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

Despite the current prominence of cobalt oxides as 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 absorption spectroscopy to investigate and discuss the previously unaddressed effect of different OH- concentrations in the range from 1 M to 0.016 M on the population of Co4+ in thin films of CoOx and its concomitant impact on their OER performance. Evidence for Co4+ is provided by in-situ X-ray absorption spectroscopy. Our UV-Vis absorption spectroscopic findings indicate that, not only can the overall redox conversion of Co be qualitatively monitored as a function of potential and OH- concentration, but also the formation of oxidized Co (i.e.,Co3+ and Co4+) 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 OH- concentrations above 1.2 V vs. RHE, which is consistent with voltammetric redox couples, indicating an enhancement in Co oxidation state and the consequent predominance of Co4+ under conditions of elevated OH- concentrations. Furthermore, the evolved oxygen due to OER does not depend on Co4+or OH- activity at 1.54 V vs. RHE, while a correlation with both Co4+ and OH- is identified at 1.59 V vs. RHE. 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 OH- concentrations and current densities.