Composition-Dependent Catalytic Activities of Noble-Metal-Free NiS/Ni3S4 for Hydrogen Evolution Reaction

The development of efficient noble-metal-free hydrogen evolution catalysts is quite appealing with the aim of providing cost-competitive hydrogen. Herein, nickel sulfides (NiSx) with tunable NiS/Ni3S4 molar ratios were synthesized via a simple hydrothermal method. Detailed electrochemical studies under neutral conditions indicated that the electrocatalytic property of NiSx catalysts was determined by the composition. Notably, the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 exhibited the lowest overpotential and charge-transfer resistance. As analyzed from the Tafel plots, the rate-determining step of NiSx catalysts for hydrogen generation was the Volmer step, in which the proton adsorption played a key role. Theoretical calculation revealed that NiS and Ni3S4 exhibited the metallic behaviors with different work functions. Consequently, the NiSx sample with the NiS/Ni3S4 molar ratio of 1.0 owned the most adsorbed protons, which led to the highest electrocatalytic property. Meanwhile, NiSx was demonstrated to be efficient cocatalysts to promote photocatalytic hydrogen generation. NiSx/CdS with the NiS/Ni3S4 molar ratio of 1.0 showed the best photocatalytic activity with the apparent quantum efficiency of 56.5% at 420 nm. This result was in good agreement with the electrocatalytic activities of NiSx samples, indicating the intrinsic property for efficient hydrogen generation.