In this study, carbonaceous materials such as activated carbon and activated carbon aerogel were chemically modified with a surfactant sodium oleate in order to improve their specific capacitance and energy storage in electrochemical double-layer capacitors (EDLCs). Optimal conditions for surface modification of activated carbon have been examined as a surfactant solution concentration of 0.25 wt.% together with a time of 24 h for treatment at 25 degrees C. Specific capacitance and energy density can be improved significantly by surface modification of carbon materials. The enhancement in specific capacitance and energy density is mainly attributable to improvement in wettability of carbon materials, which results in a higher usable surface area and a smaller internal resistance. The effects from surface modification become more marked at higher discharge rates, at which the internal resistance has a more important impact on the energy delivery. A two times energy density of the original carbon could be achieved for the modified carbon materials at a high discharge rate, which indicates that the modified carbons are more suitable in EDLCs for high current applications. In addition, the modified carbon materials possess excellent cycle stability (the capacity decay was only 4% after 20,000 cycles). (c) 2005 Elsevier Ltd. All rights reserved.