Combining renewable energy technologies with water electrolyzers to produce green hydrogen should provide a very promising strategy for future sustainable and pollution-free energy supply.In this study,In this study,one-dimensional(1-D)and low-cost nickel sulphide(NiS2)was developed as a support material for the α-Co(OH)2 nano-sheets since NiS2 usually possesses higher conductivity than its corresponding oxide.1-D NiS2 nano-wires were directly grown on the Ni foam,which acted as a current collector,and the interconnected nano-wires formed a 3-D network.Subsequently,2-D α-Co(OH)2 nano-sheets were synthesized onto the 1-D NiS2 nano-wires to form core-shell structured Ni3S2@Co(OH)2 composite materials.Due to the synergetic effect between Ni3S2 and Co(OH)2 as well as the unique hierarchical structure,the obtained 1-D and core-shell structured Ni3S2@Co(OH)2 composite material demonstrated good electrochemical activity towards both the HER and OER.In our condtions,the Ni3S2@Co(OH)2 catalyst exhibited a low onset potential and charge transfer resistance together with excellent stability towards the HER.In terms of the onset potential and stability towards the OER,it was observed that the Ni3S2@Co(OH)2 was comparable to that of the RuO2.Due to its well-defined bifunctionality,it was found that an asprepared water electrolyzer using Ni3S2@Co(OH)2 on both the cathode and anode could deliver a constant cell voltage of 1.64 V at 10 mA.cm-2 for 20 h.This study also indicated that a core-shell structure with ultrathin Co(OH)2 nano-sheets covered on the surface of Ni3S2 nano-wires may act as a promising bifunctional HER and OER catalyst for producing green hydrogen via water splitting.Moreover,the development of such bifunctional electrocatalysts for water electrolyzers could increase their performance and decrease their cost,and might also simplify the electrolyzer′s balance of plant.