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 reaction on cobalt oxides requires insights into the oxide-solution interface structure and composition under reaction conditions. We here present operando studies of electrodeposited epitaxial thin films with planar surface morphology that couple X-ray absorption spectroscopy, surface X-ray diffraction, and electrochemical measurements. This enabled us to disentangle bulk and surface contributions of the XAS signal and to correlate the cobalt oxidation state with the surface structure of cobalt oxide films. In the case of Co₃O₄(111) films, we show a one-to-one correlation between the Co oxidation state increase in the pre-OER potential range and the potential-dependent thickness of the reversibly formed amorphous layer on the oxide surface. From this correlation, we conclude that this amorphous layer is exclusively composed of Co³⁺. In the case of CoOOH(001) films, we show that no such surface amorphization takes place and that the small oxidation state change with potential may be attributed to the progressive deprotonation of the surface Co-OH groups. For both oxides, the amount of Co⁴⁺ remains below the detection limit.