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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 Co 4+ 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 Co 4+ in thin films of CoO x and its concomitant impact on their OER performance. Evidence for Co 4+ 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.,Co 3+ and Co 4+ ) 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 Co 4+ under conditions of elevated OH − concentrations. Furthermore, the evolved oxygen due to OER does not depend on Co 4+ or OH − activity at 1.54 V vs. RHE, while a correlation with both Co 4+ 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 Co 4+ in facilitating the alkaline OER at high OH − concentrations and current densities.
Du et al. (Sun,) studied this question.