| Summary: | Developing an efficient multi-functional electrocatalyst with high efficiency and low cost to replace noble metals is significantly crucial for the industrial water electrolysis process and for producing sustainable green hydrogen (H<sub>2</sub>) fuel. Herein, ultrathin CuCo<sub>2</sub>S<sub>4</sub> nanosheets assembled into highly open three-dimensional (3D) nanospheres of CuCo<sub>2</sub>S<sub>4</sub> (Cu/Co = 33:67) were prepared by a facile one-pot solvothermal approach and utilized as a bifunctional electrocatalyst for efficient overall water splitting. The as-prepared CuCo<sub>2</sub>S<sub>4</sub> is characterized structurally and morphologically; the BET surface area of the CuCo<sub>2</sub>S<sub>4</sub> (Cu/Co = 33:67) catalyst was found to have a larger specific surface area (21.783 m<sup>2</sup>g<sup>−1</sup>) than that of other catalysts with a Cu/Co ratio of 67:33, 50:50, and 20:80. Benefiting from a highly open structure and ultrathin nanosheets with excellent exposure to catalytically active sites, CuCo<sub>2</sub>S<sub>4</sub> (Cu/Co = 33:67) is identified as an efficient catalyst for the proton reduction and oxygen evolution reactions in 1 M KOH with an overpotential of 182 and 274 mV at 10 mA cm<sup>−2</sup>, respectively. As expected, a low cell voltage of 1.68 V delivers a current density of 10 mA cm<sup>−2</sup>. Stability and durability are also greatly enhanced under harsh alkaline conditions. Therefore, this work provides a simple strategy for the rational design of spinel-based transition metal sulfide catalysts for electrocatalysis.
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