Hierarchical Ni-Co-S@Ni-W-O core–shell nanosheet arrays on nickel foam for high-performance asymmetric supercapacitors

Nickel cobalt sulfides (Ni-Co-S) have attracted extensive attention for application in electronic devices owing to their excellent conductivity and high electrochemical capacitance. To facilitate the large-scale practical application of Ni-Co-S, the excellent rate capability and cyclic stability of these compounds must be fully exploited. Thus, hierarchical Ni-Co-S@Ni-W-O (Ni-Co-S-W) core/shell hybrid nanosheet arrays on nickel foam were designed and synthesized herein via a facile three-step hydrothermal method, followed by annealing in a tubular furnace under argon atmosphere. The hybrid structure was directly assembled as a free-standing electrode, which exhibited a high specific capacitance of 1, 988 F·g^-1 at 2 A·g^-1 and retained an excellent capacitance of approximately 1, 500 F·g^-1 at 30 A·g^-1, which is superior to the performance of the pristine Ni-Co-S nanosheet electrode. The assembled asymmetric supercapacitors achieved high specific capacitance (155 F·g^-1 at 1 A·g^-1), electrochemical stability, and a high energy density of 55.1 W·h·kg^-1 at a power density of 799.8 W·kg^-1 with the optimized Ni-Co-S-W core/shell nanosheets as the positive electrode, activated carbon as the negative electrode, and 6 M KOH as the electrolyte.