The electrochemical capacitor of graphite/activated carbon (AC) using quaternary alkyl ammonium-based organic electrolytes has been proposed. The charge storage mechanisms involve the adsorption of anions into pores of the AC positive electrode and the intercalation of ammonium cations into crystal lattice of the graphite negative electrode, respectively. The intercalation processes of ammonium cations have been investigated by in situ XRD measurements on graphite negative electrodes and the effect of cation size has been intensively addressed. Among the four quaternary alkyl ammonium cations, the intercalation of the biggest cation tetrabutyl ammonium demonstrates the largest degree of crystal lattice expansion of graphite, but the best cycle-ability. By contrast, the insertion of second big cation tetraethyl ammonium into graphite gives the smallest expansion degree of graphite crystal lattice, but poor cycle performance of capacitor. A mechanism has been proposed to explain the above phenomenon. (C) 2011 Elsevier B.V. All rights reserved.