Sustainable conversion of agricultural biomass waste into electrode materials with enhanced energy density for aqueous zinc-ion hybrid capacitors

Herein, we demonstrate the facile, economic and highly scalable conversion of (N, O and Fe)-self-doped heteroatom Solanum melongena (SM) biomass into highly electro-active carbon material for symmetric supercapacitor and zinc-ion hybrid capacitor (ZIHC) applications. The SM is ubiquous agri-bio-waste that inherently possess electro-active constituents and upon conversion resulted in high surface area, self-doped heteroatoms (N, O and Fe) through one step pyrolysis. A synergetic combination of N, O and Fe co-doping and relatively high-surface-area features that offers widespread interfacial active sites for charge storage and fast kinetics of electrochemical reactions in both supercapacitor and battery-type capacitor to maximize energy and power output. The symmetric supercapacitor is fabricated using 6 M KOH, delivering high specific capacitance of 154.64 F/g at 0.1 A/g current density. Long cycling performance of device exhibits 99 % retention over 50,000 charge and discharge cycles at 10 A/g. While SM carbon as the cathode in zinc ion hybrid capacitor (ZIHC) endows excellent Zn2+ ion storage capacity with 2 M ZnSO4·7H2O as electrolyte. The ZIHC delivered an excellent specific capacitance of 313.08 F/g, a substantial energy density output of 141.35 W h/kg and a power density of 6935.38 W/kg. In addition, ZIHC exhibited better cyclic stability with 1.92 % loss over 20,000 charge and discharge cycles at 4 A/g. The ZIHC device delivered excellent output in wearable devices and in lightning the LED (25.04 min). This study provides ubiquitous agri-waste-derived carbon that can be outstanding electrode material in energy storage devices owing to their hierarchal pore configuration, graphitization degree and self-doping of heteroatoms.