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

Abstract

Highly active oxygen evolution reaction (OER) electrocatalystsbased on abundant and less expensive transition metal oxides mustbe developed for the commercialization and wide application of waterelectrolyzers in large-scale energy storage systems. Among the non-preciousmetal group OER catalysts, spinel-type cobalt oxide has attractedattention owing to its superior theoretical/empirical activity andstability at relatively low costs, and the substitution of cobaltions with other metal ions is also considered as a promising approachto improve the intrinsic activity of cobalt oxide. However, many studieshave not considered the exact geometrical site occupancy and oxidationstates of substituted metal ions. Therefore, the role and effect ofsubstituted metal ions are still unclear, and it is difficult to identifythe activity descriptor in OER, although such identification wouldbe extremely important to guide the design of a highly active non-preciousmetal group OER catalyst. Herein, we report the origin of the enhancedOER activities of cobalt-based spinel-type metal oxides with preciselycontrolled substitution sites and oxidation states. One of the Co3+ ions in the octahedral sites was selectively substitutedby Cr3+ and Mn3+ ions using the nanocastingmethod. The synthesized CrCo2O4 showed 5.4 timesenhanced electrocatalytic OER mass activity at 1.6 V-RHE compared to that of Co3O4, whereas MnCo2O4 showed mass activity similar to that of Co3O4. The more oxophilic property of Cr facilitatesthe adsorption of oxygen species on the surface, thereby increasingthe surface hydroxylation and reducing the charge-transfer resistance,leading to increased electrocatalytic OER activity.