In situ UV–Vis absorption spectroscopy study of the water electrooxidation on cobalt oxide catalysts
Jia Du, Joaquín Morales‐Santelices, Omeshwari Yadorao Bisen, Denis Antipin, Dulce M. Morales, Marcel Risch
Abstract
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.