Mn-substituted spinel zinc cobaltite nanoplates as electrode materials for solid-state asymmetric supercapacitors

Metal oxides with a spinel structure are attracting increasing attention for use in the storage and conversion of energy. Based on their high porosity with good chemical stability, and high surface area, they are considered a superior material in energy storage applications. The Mn-substituted spinel zinc-cobalt oxide (zinc manganese cobaltite; ZnMnCo₂O₄ (ZMC)) with a uniform plate-like nanostructure and tea-waste biomass-derived active carbon (TWAC) were prepared via simple hydrothermal and chemical methods. The synthesized ZMC and TWAC were utilized as electrode materials for the anodes and cathodes in a solid-state supercapacitor. In the half-cell setup, the Mn-substituted ZnCo₂O₄ (ZMC-5) electrode provided a specific capacitance (Cs) of 791 F/g at a current density (CD) of 0.5 A/g using 0.5 M Na₂SO₄ as the electrolyte. In addition, the life cycle performance of the ZMC-5 electrode delivered a retention capacity of 88 % after 10,000 charge and discharge (GCD) cycles with 94 % coulombic efficiency (CE). The negative electrode (TWAC) shows a Cs of 485 F/g at 0.5 A/g. The solid-state asymmetric supercapacitor (SSAS) was fabricated using ZMC, TWAC, and PVA/Na₂SO₄ as the anode, cathode, and electrolyte. The assembled ZMC-5/TWAC device exhibited high Cs and 62 Wh/kg @ power density (PD) of 2000 W/kg. The constructed ZMC-5//TWAC devices were able to glow up 25 LEDs (red) in series connection for 5 min.