Crystal-field modulation activates dormant tetrahedral Co sites for efficient alkaline hydrogen evolution

Water activation in the Volmer step is a major kinetic bottleneck in alkaline hydrogen evolution, yet the catalytic potential of many lattice sites remains underutilized. Here, we develop an oxalic-acid-assisted treatment to modulate the surface coordination environment of spinel Co3O4 while preserving its crystalline framework. The optimized U-Co3O4-2 electrode requires an overpotential of 19 mV to reach 10 mA cm-2 in 1.0 M KOH and sustains operation for at least 1000 h. Density functional theory indicates that coordination unsaturation reconstructs the local crystal field and selectively lowers dz2 occupation at tetrahedral Co sites. The resulting orbital redistribution reduces the population of Co–H2O antibonding states, strengthens Co–H2O coupling, and makes dissociative water activation thermodynamically more favorable. Thus, tetrahedral Co sites that are weakly active in pristine Co3O4 can be converted into catalytically competent water-activation centers. These results identify coordination-induced crystal-field modulation as a strategy for increasing lattice-site utilization in non-noble-metal alkaline HER catalysts.