NiCo₂O₄ Nanoneedle-Coated 3D Reticulated Vitreous Porous Carbon Foam for High-Performance All-Solid-State Supercapacitors

A binder-free, electrically conducting nickel cobalt oxide (NiCo₂O₄)-reticulated vitreous carbon (RVC) foam (NiCo₂O₄@RVC) electrode was prepared by template carbonization of open-cell polyurethane foam followed by the hydrothermal growth of NiCo₂O₄ nanoneedles, leading to the formation of a hierarchical porous electrode. The growth of NiCo₂O₄ nanoneedles (length and diameter) on RVC foam was found to depend on hydrothermal coating time, which varied between 6 and 12 h. However, optimally grown NiCo₂O₄ nanoneedles for 8 h on an RVC foam with an average diameter of 77(±9) nm and length of ∼2 μm exhibited the lowest charge-transfer resistance, resulting in the areal capacitance (C_a) of ∼2.45 F/cm² at a scan rate of 5 mV/s. A symmetric supercapacitor (SC) device exhibited a maximum C_a of 1.22 F/cm² at a current density of 1 mA/cm² and an energy density of 2.51 W h/kg at a power density of 30 W/kg. The SCs showed a capacitance retention of ∼97% after 10,000 galvanostatic charge/discharge (GCD) cycles, apparently due to a highly stable NiCo₂O₄ structure on the RVC network structure, which was ascertained by various characterization techniques after the GCD cycles. Further, the SC module, comprising three devices in series, successfully lights up an LED, demonstrating the energy storage capability of these electrodes in real applications. Owing to its excellent electrochemical performance, the NiCo₂O₄@RVC electrode offers a low-cost and efficient alternative material in energy storage applications.