights the significance of identifying the intrinsic active sites under realistic reaction conditions. Therefore, our study opens a new avenue for designing transition metal sulfide catalysts via the promotion of a phase transition with high activity.

We will present the unique potential and effectiveness of rapid thermal processing (RTP) for overcoming two significant challenges in the development of oxide thin-film photoelectrodes. The first challenge is the need to bypass the normal temperature limit for the most commonly used glass-based F:SnO₂ substrates (FTO, ~ 550 °C) in photoelectrochemistry and photovoltaics research. This limit is a problem since many multinary metal oxides require high processing temperatures (to at least ≈ 850 – 1000 °C in ambient pressure) to obtain the desired high density, high crystallinity, and low defect density.1 The second challenge is synthesizing multinary metal oxides that require a narrow range of optimal processing conditions (e.g., temperatures, heating times, and gas environment) in order to avoid undesired phase transformations and impurities segregation. We will show that RTP can overcome the first challenge by flash-heating Ta₂O₅, TiO₂, and WO₃ photoelectrodes to 850 °C without damaging the FTO, and the second challenge with the fabrication of ternary α-SnWO₄, which recently attracted attention as a potential candidate for solar water oxidation. In both established and promising materials, heating by RTP resulted in superior crystallinity, electronic properties, and performance when compared with conventional furnace heating of similar photoelectrodes, culminating in a new performance record for α-SnWO₄ for sulfite oxidation.

For photoelectrochemical energy conversion, metal nitride semiconductors have the potential to overcome several limitations associated with the more intensively investigated class of metal oxides. Among these materials, Ta₃N₅ is especially promising. However, it is commonly synthesized by nitridation of Ta₂O₅ films in ammonia atmosphere at high temperatures, which results in high concentrations of residual oxygen, nitrogen vacancies, and low-valent Ta cations within the Ta₃N₅ lattice. These defects often dominate the (opto)electronic properties of Ta₃N₅ photoelectrodes, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport mechanisms. Here, we deposit tantalum nitride thin films by reactive magnetron sputtering and explore the role of subsequent NH₃ annealing.[1] This synthesis process leads to thin films with near-ideal stoichiometry, as well as significantly reduced native defect and oxygen impurity concentrations compared to the commonly used nitridation of Ta₂O₅. By analyzing structural, optical, and photoelectrochemical properties as a function of NH₃ annealing temperature, we provide new insights into the basic semiconductor properties of Ta₃N₅, as well as the role of defects on its optoelectronic characteristics. For example, the high material quality enables us to unambiguously identify the nature of the Ta₃N₅ bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. Improved understanding of not only the basic properties of this material, but also of how defect concentrations can be optimized, provides a path to high efficiency photoelectrodes.

Heterogeneous electrochemical processes, including photoelectrochemical water splitting to evolve hydrogen using electrocatalyst-coated semiconductors, are driven by the accumulation of charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. Conventional electrochemical techniques measure/control potentials at the conductive substrate or semiconductor ohmic contact, but are unable to isolate processes and electrochemical potentials at the surface during operation. I will present our recent work demonstrating that the nanoelectrode tip of an atomic-force-microscope cantilever can effectively sense the surface electrochemical potential of electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe₂O₃ and BiVO₄. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. Recent results on Si, InP, and oxide based electrocatalyzed semiconductors will be discussed.

Perovskite oxides (ABO₃) are highly active for the oxygen evolution reaction (OER). Activity is observed to correlate with changes in bulk electronic structure parameters, such as metal-oxygen covalency and transition metal oxidation state. We employ spectroscopic approaches to consider such descriptors at the surface of perovskite oxides in situ, as well as their implications on the formation of surface adsorbates proposed to act as reaction intermediates during the OER.

Many surface science techniques, such as X-ray photoelectron spectroscopy (XPS), collect information from inherently surface-sensitive low-energy processes, requiring operation in ultrahigh vacuum. This constraint is lifted for ambient pressure XPS, which can probe the surface in equilibrium with the gas phase at pressures up to ~a few Torr, or with thin liquid layers using a higher incident photon energy. This presentation will discuss the insights obtained with this technique regarding the electronic structure of oxide electrocatalysts in an oxidizing or humid environment, as well as the reaction intermediates of relevance to electrocatalysis.¹ We will then extend the technique to probe electrocatalysts in operando,² driving current through a thin layer of liquid electrolyte and employing a tender X-ray source.

This paper will focus on two key areas in solar fuels: (1) the development and application of advanced in-situ methods to achieve new fundamental insights, and (2) the design and development of new catalysts and chemical processes driven by renewables.

There are many challenges in the synthesis of solar fuels. This paper will describe efforts towards two reactions of interest, the hydrogen evolution reaction (HER) and the CO₂ reduction reaction (CO₂RR). First, we will cover research activities aimed at developing synchrotron-based methods aimed at shedding light onto catalysts that drive these important reactions, including Pd and Cu. Next, we will discuss efforts to design and develop advanced catalysts and their translation towards devices and technology in the solar fuels sector. The goal is ultimately to develop high performance systems with the desired efficiency, selectivity, and reaction rates for new technologies that can contribute to the fuels and chemicals sector. The role of theory and experiment collaboration will be highlighted.

Many industrial chemical processes involve a high-energy demand (often still derived from fossil fuels), toxic reactants, and the production of high amounts of waste. Therefore, the development of more efficient, less hazardous technologies, based on renewable energies, has become one of the most challenging topics for chemical synthesis. For achieving these goals, the combination of catalysis with electrochemical methods, that is, electrocatalysis, can play a very important role [1]. With electrochemical methods, toxic and dangerous chemicals can be replaced with clean electrons, the efficiency and selectivity of the reactions can be tuned by choosing the applied potential, and more importantly, the energy used can come from renewable sources like wind or solar.

