In-situ electrochemical pretreatment of hierarchical Ni3S2-based electrocatalyst towards promoted hydrogen evolution reaction with low overpotential.

The improvement of electrocatalysts in hydrogen evolution reaction (HER) activity can be achieved by two strategies: (i) to enhance the intrinsic catalytic activity, and (ii) to improve the active-site exposure. In this work, nickel sulfide (Ni3S2), as an efficient pre-electrocatalyst, is developed by a facile, mild and in-situ electrochemical (EC) pretreatment for enhancement in HER activity. It is found that after the EC treatment, the required overpotential for HER greatly decreases from 288 mV (pristine-Ni3S2) to 169 mV (EC-Ni3S2) at 10 mA cm-2 in 1.0 M KOH, and is stable for over 25 h with nearly unchanged structure and chemical composition. Two possible mechanisms are suggested to explain this enhancement: (i) the oxygen-doped sulfur-deficient Ni3S2 (Ni3S2-xOx) layer should have been formed on the surface of Ni3S2 nanosheets, which brings about the electronic environment modulation, resulting in the optimized hydrogen (H*) adsorption/desorption and higher electrical conductivity; (ii) the developed nanosheets/microspheres structure with poor crystallinity increases the number of active sites in hydrogen generation. The promotion in HER performance of Ni3S2 electrocatalyst via EC pretreatment may exploit the potential for nickel sulfide and other transition metal dichalcogenide (TMD) electrocatalysts to achieve high efficiency and stability for electrochemical energy conversion and storage.