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

## TL;DR

This study explains why NiMo alloys are highly effective for hydrogen production in alkaline conditions by identifying the active sites and their cooperative mechanism.

## Contribution

The paper provides a definitive mechanistic framework for NiMo alloy activity in alkaline HER, resolving prior controversies.

## Key findings

- O-covered Ni3Mo(111) surfaces show low water dissociation barriers and near-thermoneutral hydrogen adsorption.
- A synergistic mechanism between Mo and Ni sites explains the superior HER performance of NiMo alloys.
- Microkinetic modeling confirms the mechanism with predictions matching experimental exchange current densities.

## Abstract

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 MoOx as the active site. This work establishes a definitive mechanistic framework that reconciles prior controversies and provides rational design principles for HER catalysts.

This study resolves the controversy regarding the active origin of Nickel‐molybdenum (NiMo) alloys for alkaline hydrogen evolution. By combining surface Pourbaix diagrams with microkinetic modeling, we demonstrate that the O‐covered Ni3Mo surface drives activity through a synergistic mechanism: Mo sites facilitate water dissociation while adjacent Ni sites optimize hydrogen evolution, definitively ruling out MoOx as active species.