First-Principles Prediction of the Protective Effect of Sulfur in Composite Ni/Ni3S2 Hydrogen Evolution Catalysts

Nickel-based compound catalysts have been shown to have significantly higher alkaline hydrogen evolution activity than pure Ni. We investigate the composite Ni/Ni¬3S2. First, simulated annealing of a previously generated Ni/Ni3S2 heterostructure uncovered S-diffusion to the Ni surface. Grand canonical calculations indicate pristine Ni may be lightly covered by OH at alkaline conditions, in contrast to Pt/Pd, and reveal that S prevents OH adsorption. Static calculations showed no significant effect on kinetics. Using metadynamics to dynamically sample the Volmer step, we show an alignment in entropic effects on Ni and Pt. Low-temperature sampling on Ni resulted in a significantly lower barrier indicating a dependence on solvation consistency. Our characterization of the Ni and Pt interfaces from unbiased ab initio molecular dynamics show similar solvent structures, further supporting this line of thinking. Finally, calculations on OH- and S-modified Ni surfaces found that the former had a greater Volmer barrier by 0.16 eV. This OH-modified surface had a significantly less consistent solvation environment, with an average hydrogen bond lifetime 14 fs less than that of the S-modified surface. These results suggest that the enhanced activity of composite Ni/Ni3S2 originates from adsorbed S on Ni preventing OH-binding and the associated decline in interfacial solvation.