Functional Group-Dependent Supercapacitive and Aging Properties of Activated Carbon Electrodes in Organic Electrolyte

The concentrations of surface functional groups on activated carbon (AC) are manipulated via heat treatment at various temperatures. The carboxyl (O–C = O) population clearly decreases at 600 °C, whereas the lactone (RO–C = O) and phenol (C–OH) populations decrease if the temperature exceeds 750 °C. Their effects on electrode capacitance, leakage current, and gas evolution are systematically investigated in 1 M tetraethylammonium tetrafluoroborate/propylene carbonate electrolyte. The assembled symmetric supercapacitors are also subjected to an aging test, where the cells are held at 2.5 V and 70 °C. The decreased functional group populations significantly reduce gassing and improve the cell durability; the mechanisms are explored using electrochemical impedance spectroscopy and post-mortem SEM. Nevertheless, the AC surface area drops dramatically at 850 °C, resulting in a considerable reduction in capacitance. A rational control of heat-treatment temperature is critical for obtaining AC with balanced supercapacitor performance.