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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 Li 2 Co 2 O 4  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 Co 3+  to Co 4+  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 Co 4+  site, rather than the Co 3+  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 during the oxygen evolution reaction