Effect of microcrystallite structures on electrochemical characteristics of mesoporous carbon electrodes for electric double-layer capacitors

The effect of microcrystallite structures on electrochemical characteristics of mesoporous carbon electrodes was investigated by using nitrogen gas adsorption method, high resolution transmission electron microscopy, X-ray diffractometry (XRD), AC-impedance spectroscopy and cyclic voltammetry (CV). For this purpose, the carbon specimens with various microcrystallite structures were prepared using different carbonaceous precursors and varying heat-treatment temperature. The sizes of the microcrystallite in a- and c-axis direction, La and Lc, were quantitatively estimated from Scherrer's equation using the XRD patterns. From the analysis of the impedance spectra, it was found that the value of constant phase element exponent α decreased with increasing Lc/La, indicating that the frequency dispersion of capacitance increases with increasing amounts of the edge planes, i.e., with increasing surface inhomogeneity. Moreover, the value of specific double-layer capacitance Cd determined at the lowest frequency in the impedance spectra increased with increasing Lc/La, which is attributable to the higher value of Cd for the edge planes than that for the basal planes. From the results of the CVs, it is concluded that the ion penetration into the pores is closely related to Lc/La during double-layer charging/discharging of the carbon specimens, that is, the larger the value of Lc/La is, the lower exhibits the rate capability γ, thus causing more retardation of the ion penetration into the pores.