Unraveling the Cobalt Oxidation State at the Surface of Epitaxial Cobalt Oxide Films during the Oxygen Evolution Reaction by Operando X-ray Absorption Spectroscopy/Surface X-ray Diffraction

Better understanding of the oxygen evolution reactionon cobaltoxides requires insights into the oxide-solution interfacestructure and composition under reaction conditions. We here present operando studies of electrodeposited epitaxial thin filmswith planar surface morphology that couple X-ray absorption spectroscopy,surface X-ray diffraction, and electrochemical measurements. Thisenabled us to disentangle bulk and surface contributions of the XASsignal and to correlate the cobalt oxidation state with the surfacestructure of cobalt oxide films. In the case of Co3O4(111) films, we show a one-to-one correlation between theCo oxidation state increase in the pre-OER potential range and thepotential-dependent thickness of the reversibly formed amorphous layeron the oxide surface. From this correlation, we conclude that thisamorphous layer is exclusively composed of Co3+. In thecase of CoOOH(001) films, we show that no such surface amorphizationtakes place and that the small oxidation state change with potentialmay be attributed to the progressive deprotonation of the surfaceCo-OH groups. For both oxides, the amount of Co4+ remainsbelow the detection limit.