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 (Ca) of ∼2.45 F/cm2 at a scan rate of 5 mV/s. A symmetric supercapacitor (SC) device exhibited a maximum Ca 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.