Metal-oxygen covalency engineering activates stable lattice oxygen in NiCo₂O₄ spinel for efficient oxygen evolution

Transition-metal spinels hold immense promise for the oxygen evolution reaction (OER), yet their inherently strong metal-oxygen bonds severely restrict the activation of lattice oxygen, locking the reaction into the kinetically sluggish adsorbate evolution mechanism (AEM). Herein, we propose a metal-oxygen covalency engineering strategy to modulate the octahedral sites of NiCo₂O₄ spinel by modifying the tailored Ru and Fe bimetallic ions. The preferential incorporation of high-valent Ru significantly enhances M-O bond covalency and upshifts the O 2p band center, lowering the thermodynamic barrier for the lattice oxygen mechanism (LOM) pathway. Concurrently, the synergistic Fe species functions as an electronic buffer that effectively suppresses metal dissolution and stabilizes the lattice framework, thereby preventing structural collapse. Impressively, the optimal RuFe-NiCo₂O₄ catalyst bypasses the inherent AEM scaling relationships, delivers an exceptionally low overpotential of 390 mV at 500 mA cm^-2 and retains excellent catalytic stability for 800 h in a prototype anion exchange membrane water electrolyzer (AEMWE). This work establishes a highly rational and feasible route for the design of next-generation, industrial-grade water splitting electrocatalysts.