Nickel sulfides for electrocatalytic hydrogen evolution under alkaline conditions: a case study of crystalline NiS, NiS2, and Ni3S2 nanoparticles

Electrocatalytic water splitting to produce H-2 plays an important role in the capture, conversion, and storage of renewable energy sources, such as solar energy and wind power. As the reductive half reaction of water splitting, H-2 evolution reaction (HER) suffers from sluggish kinetics, and hence competent HER catalysts are needed. Despite being excellent HER catalysts, noble metal-based catalysts (i.e. Pt) are too expensive to be economically competitive. Therefore, low-cost catalysts composed of solely earth-abundant elements have attracted increasing attention these years, among which nickel-based HER catalysts, particularly nickel chalcogenides, are considered as promising candidates. Although many nickel chalcogenides, including NiS, NiS2, and Ni-3 S-2, have been reported for hydrogen evolution, their intrinsic catalytic activities have never been investigated and compared in detail under the same conditions. Most of the previous investigations were limited to only one species of nickel chalcogenides under very unique conditions, rendering a fair comparison of their HER activities impossible. Herein, we report the preparation and characterization of three crystalline nickel sulfides, NiS, NiS2, and Ni3S2, with comparable crystal sizes and specific surface areas. Detailed electrochemical studies under strongly alkaline conditions coupled with theoretical computations were performed to probe their intrinsic HER activities, resulting in the order of Ni3S2 > NiS2 > NiS. The superior HER performance of Ni3S2 mainly stems from the combined effect of large electrochemically active surface area and high conductivity (metallic conductor vs. semiconductor).