A profound understanding of the solid/liquid interface is central in electrochemistry and electrocatalysis, as the interfacial properties ultimately determine the electro-reactivity of a system. Although numerous electrochemical methods exist to characterize this interface under operating conditions, tools for the in-situ observation of the surface chemistry, that is, chemical composition and oxidation state, are still scarce, and currently exclusively available at synchrotron facilities. Here, we demonstrate the ability of laboratory-based near-ambient pressure X-ray photoelectron spectroscopy to track changes in oxidation states in-situ with respect to the applied potential. In this proof-of-principle study with polycrystalline gold (Au) as the best-studied electrochemical standard, we show that during the oxygen evolution reaction (OER), at high OER overpotentials, Au3+ governs the interfacial chemistry, while, at lower overpotentials, Au+ dominates.
