3D meso/macroporous Ni^sub 3^S^sub 2^@Ni composite electrode for high-performance supercapacitor

Ni3S2@Ni composite electrode for supercapacitor is synthesized via a one-step solvothermal treatment of Ni foam at 120 °C, exhibiting a 3D meso/macroporous network structure. The structure contains Ni3S2 nanosheets with 15–20 nm in thickness and meso/macropores with an average pore size of 24.69 nm, and its formation is regulated through the slow release of S2− ions and the solid/liquid interfacial reactions in absolute ethanol. At a current density of 17.15 A g−1, the composite electrode demonstrates a high specific capacitance of 945.71 F g−1, and at various current densities below 17.15 A g−1, it retains capacitance retention ratios above 100% after 2000 charge/discharge cycles. A two-step oxidation and three-step reverse reduction process occurs during the reversible Faradaic reaction of Ni3S2 in KOH aqueous solution, which is due to the valence transitions of Ni0 in Ni3S2 between Ni0 and Ni3+. The homogeneous cracking in Ni3S2 layer is a critical factor for achieving its long-term cycling stability, however, the cycling results in its amorphization. An asymmetric supercapacitor is assembled using Ni3S2@Ni composite electrode as the positive electrode and Ni foam as the negative electrode, which delivers an energy density of 55.79 Wh kg−1 at the power density of 938.98 W kg−1.