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