We studied the electrochemical hydrogen storage properties of activated carbon (AC) material mixed with copper. The discharge capacity of AC-Cu electrode which reached 510 mAh/g after 3 84 cycles, is much higher than that of the CNT-Cu electrodes. The plateau of discharge potential for AC-Cu electrode was very long and flat and reached -0.88 V vs. Hg/HgO, which was far from the potential of copper oxidation. The discharge plateau gradually appeared and continually lengthened with the increase of cycle number. Cyclic voltammetric experiments showed that the adsorption and desorption of hydrogen occurred on the surface of activated carbon and the active site increased with the increase of cycle number. The mechanism for electrochemical storage of hydrogen in AC-Cu electrode may be mainly physisorption. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.