The impurity level, surface functional group concentration, and specific surface area of activated carbons (ACs) are manipulated via acid rinsing, post heat treatment, and activation time, respectively. The effects of these factors on the electrode gravimetric/volumetric capacitances, leakage current, and gas evolution amount in 1 M triethylmethylammonium tetrafluoroborate/propylene carbonate electrolyte are systematically investigated. The impurity and surface functional groups do not significantly influence the electrode capacitances (regardless of charge–discharge rates), but increase the leakage current and gas evolution amount of the constructed supercapacitors. Increasing the specific surface area of ACs increases the capacitor gravimetric energy density and power density, but decreases the corresponding volumetric performance. The cyclic stability of various ACs is evaluated at 25 and 70 °C. Property variations of the electrodes upon cycling are examined using scanning electron microscopy and electrochemical impedance spectroscopy.