Unravelling the volumetric performance of activated carbons from biomass wastes in supercapacitors

With the awareness of the need for low-cost carbons for supercapacitors, the recycling of biomass wastes as precursors of activated carbons is focusing great interest. The comparison of a diversity of carbons derived from residues of industrial processing of grapes, apples and cherries reveals the utility of physical activation to achieve better volumetric performance in aqueous supercapacitors. Whereas the high porosity development of the materials obtained by KOH activation (mostly around 1500-2000 m(2) g(-1)) provides a superior gravimetric capacitance up to nearly 300 F g(-1) in 2 M H2SO4, the low density of the electrodes reduces the volumetric capacitance to 60-130 Fcm(-3). Porous carbons produced by steam or CO2 reaches higher efficiency for compact devices. The physically activated materials display an ultramicroporous structure with standard surface areas of around 900 m(2) g(-1) and, therefore, their gravimetric capacitance is limited to 100-150 F g(-1). However, harnessing the balance of porosity, density and good packaging, the corresponding electrodes reach 80-170 F cm(-3), without sacrificing power delivery by the supercapacitor. Higher carbon yield and lower environmental impact represent additional advantages of the physical activation to produce biomass wastes derived-carbons.