Ni(OH)₂ Templated Synthesis of Ultrathin Ni₃S₂ Nanosheets as Bifunctional Electrocatalyst for Overall Water Splitting

Ultrathin nickel (Ni)-based sulfide nanosheets have been reported as excellent electrocatalysts for overall water splitting; however, the uncontrollability over thickness due to the nonlayered structure still hampers its practical application. Herein, a simple topochemical conversion strategy is employed to synthesize cobalt-doped Ni₃S₂ (Co-Ni₃S₂) ultrathin nanosheets on Ni foam. The Co-Ni₃S₂ nanosheets are controlled synthesized by using Co-Ni(OH)₂ ultrathin nanosheets as templates with anneal and sulfurization treatment, showing exceptional electrocatalytic activity. This template-assisted method can also be applied to obtain Ni, NiO, and NiP_x nanosheets, providing a universal strategy to synthesize ultrathin nanosheets of nonlayered materials. The overall water splitting of this Co-Ni₃S₂ ultrathin nanosheets achieves a low voltage of 1.54 V at a current density of 10 mA cm⁻² and high durability in 1 m KOH, comparable to the best performance of electrochemical water splitting ever reported. The detailed structural transformation of Ni-based sulfides in the catalytic process and its mechanism are further explored both experimentally and theoretically.