KPF6 dissolved in Propylene carbonate (PC) has been proposed as an electrolyte for activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitor has been tested in XPF6-PC or XBF4-PC electrolytes (X stands for alkali or quaternary alkyl ammonium cations). The AC/graphite capacitor using KPF6-PC electrolyte shows an excellent cycle-ability compared with other electrolytes containing alkali ions. The big decomposition of the PC solvent at the AC negative electrode is considerably Suppressed in the case of KPF6-PC, which fact has been correlated with the mild solvation of K+ by PC solvent. The relationship between the ionic radius of cation and the corresponding specific capacitance of AC negative electrode also proves that PC-solvated K+ ions are adsorbed on AC electrode instead of naked K+ ions. (C) 2009 Elsevier B.V. All rights reserved.