Fine-tuning cobalt electronic states of CoOOH-based materials toward adsorbate evolution, and multi-pathway oxygen production.

Developing highly efficient CoOOH-based oxygen evolution catalysts requires the electronic state of cobalt to be fine-tuned in order to optimize the adsorption energies of key intermediates (including *O and *OOH) at the cobalt sites. However, this issue has not been sufficiently addressed in previous studies. A strategy for chemical modification followed by electrochemical reconstruction has been developed to synthesize Mo, Fe co-doped CoOOH anchored on Ni foam (Mo, Fe-CoOOH/NF), which acts as an efficient electrocatalyst in the oxygen evolution reaction (OER). Introducing Mo and Fe species into CoOOH precisely regulates the electronic states of cobalt, shifting from localization to enhanced delocalization. This is due to a substantial alteration of the cobalt atoms' chemical environment within CoOOH, which consequently promotes adsorbate evolution towards efficient OER. Our results reveal that, in addition to the adsorbate evolution mechanism (AEM), other multi-pathway oxygen production mechanisms exist in Mo, Fe-CoOOH/NF as well, including the oxide pathway mechanism (OPM) and lattice oxygen mechanism (LOM) based on the Co-Fe bimetallic active sites. Furthermore, the OPM might contribute more to Mo, Fe-CoOOH/NF than the LOM because the OPM is thermodynamically more favorable according to the results of the theoretical calculations. Lastly, introducing Mo and Fe species into CoOOH not only enhances the reactivity of Co sites by reducing the strength of the Co-O bond, but also facilitates electron transfer during the OER by narrowing the bandgap of CoOOH-based materials. Thanks to these advantages, the overpotential required for Mo, Fe-CoOOH/NF to achieve a current density of 300 mA cm-2 in 1.0 M KOH is just 245 mV vs. RHE, with good OER stability. This work has important applications in the fields of electrocatalysis and energy conversion.