The electrochemical properties of electric double-layer capacitors (EDLCs) made with plasma-treated powdered activated carbon (treated using a dielectric barrier discharge) were examined using cyclic voltammetry (CV), Cole-Cole plots, and X-ray photoelectron spectroscopy (XPS). The dielectric barrier discharge method, which operates at atmospheric pressure, dramatically reduces the processing time and does not require vacuum equipment, making it a more practical alternative than low-pressure plasma treatment. The experimental data indicate that the specific capacitance of the EDLCs could be improved by oxygen plasma treatment. Capacitance of EDLCs made with activated carbon treated for 15 s showed 193.5 F/g that 20% increase in the specific capacitance relative to untreated EDLCs. This result indicates that the plasma treatment yields EDLCs that are suitable for high-energy applications. The enhancement of capacitance was mainly attributed to an increase in the BET surface area of the activated carbon and the creation of carboxyl groups on the surface of the carbon. The carboxyl groups induced oxidation-reduction reactions in the presence of O-2 which was included in the operation gas. In addition, the carboxyl groups improved the penetration of the electrolyte solution into the carbon electrodes. (C) 2012 Elsevier Ltd. All rights reserved.