Various kinds of activated carbon/activated carbon fibers were used in the evaluation of electrical double layer capacitors using the method of image analysis. The appropriate hydrated ion structures in an aqueous system of H 2 SO 4 /H 2 O and an organic system of LiClO 4 /polypropylene carbonate were calculated using the software Cerius 2 ͑ver. 3.8͒. The capacitance obtained varied with the electrolyte used, even though the capacitor material remained the same. The relationship between the pore size and the electrolyte ion diameter is discussed.
Activated milled mesophase carbon fibers (AC-mMPCF, MP-series) show a higher specific capacitance in spite of a smaller specific surface area than those of powder-type activated carbons (AC-series). This phenomenon can be interpreted to mean that it is difficult to predict the capacitance of an electric double-layer capacitor (EDLC) knowing just the surface area and the pore size. More information is needed about other inherent characteristics of the samples, for example, the equivalent series resistance (ESR), shape of the pores, etc. We investigate here other characteristics of the samples. Consequently, it was deduced that the MP-series of EDLCs have a slit pore shape, which affects the accessibility of the electrolyte ion onto the electrode surface. Moreover, the MP-series of materials have suitable ESR values, and these material properties themselves should be considered as factors that affects the deterioration of the specific capacitance of EDLCs.
A new application to electric double-layer capacitor fields of Saran® resin using samples obtained directly from the production line was tried and investigated. These samples are prepared through a simple process by only heat-treatment with no additional activation process. Homogeneous polyvinylidenechloride ͑PVDC͒ based carbon heat-treated at 700°C showed a maximum specific capacitance of 92.5 F/g at 1 mA/cm 2 of current density ͑which is obtained by a unit cell system by two electrodes, equivalent to 370 F/g in a conventional three-electrode system͒. Other blended samples show a better capacitance at high current density over 300 mA/cm 2 than that of homogeneous PVDC, although the blends show poor characteristics at relatively low discharge current density. To clarify the relation between the pore size distribution and the capacitance, image analysis of transmission electron microscopy photographs was performed. The pore size distribution of each Saran resin is shifted according to the kind of blended polymer. The pore size distribution of Saran-based carbon is controllable by mixing with a copolymer. The simulation result of the electrolyte ion was used to clarify the capacitance behavior.
The heat-treatment retention time effect of carbonized polyvinylidenechloride (PVDC) was investigated. Homogeneous PVDC with a crystallite size of 267 Å was used as a precursor material for an electric double-layer capacitor electrode. The P-120m material, which was heat treated for 120 min at 700 °C, shows a larger specific capacitance than any other material in this study. It shows the largest values reported up to now, reaching values as high as 100.2 F/g for a two-electrode system, which is equivalent to 400.8 F/g in a conventional three-compartment electrode system. It is difficult to distinguish the difference in the pore-size distribution by way of gas adsorption as the retention time is varied. However, the difference can be clarified using a novel method based on the analysis of transmission electron microscopy images. As the retention time for heat treatment increases, the pore size grows through the coalescence of small pores. Furthermore, a new concept for the electric double-layer capacitance is suggested on the basis of analysis of the transmission electron microscopy observations.
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