An energy storage device including an active electrolyte, a first electrode and a second electrode is provided. The active electrolyte contains protons and ion pairs with a redox ability. The first electrode and the second electrode coexist in the active electrolyte and are separated from each other. The first electrode and the second electrode respectively include an active material producing a redox-reaction with the active electrolyte or an active material producing ion adsorption/desorption with the active electrolyte. The active electrolyte receives electrons from the first electrode and/or the second electrode so as to perform a redox-reaction for charge storage.