Oxophilicity Induced\nSurface Hydroxylation to Promote\nOxygen Evolution in Selectively Substituted Spinel-Type Cobalt Oxides

Abstract

Highly active oxygen evolution reaction (OER) electrocatalysts\nbased on abundant and less expensive transition metal oxides must\nbe developed for the commercialization and wide application of water\nelectrolyzers in large-scale energy storage systems. Among the non-precious\nmetal group OER catalysts, spinel-type cobalt oxide has attracted\nattention owing to its superior theoretical/empirical activity and\nstability at relatively low costs, and the substitution of cobalt\nions with other metal ions is also considered as a promising approach\nto improve the intrinsic activity of cobalt oxide. However, many studies\nhave not considered the exact geometrical site occupancy and oxidation\nstates of substituted metal ions. Therefore, the role and effect of\nsubstituted metal ions are still unclear, and it is difficult to identify\nthe activity descriptor in OER, although such identification would\nbe extremely important to guide the design of a highly active non-precious\nmetal group OER catalyst. Herein, we report the origin of the enhanced\nOER activities of cobalt-based spinel-type metal oxides with precisely\ncontrolled substitution sites and oxidation states. One of the Co³⁺ ions in the octahedral sites was selectively substituted\nby Cr³⁺ and Mn³⁺ ions using the nanocasting\nmethod. The synthesized CrCo₂O₄ showed 5.4 times\nenhanced 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 facilitates\nthe adsorption of oxygen species on the surface, thereby increasing\nthe surface hydroxylation and reducing the charge-transfer resistance,\nleading to increased electrocatalytic OER activity.