Covalency competition in Ni-O-Mo bridging sites with enhanced Lewis acidity for efficient alkaline hydrogen evolution

The inherently slow kinetics of water dissociation during the alkaline hydrogen evolution reaction (HER) severely restrict the performance of non-noble metal catalysts. Herein, a NiMoO₄ catalyst featuring electron-deficient Lewis acidic Ni sites, induced by asymmetric covalency competition in the Ni-O-Mo bridging structure, is developed to promote the adsorption, activation, and dissociation of water in alkaline HER. Experiments and calculations confirm that the stronger Mo-O covalency versus Ni-O in NiMoO₄ drives electron redistribution from Ni to Mo via the bridging oxygen, rendering the Ni sites electron-deficient and significantly enhancing their Lewis acidity. With their vacant d orbitals, the Ni sites with enhanced Lewis acidity are capable of accepting lone-pair electrons from the oxygen atom of water molecule, thereby strengthening the adsorption and enrichment of interfacial H₂O. This coordination simultaneously promotes the conversion and activation of interfacial water and weakens the H-OH bond, significantly lowering the water dissociation free energy. Therefore, NiMoO₄ delivers a low overpotential of 317 mV at 100 mA cm^-2, and the assembled electrolyzer requires only 1.79 V to achieve 500 mA cm^-2 while maintaining robust durability for over 500 h. This work establishes a covalency competition mechanism to modulate Lewis acidity in bimetallic oxide bridging structures, providing an effective strategy for developing efficient electrocatalysts for alkaline HER.