Since both EDLC and pseudocapacitan

Since both EDLC and pseudocapacitance are surface phenomena, high-surface-area mesoporous carbon and activated carbons (specific surface area: 1000–2000 m2 g−1) have been widely used as electrode materials in both academic and commercial supercapacitors 16, 17, 18, 19. Taking a specific surface area of 1000 m2 g−1 for carbon as an example, its ideal attainable capacitance could be 200–500 F g−1. However, the practically obtained values are of only a few tens of F g−1. Activated carbons have a wide pore size distribution, consisting of micropores (50 nm) 17, 18, with most of the surface area of activated carbons being on the scale of micropores [20]. Pores of this size are often poorly or non-accessible for electrolyte ions (especially for organic electrolytes), and thus are incapable of supporting an electrical double layer. By contrast, mesopores contribute the most to the capacitance in an electrical double-layer capacitor 21, 22, 23. However, recent experimental and theoretical studies have demonstrated that charge storage in pores 0.5–2 nm in size (smaller than the size of solvated electrolyte ions) increased with decreasing pore size due to the closer approach of the ion center to the electrode surface in the smaller pores 18, 24, 25, 26. Pores less than 0.5 nm wide are too small for double layer formation [26]. Currently available activated carbon materials have a high surface area but unfortunately a low mesoporosity, and hence a limited capacitance due to a low electrolyte accessibility [20]. This translates to the limited energy density of the resultant supercapacitors (Eqn 3). The low electrolyte accessibility of activated carbons, coupled with their poor electrical conductivity, produces a high internal resistance and hence a low power density for the capacitors (Eqn 4) [20]. Consequently, a limited energy density (4–5 Wh kg−1) and a limited power density (1–2 kW kg−1) have been obtained for currently available supercapacitors based on the activated carbon electrodes [20]. Clearly, therefore, new materials are needed to overcome the drawbacks of activated carbon electrode materials to improve the performances for supercapacitors.