Coffee-Waste-Based ZnCl2 Activated Carbon in High-Performance Supercapacitor Electrodes: Impact of Graphitization, Surface Morphology, Porosity and Conductivity

Activated carbon (AC) electrodes from coffee waste (CW) were earlier assessed in supercapacitors but showed lower supercapacitor performance in terms of specific capacity (Cs), specific power (Ps) or both, compared to other biowastes. This work describes how CW-based AC electrode performance may be improved if carefully prepared. Careful processing yields higher graphitization, carbon content (aromaticity), conductivity and porosity free of any residues. Thus, AC electrodes will exhibit higher Cs and Ps simultaneously. CW was first pyrolyzed (CPyrol) and then chemically activated by ZnCl2 (ACChem). Both materials were characterized using SEM, TEM, BET, FT-IR spectra, Raman spectra and XRD. The ACChem exhibited much higher graphitization, crystallinity, specific surface area (SSA), porosity and conductivity. From cyclic voltammetry, the ACChem electrode exhibited a Cs of 261 F/g, an energy density of 18.3 Wh/kg and a Ps of 360 W/kg at 0.33 A/g. From galvanostatic charge–discharge, there was a stable Cs of 150 F/g at 0.33 A/g over 5000 charge–discharge cycles. From electrochemical impedance spectroscopy, the Cs was ~180 F/g, with a low equivalent series resistance (ESR) of 0.56 Ω at a frequency of 0.01 Hz, compared to the literature. The ACChem electrode was superior to the CPyrol electrode and to earlier CW-based AC counterparts, with much lower resistance. Moreover, the electrode competed with other biowaste-based electrodes.