On the Origin of Superior Hydrogen Evolution Activity on Composite Ni/Ni3S2 Catalysts: A First Principles Investigation

Nickel-based compound catalysts have been shown to have significantly higher alkaline HER activity than pure Ni. Here we attempt to explain this behavior with composite Ni/Ni-3S2. First, simulated annealing of a previously generated Ni/Ni3S2 heterostructure uncovered S-diffusion to the Ni surface. GCDFT calculations show that pristine Ni is likely to be lightly modified by adsorbed OH at alkaline conditions, in contrast to other metals, and reveal that the OH- and S-modified surfaces are mutually exclusive. These light modifications did not significantly change the activity of the Ni surface. Seeing no effect on the static Volmer step (the RDS of alkaline HER), we performed dynamic calculations of the alkaline Volmer barrier. Using metadynamics to sample the reaction along our defined collective variable (CV), we first estimate the barrier on Ni and Pt, obtaining a similar gap between static and dynamic barriers. Low temperature (100K) sampling on Ni found a significantly lower barrier relating to the more consistent hydrogen-bondng at low temperatures. Our characterization of the Ni and Pt interfaces from unbiased AIMD showed similar solvent structures, further supporting this line of thinking. Finally, calculations on OH- and S-modified Ni surfaces found that the former surface 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. From these results, we contend that the enhanced activity of composite Ni/Ni3S2 catalysts originates from adsorbed S on Ni preventing OH-binding and the associated decline in interfacial solvation. Our calculations emphasize the importance of interface dynamics on electrochemical reaction barriers.