Octahedral-coordinated Co₃O₄ for water electrolysis in acid

The development of highly active and stable non-noble metal oxide catalysts to replace iridium-based materials for efficient acidic water electrolysis is crucial^1-3. However, traditional spinel cobalt oxide suffers from intrinsic performance limitations from coexistence of inactive tetrahedral (T_d) and highly active octahedral (O_h) coordination sites^4,5. Here we report a new trigonal-phase Co₃O₄ (Tri-Co₃O₄) produced by a vacuum-mediated molten-alkali mechanochemical method, which shows edge-shared [CoO₆] octahedral coordination with the space group P-3m1 (164). The three-layer compact structure provides Co²⁺ and Co³⁺ located in octahedral coordination in the ratio 1:2. Tri-Co₃O₄ achieves a low overpotential of 269 millivolts (mV) at the current density of 10 mA cm^-2 in the acidic oxygen evolution reaction (OER), 181 mV less than spinel-type Co₃O₄. It also achieves a current density exceeding 1,800 mA cm^-2 at a cell voltage of 1.80 V in proton-exchange membrane water electrolysis (PEMWE) devices. The catalytic mechanism shows that the 2D layered structure with edge-shared octahedral coordination can effectively optimize the adsorption of intermediates and reduce the dissolution of Co, thereby substantially improving the activity and stability of the non-noble metal catalysts.