Physically and Chemically Activated Carbon from Coffee Waste in High Performance Supercapacitor Electrodes

Biowaste sources, like coffee wastes (CWs), are considered for advanced materials. CW was assessed to prepare activated carbon for supercapacitor electrodes, but showed low performance compared to other counterparts. Enhanced CW activation process may help address this limitation. In this study, CW was subjected to two activation methods: physical (pyrolysis) activation and physical-chemical activation using ZnCl2, to determine the more effective approach. Both resulting materials were analyzed using SEM, TEM, BET, Raman spectroscopy and XRD. The physical-chemical activation yielded activated carbon with better characteristics, including a higher specific-surface area (SSA) of ~830 m²/g, compared to 458 m²/g from the physical activation. With its promising potential for supercapacitor electrodes, the former activated carbon was singled out for supercapacitor study. Cyclic voltammetry revealed a specific capacitance of 261 F g-1, and energy density of 18.3 Wh/kg, with power density of 360 W/kg at a current density 0.33 A/g. Charge-discharge studies showed a specific capacitance of 150 F g-1 at the same current density. Electrochemical impedance spectroscopy indicated a specific capacitance of 180 F g-1, with an equivalent series resistance of 0.56 Ω at a low frequency of 0.01 Hz. The electrode exhibited exceptional characteristics, maintaining high stability over 5,000 charge-discharge cycles.