Highly Efficient Oxygen Vacancy-Enriched MD-NiMoO₄: A Surface Reactivity Descriptor for Energy-Saving Green Hydrogen Generation

Herein, we report a simple mechanochemical dehydration (MD) method to synthesize MD-NiMoO 4, which is treated with NaBH 4 to generate oxygen vacancy-enriched MD-NiMoO 4 –SBH to understand the role of oxygen defects on water splitting performance. Among the as-prepared catalysts, MD-NiMoO 4 –SBH, MD-NiMoO 4, MD-NiO, and MD-MoO, MD-NiMoO 4 –SBH exhibits superior activity by demanding a small overpotential (OER/HER) of 241/108 mV with a Tafel slope of 97/85 mV dec –1 (MD-NiMoO 4: 293/156 mV; Tafel slope: 114/93 mV dec –1 ) and is stable for ∼100 h. The activation energies were calculated for electrolysis using MD-NiMoO 4 –SBH (32 kJ/mol) and MD-NiMoO 4 (28 kJ/mol). The calculated integrated areas of 5.41 × 10 –5 AV (MD-NiMoO 4 –SBH) and 3.61 × 10 –5 AV (MD-NiMoO 4 ) confirmed MOOH* formation, and the number of active sites was found to be 3.484 × 10 –16 (MD-NiMoO 4 ) and 6.540 × 10 –16 (MD-NiMoO 4 –SBH). The superaerophobicity of MD-NiMoO 4 –SBH was corroborated by rapid gas bubble growth as well as fast evolution from the catalyst surface. The bifunctional electrolyzer with MD-NiMoO 4 –SBH (1.48 V) and MD-NiMoO 4 (1.59 V) released vigorous gas bubbles in the alkaline electrolyte and exhibits robust durability, which could be employed in water splitting devices for practical utility. As observed from the electrochemical results, the MO theory reveals that the band gap state-assisted antibonding state formation, having low electron occupancy to favor OER and HER kinetics.