Structural features necessary to obtain a high specific capacitance in electric double layer capacitors

Two classes of carbon materials with different microstructures were chosen for electric double layer capacitors (EDLCs). One is a mesophase pitch-based carbon fiber (MPCF). The other is an isotropic pitch-based carbon fiber (IPCF). Both series of samples were activated by KOH with varying the KOH addition fraction. The MF-series prepared from MPCFs by several KOH fractions exhibited a good capacitance in comparison with the IF-series prepared from IPCFs, although the former had smaller specific surface areas than those of the latter. The specific capacitance per weight (F/g) of both samples increased with an increase in KOH fraction. Further evaluation of the capacitance using the specific capacitance per unit area elucidates the efficiency of the surface area. The MF-series prepared by the addition of 200-250 wt.% KOH showed a larger capacitance per unit volume and surface area under the same charge/discharge conditions, despite their smaller specific surface areas. (C) 2004 Elsevier Ltd. All rights reserved.