Oxophilicity Induced Surface Hydroxylation to Promote Oxygen Evolution in Selectively Substituted Spinel-Type Cobalt Oxides

Abstract

Highly active oxygen evolution reaction (OER) electrocatalysts based on abundant and less expensive transition metal oxides must be developed for the commercialization and wide application of water electrolyzers in large-scale energy storage systems. Among the non-precious metal group OER catalysts, spinel-type cobalt oxide has attracted attention owing to its superior theoretical/empirical activity and stability at relatively low costs, and the substitution of cobalt ions with other metal ions is also considered as a promising approach to improve the intrinsic activity of cobalt oxide. However, many studies have not considered the exact geometrical site occupancy and oxidation states of substituted metal ions. Therefore, the role and effect of substituted metal ions are still unclear, and it is difficult to identify the activity descriptor in OER, although such identification would be extremely important to guide the design of a highly active non-precious metal group OER catalyst. Herein, we report the origin of the enhanced OER activities of cobalt-based spinel-type metal oxides with precisely controlled substitution sites and oxidation states. One of the Co3+ ions in the octahedral sites was selectively substituted by Cr3+ and Mn3+ ions using the nanocasting method. The synthesized CrCo2O4 showed 5.4 times enhanced electrocatalytic OER mass activity at 1.6 VRHE compared to that of Co3O4, whereas MnCo2O4 showed mass activity similar to that of Co3O4. The more oxophilic property of Cr facilitates the adsorption of oxygen species on the surface, thereby increasing the surface hydroxylation and reducing the charge-transfer resistance, leading to increased electrocatalytic OER activity.