An advanced hybrid electrochemical capacitor (HEC) has been proposed that uses a wide potential range at the positive electrode (cathode). The conventional HEC uses Li-dopable carbon as the negative electrode (anode) and activated carbon as the cathode, where the potential ranges from 3 to 4.5 V vs Li/Li+. In the advanced HEC, the potential range at the cathode is extended to the whole potential window of activated carbon, specifically from 1.5 to 4.5 V vs Li/Li+. This gives higher energy and power densities as a result of a reduction in the amount of cathode material and electrolyte.