In situ UV-Vis spectroscopy study of the water electrooxidation on cobalt oxide catalysts

Despite the current prominence of cobalt oxides as the state-of-the-art electrocatalysts for the alkaline oxygen evolution reaction (OER), there is a lack of unambiguous demonstration for the presence and the role of Co4+ prior to/during the OER. Here, we combine electrochemistry with in situ UV-Vis spectroscopy to investigate and discuss the previously unaddressed effect of different electrolyte pH in the range from pH 12.2 to pH 14.0 on the population of Co4+ in thin films of CoOx and its concomitant impact on their OER performance. Our UV-Vis spectroscopic findings indicate that, not only can the overall redox conversion of Co be qualitatively monitored as a function of potential and pH, but also the Co3+ oxidation assigned to a peak at 800 nm can be more quantitatively tracked in situ via stepped potential spectroelectrochemistry; with their optical signals becoming stronger at higher electrolyte pH within Co3+/ Co4+ conversion region, which is consistent with voltammetric observations that manifest heightened Co3+ oxidation current, indicating an enhancement in Co oxidation state and the consequent predominance of Co4+ under elevated pH conditions. Furthermore, the OER activity shows a positive correlation with Co4+ density (i.e., with electrolyte pH) in the potential region that is higher than 1.57 VRHE, leading to a non-zero OH- reaction order on RHE scale in the same region. This study not only provides spectral insight into the redox chemistry of Co at OER-relevant potentials but also highlights the importance of Co4+ in facilitating the alkaline OER at high pH and current densities.