Effect of activating agents for producing activated carbon using a facile one-step synthesis with waste coffee grounds for symmetric supercapacitors

Activated carbon with large surface area, high porosity and low-cost features is widely applied as the energy storage material for supercapacitors. Activating agent is significant in activation process since the active sites amount and pore structures of activated carbon are mainly determined by activating agents. In this work, a novel one-step activating process combining the conventional carbonization and activation processes is proposed, and the activating agents of KOH, NaOH, HCI, H3PO4, ZnCl2 and FeCl3 are applied for producing activated carbon from cost-free waste coffee grounds. The surface area, pore structure, functional groups, and defect to graphene ratio of activated carbon are investigated. The KOH-activated carbon electrode shows the highest specific capacitance of 105.3 F/g, which is five-fold of that for the commercial activated carbon electrode (21.8 F/g), owing to the less hydrophobic functional groups, moderate defect to graphene ratio, large specific surface area and high porisity for the former case. The symmetric supercapacitor with the KOH-activated carbon electrode shows a specific capacitance of 19.28 F/g and the maximum energy density of 6.94 Wh/kg at the power density of 350 W/kg. The specific capacitance retention of 133% and Coulombic efficiency of higher than 80% are also attained in the 8000 times charging/discharging process. (C) 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.