Octahedral Co2+-O-Co3+ in mixed cobalt spinel promotes active and stable acidic oxygen evolution

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, we report an overpotential of 310 mV at 10 mA/cm², stable operation at 200 mA/cm² for 200 hours in a three-electrode setup, and a proton exchange membrane electrolyzer operating at 200 mA/cm² for 450 hours.