Hybrid electrochemical capacitors in aqueous electrolytes: Challenges and prospects

In an internal hybrid capacitor, at least one electrode displays battery-like charge/discharge and the other electrode stores charge reversibly at the electric double-layer (EDL). Recently, a plethora of hybrid cells in aqueous electrolytes have been proposed by coupling an EDL electrode with a battery electrode, the latter made from a variety of redox-active/redox-mediator species either dissolved in the electrolyte or adsorbed/immobilized in nanoporous electrodes. This review presents current opinions, discusses challenges, and supplies recommendation about the hybrid cells with aqueous electrolytes and carbon electrodes.