Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l−1 H2SO4 and 6 mol l−1 KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m2 g−1. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m2 g−1, the higher BET surface area the higher specific capacitance (F g−1) for both electrolytes. The surface capacitance (μF cm−2) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g−1 and a relatively high surface capacitance about 16 μF cm−2 measured in H2SO4 solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H2SO4 than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm−2). However, in the case of H2SO4 the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm−2). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm−2.