An Ultrathin Hydroxide Film Governs the Stability and Reactivity of Gold for the Oxygen Evolution Reaction.

Surface oxidation of noble metals under electrochemical conditions has a decisive impact on the performance of electrocatalysts for electrolyzers and fuel cells. However, the experimental detection of such oxides on well-defined model electrodes under operando conditions is a major challenge, limiting our ability to establish clear structure-performance relationships. Using surface-sensitive total-reflection X-ray absorption spectroscopy in real time during cyclic voltammetry, combined with inductively coupled plasma mass spectrometry dissolution measurements, we demonstrate that an ultrathin (~5 Å) hydroxide film forms on the gold (111) surface prior to the onset of the oxygen evolution reaction. This film, composed of Au3+ in similar coordination as in the bulk oxyhydroxide, thus governs its catalytic properties, dissolution, and restructuring. The results highlight the importance of surface oxidation in water splitting catalysis even for Au(111), the most inert surface of the most inert noble metal.