The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li2Co2O4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co3+ to Co4+ accompanied by spontaneous delithiation, whereas the edge-shared Co–O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co4+ site, rather than the Co3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity.
Voltage-and time-dependent valence state transition in cobalt oxide catalysts observed by in-operando soft-x ray spectroscopy during oxygen evolution reaction.pdf
