Commercial activated carbon (AC) was improved and its performance in electric double layer (dl) capacitors was evaluated as compared to the original AC. Improvement of AC and its optimization were carried out through CO2 reactivation using a NiCl2 catalyst at different temperatures. After improvements, both specific capacitance and high rate capability of capacitors were increased. For AC improved at 850degrees C for 3 h, its discharge specific capacitance increases up to 51.84 F/g, an increase of about 20% as compared to the original AC. The electric dl capacitor with improved AC as active material can be cycled at very high current densities and its resistance capacitance time constant is much lower than that with original AC, proving that its high rate capability was improved significantly. Moreover, the significant increase in electrical conductivity for improved AC suggested that it is not necessary to use a conductor within the polarizable electrodes. Improved AC seems to be a suitable material for the fabrication of electric dl capacitors with high power density. (C) 2002 The Electrochemical Society.