We propose a novel electrochemical capacitor (EC) that used electrolyte-based redox reactions of halogen species, which are induced by a novel pretreatment of an activated carbon positive electrode. This treatment, consisting of an impregnation of pores at a positive electrode of activated carbon fiber cloth (ACFC) with a halogenated solution such as bromine water before cell assembly, is simple and convenient. An aqueous sodium bromide solution is applied to an EC cell containing the treated positive electrode and a non-treated negative electrode of ACFC. Few studies have so far obtained ECs using an “electrolyte” charge storage system with practical performance due to diffusive loss of charged species from an electrode into an electrolyte or its shuttle migration between electrodes. Nonetheless, we have attained excellent capacitor performance with such a concept by suppressing an undesirable diffusion of electrolyte reactive species. This concept, electrolyte-based charge storage, is applicable to not only the above aqueous EC but also non-aqueous EC and lithium-ion capacitor application with dramatically improved performance.