The electrocatalytic reduction of CO₂ to produce sustainable fuels and chemicals is attracting great attention. Cu-based catalysts can lead to the production of a range of different molecules, and interestingly the product selectivity strongly depends on the preparation history, although it is not fully understood yet why. We report a novel strategy that allowed us to prepare Cu nanoparticle on carbon catalysts with similar morphologies, but prepared by in-situ reduction of either supported CuS, Cu₂S or CuO nanoparticles. For the first time the evolution of the Cu species was followed under CO₂ and H⁺ reduction conditions using in-situ X-ray absorption spectroscopy. Excellent electrochemical contact between the Cu-based nanoparticles, the carbon support and the carbon-paper substrate was observed, resulting in metallic Cu as the predominant phase under typical electrochemical CO₂ reduction conditions. Even covering less than 4% of the H₂ producing carbon support with Cu-sulfide derived nanoparticles allowed to steer the selectivity to a maximum of 12% Faradaic efficiency for the production of formate. Clear differences between the catalysts derived from CuS, Cu₂S or CuO nanoparticles were observed, which was ascribed to the presence of residual sulfur in the catalysts.

X-ray absorption spectroscopy (XAS) offers the unique possibility to study metal electrocatalysts such as silver and copper while they are performing electrochemical carbon dioxide (CO2) reduction. In this work, we present an approach to perform operando XAS experiments on an electrochemical cell performing CO2 reduction with a gas diffusion electrode (GDE) as cathode. The experimental set-up, advantages and drawbacks, XAS data analysis, and XAS theory are discussed. Results on copper and silver GDEs obtained through the presented procedures are then presented and discussed. Structural and compositional catalyst data acquired under operando conditions can help further density functional theory calculations, and catalytic, and systems studies on CO2 reduction. Structural and compositional data including crystallite size were obtained while performing high current density (up to 200 mA cm-2) CO2 reduction. On the silver catalysts at higher than 100 mA cm-2 applied current density, a Ag-X contribution was found and is ascribed to Ag-O. For both silver and copper, the XAS experiments revealed that the crystallite size of the ex situ samples is smaller than the samples during CO2 reduction. Furthermore, metal particle size polydispersity was found in the silver catalysts by comparing the obtained coordination numbers with theoretical values. The operando EXAFS data was of such high quality (k: 3-14 Å-1) that four shells could be fitted. The value of combining ex situ material characterisation and electrocatalyst performance data with operando XAS experiments is discussed and found to be of great importance to further CO2 reduction research.

Photocharging has recently been demonstrated as a powerful method to improve the photoelectrochemical water splitting performance of different metal oxide photoanodes, including BiVO₄. In this work, we use ambient-pressure X-ray Raman scattering (XRS) spectroscopy to study the surface electronic structure of photocharged BiVO₄. The O K edge spectrum was simulated using the finite difference method near-edge structure program package, which revealed a change in electron confinement and occupancy in the conduction band. These insights, combined with ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy analyses, reveal that a surface layer formed during photocharging creates a heterojunction with BiVO₄, leading to favorable band bending and strongly reduced surface recombination. The XRS images presented in this work exhibit good agreement with soft X-ray absorption near-edge structure spectra from the literature, demonstrating that XRS is a powerful tool to study the electronic and structural properties of light elements in semiconductors. Our findings provide direct evidence of the electronic modification of a metal oxide photoanode surface as a result of the adsorption of electrolyte anionic species under operating conditions. This work highlights that the surface adsorption of these electrolyte anionic species is likely present in most studies on metal oxide photoanodes and has serious implications for the photoelectrochemical performance analysis and fundamental understanding of these materials.

The field of electrochemical CO₂ conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO₂ have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO₂ are reviewed, including challenges associated with the detection of reaction intermediates.

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO₂ reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts-a nanostructured catalyst with improved CO₂ reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO₂ reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag⁰, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems.

The electrocatalytic reduction of CO₂ to chemical fuels has attracted significant attention in recent years. Among transition metals, silver shows one of the highest faradaic efficiencies for CO formation as the main reaction product; however, the exact mechanism for this conversion is not fully understood. In this work, we study the reaction mechanism of silver as a CO₂ reduction catalyst using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) during electrochemical cycling. Using ATR-FTIR, it is possible to observe the reaction intermediates on the surface of Ag thin films formed during the CO₂ electroreduction reaction. At a moderate overpotential, a proton coupled electron transfer reaction mechanism is confirmed to be the dominant CO₂ reduction pathway. However, at a more negative applied potential, both the COO^- and the COOH intermediates are detected using ATR-FTIR, which indicates that individual proton and electron transfer steps occur, offering a different pathway than at lower potentials. These results indicate that the CO₂ reduction reaction mechanism can be potential dependent and not always involving a concerted proton coupled electron transfer, opening alternative pathways to optimize efficient and selective catalysts for desired product formation. (Graph Presented).