Synergistically Active NiCo₂S₄ Nanoparticles Coupled with Holey Defect Graphene Hydrogel for High-Performance Solid-State Supercapacitors

Nickel cobalt sulfide nanoparticles embedded in holey defect graphene hydrogel (HGH) that exhibit highly porous structures and uniform nickel cobalt sulfide nanoparticle sizes are successfully prepared by a facile solvothermal–hydrothermal method. As an electrode material for supercapacitors, the as-prepared NiCo₂S₄@HGH shows ultra-high specific capacitances of 1000 F g⁻¹ and 800 F g⁻¹ at 0.5 and 6 A g⁻¹, respectively, owing to the outstanding electrical conductivity of HGH and high specific capacitance of NiCo₂S₄. After 2100 charge/discharge cycles at a current density of 6 A g⁻¹, 96.6 % of the specific capacitance was retained, signifying the superb durability of NiCo₂S₄@HGH. Moreover, remarkable specific capacitance (312.6 F g⁻¹) and capacity retention (87 % after 5000 cycles) at 6 A g⁻¹ were displayed by the symmetric solid-state supercapacitor fabricated by using NiCo₂S₄@HGH electrodes. These auspicious supercapacitor performances demonstrate that the as-developed solvothermal–hydrothermal approach can be widely used to prepare graphene-coupled binary metal sulfides for high-performance supercapacitor applications.