Unraveling the Origin of Exceptional Activity in NiMo Alloys for Alkaline Hydrogen Evolution

Nickel-molybdenum (NiMo) alloys show benchmark alkaline hydrogen evolution reaction (HER) activity, yet the active phase and mechanism remain debated. Here, we resolve this ambiguity by combining density functional theory with surface Pourbaix diagrams to model the catalyst under realistic operando conditions. We find that catalyst surfaces are reconstructed by oxygen and that a clear synergistic mechanism emerges: Mo sites catalyze the rate‑determining Volmer step (water dissociation), while adjacent Ni sites provide near-optimal binding for hydrogen evolution. This synergy is most pronounced on the O-covered Ni₃Mo(111) facet, which exhibits a low water dissociation barrier (ΔG_a = 0.65 eV) and near-thermoneutral hydrogen adsorption (ΔG_H = -0.01 eV), explaining its superior performance. Furthermore, our microkinetic model quantitatively validates this mechanism by predicting an exchange current density in excellent agreement with experimental values. Our findings also challenge the recent assignment of MoO_x as the active site. This work establishes a definitive mechanistic framework that reconciles prior controversies and provides rational design principles for HER catalysts.