First-Principles Prediction of the Protective Effect of Sulfur in Composite Ni/Ni ₃ S ₂ Hydrogen Evolution Catalysts

Nickel-based compound catalysts have been shown to exhibit superior alkaline hydrogen evolution activity compared to that of pure Ni. This study examines the underlying origins of the enhancement using the Ni/Ni3S2 composite. Simulated annealing of a Ni/Ni3S2 heterostructure uncovers S diffusion to the Ni surface. Further first-principles calculations reveal that these surface S atoms inhibit OH adsorption, while Ni surfaces can be partially covered by OH under alkaline conditions. At low coverages, the presence of S/OH exerts no significant influence on the enthalpic barrier of the rate-determining Volmer step. However, metadynamics simulations demonstrate that OH adsorption at low coverage increases the free-energy barrier by 0.16 eV relative to pristine Ni, whereas surface S has no effect. The free-energy barrier tends to increase with temperature as a result of entropic contributions. Hydrogen bond lifetime analysis demonstrates that adsorbed OH perturbs hydrogen bond networks near the catalyst surface, leading to the increase in the free-energy barrier. Our results suggest that the enhanced activity of Ni/Ni3S2 may largely originate from the suppression of OH binding induced by surface S atoms.