The energy storage process for amorphous hydrated manganese dioxide (MnO2) is suggested as fast faradaic reactions occurring at the solid electrode surface with the reduction from Mn4+ to Mn3+. In order to understand the role of cations of the electrolyte for the MnO2 electrode as a pseudocapacitor in aqueous KCI solution, we monitored the change of the capacitance by varying the concentration of the KCI electrolyte, the cation of the electrolyte, the pH of the solution, and the solvent. The charge storage for the metal oxide electrode such as MnO2 is concluded to involve a fast redox reaction through both potassium ion exchange, MnO2 + deltaK(+) + deltae(-) double left right arrow MnO2-delta(OK)(delta) and proton exchange, MnO2 + deltaH(+) + deltae(-) double left right arrow MnO2-delta(OH)(delta) dependent upon the availability of cations in the electrolyte. The contribution of proton to the pseudocapacitive process is not negligible in aqueous solution. (C) 2004 Elsevier Ltd. All rights reserved.