In Situ Identification and Time-Resolved Observation of the Interfacial State and Reactive Intermediates on a Cobalt Oxide Nanocatalyst for the Oxygen Evolution Reaction

Cobalt oxide (assigned as CoOx) is an efficient oxygen evolution reaction (OER) nanocatalyst, which has been extensively studied as a replacement to noble metal-based catalysts. The recent observations and understandings for the interfacial state, adsorbed intermediate products, and rate-determining steps (RDS) on CoOx, however, have remained elusive because of the dynamic transformation of different Co ions and the transient nature of the intermediates formed during the OER process. In this work, we propose that under the chosen experimental conditions, the redox process between Co(III) and Co(IV) species does not follow a proton-coupled electron transfer mechanism that is thought to be common prior to the OER, but it involves a proton-decoupled electron transfer, clarified by isotope labeling experiments and in situ electrostatic modulation. The interfacial state of CoOx is negatively charged prior to the formation of Co(IV)═O species. The theoretical concentration of the resulting Co(IV)═O species is approximately 0.1229 × 1019 cm–2. The Co(IV)═O species are demonstrated to directly regulate the OER performance. Moreover, we experimentally monitor the dynamic evolution behaviors of Co(IV)═O, Co(O)O–, OOH*, and O2–* intermediates during the OER with in situ time-resolved infrared spectroscopy, and the following elementary step OOH* + OH– → OO–* + H2O is likely to be the unexpected RDS in the OER process.