Supercapacitors—also called electrochemical capacitors—are a new type of elec- trochemical energy storage system applied for harvesting energy and delivering high power in a short time. Their main energy storage mechanism is based on charging an electrical double-layer (EDL) at the electrode-electrolyte interface of high surface area electrode materials. They have attracted attention for a variety of applications, especially in hybrid systems combining with batteries and fuel cells, because of their high power density, excellent cyclic stability, and rapid response to external loading on a powertrain [1–4]. However, their main disadvantage is the relatively low energy density (5–6 Wh/kg based on activated carbon [AC]), which is significantly lower than that of a lithium ion rechargeable battery (~150 Wh/kg). Numerous efforts have been made to resolve this problem