Swapping Catalytic Active Sites from Cationic Ni to Anionic S in Nickel Sulfide Enables More Efficient Alkaline Hydrogen Generation

Tuning active sites in catalyst design is the key to boosting the intrinsic activity of hydrogen evolution reaction (HER). Cationic Ni has been widely established as an active site in nickel sulfide due to the relatively low Gibbs free energy of hydrogen adsorption (Delta G(H*)). However, one of the big unsettled issues is whether S can be activated as a more active site than Ni in NiS2. Herein, the swapping of catalytic active sites from cationic Ni to anionic S in a hierarchical structure consisting of NiS2 nanoflowers grown on dual-phased NiS2-NiS foam (denoted as NiS2/NiS2-NiS) is shown. A combined study of theoretical calculations and X-ray photoelectron spectroscopy analysis demonstrate the remarkably antidromic electron transfer from Ni to S sites, therefore relieving the adsorption of hydrogen species and endowing a higher intrinsic activity at the S site over the Ni site. The new catalyst therefore exhibits superior HER performance, identified by doubling in the intrinsic activity and a twofold increased turnover frequency value compared to its pure NiS2 counterpart (0.028 s(-1) vs 0.015 s(-1) at the applied overpotential of 200 mV). The NiS2/NiS2-NiS electrode also demonstrates outstanding activity toward the oxygen evolution reaction and overall water splitting.