This work highlights the potential to utilize waste biomass to produce electrode materials for cost-effective energy storage systems for the development of renewable energy technologies. Porous carbon electrodes for supercapacitors were prepared from waste coffee grounds, sugar cane bagasse, sawdust, and popcorn by chemical activation with ZnCl₂. The double-layer capacitances of these carbon electrodes were evaluated in two-electrode supercapacitor cells with 1 M H₂SO₄ and 1 M tetraethyl ammonium tetrafluoroborate/acetonitrile (TEABFSO₄/AN) electrolytes. The choice of carbon precursor and activation conditions determine the electrochemical performance, with surface area, pore size distribution, electrical conductivity, and electrochemically active surface functional groups all affecting specific capacitance.

The carbons prepared from waste coffee grounds showed the highest specific capacitances in both the aqueous and organic electrolytes, with capacitances as high as 368 F/g in H₂SOSO₄ and 134 F/g in TEABFSO₄/AN. The good electrochemical performance of the waste coffee grounds carbon was attributed to a well developed porosity and the presence of electrochemically active quinone oxygen groups and nitrogen functional groups. The highest energy densities were obtained with the organic electrolyte and the waste coffee ground carbon possessing the greatest mesopore volume, with an energy density of 34 Wh/kg at low current loads and significantly an energy density of 16.5 Wh/kg was retained at high power loads.