Octahedral Co²⁺‐O‐Co³⁺ in Mixed Cobalt Spinel Promotes Active and Stable Acidic Oxygen Evolution

Abstract Cobalt (Co)‐based oxides show promising activity as precious metal‐free catalysts for the oxygen evolution reaction in proton exchange membrane water electrolysis, but the dissolution of Co has limited the durability of Co₃O₄ at industrially relevant current densities. This work demonstrates that cation in an octahedral coordination environment accounts for the oxygen evolution activity. Using a mixed inverse‐normal phase spinel Co_xGa_(3‐x)O₄ as a proof‐of‐concept example, the designed Co²⁺‐O‐Co³⁺ motifs in octahedral sites trigger oxygen evolution through a kinetically favorable radical coupling pathway. Furthermore, lattice oxygen exchange, a leading factor in catalyst structural degradation for normal Co₃O₄, is suppressed, as evidenced by isotopic labeling experiments and theoretical calculations. With the optimized catalyst, Co_1.8Ga_1.2O₄, an overpotential of 310 mV at 10 mA cm⁻² is reported, with stable operation at 200 mA cm⁻² for 200 h in a three‐electrode setup, and a proton exchange membrane electrolyzer operating at 200 mA cm⁻² for 450 h.