Impedance analysis on electric double layer capacitor with transmission line model

Abstract

A new electrode model involving the fractal structure of activated carbon used as electrode material was proposed for an electric double layer capacitor (EDLC). The fractal structure of activate carbon was simulated by branch pore structure of three sizes of cylindrical pores. Three sizes of cylindrical pores were related to macro, meso, and micro pore, since the pore size of activated carbon has wide distribution with a few modes of nm order. The impedance spectrum of EDLC describes the locus of blocking electrode in low frequency range, and the curve at an angle of almost 45° to real axis in high frequency range on the Nyquist plane. The low and high frequency ranges of the impedance spectrum were defined as a lumped constant-type and a distributed constant-type, respectively. Computer simulation of electrochemical impedance with the present electrode model was carried out to understand the relation between the impedance and the electrode structure. The contributions of five parameters to impedance spectrum were discussed, i.e., depth of pore, diameter of pore, specific resistance, the interfacial impedance at electrode/solution, and branch number. The specific resistance p influenced on the shape of impedance spectrum in distributed constant-type range significantly. On other hand, the interfacial impedance at electrode/solution interface controlled the shape of the impedance spectrum in lumped constant-type range. In the course of the curve-fitting to impedance spectrum of EDLC, the separated impedance spectra related to macro, meso, and micro pores were obtained, and the roles of these pores on electric capacity were discussed.