Non-aqueous electrochemical capacitor utilizing electrolytic redox reactions of bromide species in ionic liquid

We propose a novel electrochemical capacitor (EC) utilizing an “electrolytic” charge-storage system of bromide species. The present EC cell is fabricated with 1.0 mol dm−3 1-ethyl-3-methylimidazolium bromide dissolved in 1-ethyl-3-methylimidazolium tetrafluoroborate as an electrolyte and activated carbon fiber cloths (ACFCs) as electrodes. Although such a charge-storage concept would usually be useless due to inevitable, undesirable reactions, e.g. shuttle reaction or migration of active redox species between electrodes, we attain much-enhanced capacitance with an excellent reversibility by suppressing an undesirable diffusion process. In fact, the coulombic efficiency of the present cell is over 99.7% at 100 mA g−1. The oxidation species such as Br2 or Br3− as charge accumulators do not diffuse from a positive electrode into the electrolyte. Moreover, the present EC cell has an excellent cycleability of at least 10,000 cycles.