Three-Dimensional Hierarchically Mesoporous ZnCo₂O₄ Nanowires Grown on Graphene/Sponge Foam for High-Performance, Flexible, All-Solid-State Supercapacitors

To achieve high energy storage on three-dimensional (3D) structures at low cost, materials with high power and long cycle life characteristics have to be developed. We synthesized ZnCo₂O₄/reduced graphene oxide (rGO) binary composites in commercial sponges. ZnCo₂O₄ nanosheets were grown on the surface of GO/sponge through a hydrothermal reaction. The resulting flexible, free-standing ZnCo₂O₄/rGO/sponge electrodes were used as the electrodes in a symmetric supercapacitor. ZnCo₂O₄/rGO/sponge electrodes have a large specific capacitance of 1116.6 F g⁻¹ at a scan rate of 2 mV s⁻¹ in aqueous electrolyte. The all-solid-state flexible supercapacitor is assembled based on ZnCo₂O₄/rGO/sponge electrodes, which show excellent electrochemical performances with a specific capacitance of 143 F g⁻¹ at a current density of 1 A g⁻¹. The as-fabricated supercapacitor also exhibits excellent cycling stability (93.4 % capacitance retention after 5000 cycles) and exceptional mechanical flexibility. These results demonstrate the potential of ZnCo₂O₄/rGO/sponge as an electrode in flexible, high-performance supercapacitors.