Crystal\nFacet Regulation and Ru Incorporation of Co<sub>3</sub>O<sub>4</sub> for Acidic Oxygen Evolution Reaction Electrocatalysis

Acidic oxygen evolution reaction (OER) has long been\nthe bottleneck\nof proton exchange membrane water electrolysis. Ru- and Ir-based oxides\nare currently state-of-the-art electrocatalysts for acidic OER, but\ntheir high cost limits their widespread application. Co<sub>3</sub>O<sub>4</sub> is a promising alternative, yet the performance requires\nfurther improvement. Crystal facet engineering can effectively regulate\nthe kinetics of surface electrochemistry and thus enhance the OER\nperformance. However, the facet-dependent OER activity and corrosion\nbehavior of Co<sub>3</sub>O<sub>4</sub> have not been thoroughly\nstudied. In this study, we systematically investigated the OER performance\nand crystal facet dependency of Co<sub>3</sub>O<sub>4</sub>. The results\ndemonstrate that Co<sub>3</sub>O<sub>4</sub> with mixed {111} and\n{110} facets exhibits better OER activity and stability than Co<sub>3</sub>O<sub>4</sub> with {111} or {100} facets. The surface Co<sup>3+</sup> species are responsible for the high OER activity, but its\ntransformation to CoO<sub>2</sub> is also the root cause of the dissolution,\nleading to an activity–stability trade-off effect. The possible\napproach to addressing this issue would be to increase the Co<sup>3+</sup> contents by nanostructure engineering. To further improve\nthe performance, Ru is introduced to the best-performing Co<sub>3</sub>O<sub>4</sub>. The resulting Co<sub>3</sub>O<sub>4</sub>/RuO<sub>2</sub> heterostructure exhibits an overpotential of 257 mV at 10\nmA cm<sup>–2</sup> and can stably catalyze the OER for 100\nh.