In this talk, I will focus on how electrocatalysis can help on the knowledge generation, reaction improvement and development of alternative industrial processes. I will give examples from studies of the electrosynthesis of high value chemicals such as organic carbonates, fuels and energy dense carriers.

Ambient pressure X-ray photoelectron spectroscopy is a valuable tool for investigating surfaces and interfaces in elevated pressure conditions. We have recently constructed and commissioned a new end-station dedicated to Spectroscopic Analysis with Tender X-rays (SpAnTeX).  The SpAnTeX end-station focuses on X-ray photoelectron spectroscopy measurements of solid-liquid interfaces. It is able to operate at pressures up to 30 mbar and photoelectron kinetic energies up to 10 keV. At the heart of the SpAnTeX end-station is a SPECS PHOIBOS 150 HV NAP electron spectrometer. This new spectrometer contains two additional features that allow for measurements with lateral resolution better than 30 μm and time resolved measurements with 100 ns or less time resolution. The SpAnTeX end-station is based on a modular concept which allows for the rapid exchange of sample environment modules. To date, we have constructed two modules. One module, the dip-and-pull module, is used for investigating solid-liquid interfaces under applied bias and illumination. The second module incorporates a droplet train which facilitates investigation of liquid phase processes with time resolution ranging from the μs to ms regimes. After a technical introduction to the SpAnTeX end-station and the experimental modules, results obtained using SpAnTeX will be presented. We will present results for Electrochemically Mediated Amine Regeneration (EMAR) for CO2 capture and sequestration processes, and light induced changes at the bismuth vanadate- and silicon-aqueous electrolyte interfaces.

The kinetics of electrochemical reactions are typically analysed through Butler-Volmer analyses of current – voltage data. Such analyses have been very effective at determining electrochemical kinetics on metal electrodes. However their application to the kinetics of (photo)electrocatalytic water oxidation / reduction on metal oxides can be more challenging, due to multiple redox transitions observed in such metal oxides, the localised nature of these transitions and the complexity of the water oxidation / reduction reactions. In my talk I will address the potential of operando spectrochemistry to determine redox state population densities in metal oxides electrodes and photoelectrodes, and the use of such data to undertake rate law analyses of water oxidation / reduction. These studies will primarily be applied to Ni/Fe oxyhydroxide electrocatalysts and hematite photoanodes for water oxidation, as well as comparison with other metal oxide for both water oxidation and reduction. These studies will address the nature of the states driving water oxidation / reduction and the reaction kinetics and dependence upon population density. For example for Ni(M)OOH electrocatalysts, these studies will address the impact of metal (M) substitution on both the population densities and reaction rate constants, and how these together impact upon the overall current / voltage behaviour. A key conclusion of my talk will be that for the systems studied the kinetics of water / oxidation appear to be primarily driven by the population of states driving these reactions – and as such it more appropriate to employ rate law rather than Butler-Volmer models in analysing these kinetics. 

Systematic computational materials modeling strategies using first-principles methods allow one to describe and understand chemistries of already known materials, and, importantly, they can be used to predict new materials through a careful analysis of the surface chemistry at the atomic level. In this talk, I demonstrate how we have been able to computationally predict oxide catalyst materials for the oxygen evolution reaction (OER) — that have been experimentally synthesized, characterized and tested. I then share recent theoretical insights on the stability-activity conundrum of oxide materials and present a computationally predicted nanostructured OER catalyst-systems that exhibit a new surface activation phenomenon leveraging our theoretical understanding. I conclude with some of our efforts to understand more complex nanostructured oxide materials.

Catalytic mechanisms at electrode surfaces guide the development of electrochemically-controlled energy storing reactions and chemical synthesis. The intermediate steps of these mechanisms are challenging to identify experimentally, but are critical to understanding the speed, stability, and selectivity of product evolution.  In the laboratory group, we employ photo-triggered vibrational and electronic spectroscopy to time-resolve the catalytic cycle at a surface, identifying meta-stable intermediates and critical transition states which connect one to another.  The focus is on the highly selective water oxidation reaction at the semiconductor (SrTiO₃)-aqueous interface, triggered by an ultrafast light pulse in an electrochemical cell.  Here, I will summarize the work done to date by the group: the structure and kinetics of forming the initial intermediates that trap charge (Ti-OH^*) through the next event at microseconds, suggested to be the formation of the first O-O bond of O₂ evolution.  There will be a focus on how time-resolving the intermediates leads to experimental identification of theoretical descriptors of oxygen evolution, one of which is the free energy difference to create the first meta-stable intermediate (DG₁(OH^*)).  In so doing, reaction conditions that shift equilibria become an important, independent axis to the time & energy axes of the spectroscopy.  While many open questions remain, these experiments provide and benchmark the opportunity to quantify intermediates at an electrode surface and follow a heterogeneous catalytic cycle in time. 

Electrified interfaces, such as electrodes for electrochemical CO₂ reduction, can change their morphology as a reaction progresses and when the potential is changed. Likewise, the species in the electrolyte can change significantly while a reaction occurs. This makes operando probing important for understanding how reactions occur as these surfaces. Here, we have used electrochemical atomic force microscopy (EC-AFM) [1] and scanning electrochemical microscopy (AFM-SECM) to probe the cathode topography and the gradient in electrolyte pH near the cathode. I will discuss the experimental techniques, as well as the implications of the results on electrochemical CO₂R reactors. I will focus on solvent (H₂O) and solute (CO₂(aq), CO(aq), OH^-) activities [2], the nature of the triple-phase region in gas-diffusion electrodes (GDEs) [3], and how to optimize GDEs for selective and high current density CO₂R.