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We propose novel hybrid capacitors (HCs) with electrolyte-involved redox reactions of bromide or iodide species by pretreatment of an activated carbon positive electrode. The treatment is simple; impregnation of pores at an activated carbon fiber cloth (ACFC) as a positive electrode with bromine- or iodine-containing water before cell assembly. The treated positive electrode is applied to a HC cell with a non-treated negative electrode of ACFC and its electrochemical performance is investigated by galvanostatic cycling and leakage current tests. Few studies on such “electrolytic” charge storage systems have provided acceptable capacitor performance because of inevitable self-discharge caused by diffusion of charged species form an electrode to the other one through an electrolyte. Nevertheless, our electrolyte-redox-based HCs show excellent performance without undesirable diffusion of charged species. Moreover, the present HC utilizing a bromide redox system fulfills a practical cell voltage of 1.8 V in spite of an aqueous electrolyte system. This high voltage provides excellent energy density, which is 5 times higher than that in a conventional aqueous electric double-layer capacitor (EDLC), and 1.2 times higher even than that in a 2.7 V-class non-aqueous EDLC, while keeping high charge–discharge rate capability.

The electrochemical treatment of activated carbon electrodes has been investigated as a novel method enhancing the capacitance of electric double layer capacitor (EDLC) electrodes. The treatment at 0.7 V floating in a week at 70°C can be applied to EDLC with aqueous electrolytes. In 30 wt% H2SO4/H2O electrolyte, the treatment causes a capacitance increase in a low voltage region: 0–0.4 V. On the other hand, in 3.5 M NaBr/H2O electrolyte, it results in a remarkable capacitance increase of 2.4 times in a high voltage region: 0.4–1.0 V. These results suggest that the treatment mechanisms in the above electrolyte systems should be different. We report herein the origins and mechanisms of these capacitance increases.