Surface chemistry of model electrodes under electrocatalytic conditions : Developing Total Reflection X-ray Absorption Spectroscopy for operando studies

Electrochemical reactions like water-splitting or corrosion initiate at electrode-electrolyte interface, and in some cases can also be closely linked. In particular, the oxygen evolution reaction (OER) in electrolyzers and the oxygen reduction reaction (ORR) in fuel cells are not only sluggish but can also lead to severe catalyst degradation. A fundamental understanding of these processes is therefore essential for developing more stable and efficient electrocatalysts for green hydrogen technologies. However, probing the structure and chemistry of the electrode surface remains challenging, because these species are ultrathin, amorphous, unstable and buried beneath a thick dense electrolyte.
This thesis applies a range of ex situ, in situ and operando methods based on synchrotron X-rays, electrons and visible light to understand the surface chemistry of gold, platinum and nickel alloy 59 model electrodes under electrocatalytic conditions. Emphasis is placed on developing total reflection X-ray absorption spectroscopy (RefleXAFS) in real operando conditions, which is an underexplored approach. The main model system studied in this thesis is gold, used to compare all the techniques and to understand its stability and surface evolution during OER. Nickel alloy 59, a corrosion-resistant material catalytically active towards OER, serves especially as a secondary model system to validate and further investigate the capabilities of RefleXAFS. Finally, the role of electrochemical oxidation of platinum during ORR is investigated.
Overall, this work shows that ultrathin (hydr)oxide films control the stability and activity of noble metals and corrosion-resistant alloys under electrocatalytic conditions. These findings highlight the link between surface oxidation, electrocatalysis and metal dissolution, and underscore the necessity of operando studies and multimodal approaches to understand electrochemical interfaces.