Construction of fully octahedral-coordinated Co3O4 for efficient acidic water electrolysis
Proton exchange membrane water electrolysis (PEMWE) enables green hydrogen production from renewable electricity but relies on scarce Ir/Ru catalysts for the kinetically sluggish and acid-stable oxygen evolution reaction (OER). Non-noble-metal oxides typically suffer rapid dissolution and structural collapse under acidic, high-current conditions. Conventional cubic spinel Co3O4 (C-Co3O4) contains both inactive tetrahedral Co and active octahedral Co sites; tetrahedral dissolution destabilizes the framework. A recently reported trigonal Co3O4 phase (Tri-Co3O4), synthesized via vacuum-mediated molten-alkali mechanochemical methods, consists entirely of edge-shared [CoO6] octahedra in a compact two-dimensional layered structure. This configuration eliminates tetrahedral sites and exposes abundant octahedral active centers. Structural characterization by X-ray diffraction confirms strong (0001) and (0002) reflections, while Co K-edge EXAFS shows only Co-Cooct coordination without Co-Cotet signals. Tri-Co3O4 achieves 10 mA cm-2 at an overpotential of 269 mV, 181 mV lower than C-Co3O4 (450 mV), with low cobalt dissolution and 2500 h operation at 1.7 V in a practical PEMWE device. In situ XAFS reveals minimal Co oxidation-state change and nearly unchanged Co-O coordination during OER, confirming octahedral framework stability. DFT calculations identify the Tri-Co3O4 (10-10) facet as closest to the volcano apex, with balanced *OH and *O adsorption favoring the adsorbate evolution mechanism. Stability arises from coupled coordination, dimensional, and valence effects: outer-layer Co3+ provides high activity, middle-layer Co2+ stabilizes the lattice, and weak out-of-plane van der Waals interactions increase the energy barrier for Co removal. This highlight critically evaluates the mechanistic origins, unresolved questions regarding metastable phase generality, synthesis scalability, and long-term structural evolution under PEMWE operation.