Frequency-dependence of electric double layer capacitance without Faradaic reactions

Abstract

This report reviews the time-dependence of the double layer (DL) capacitors in the polarized potentials domains and the related subjects, which have been observed as the behavior of the constant phase element (CPE). The DL capacitance decreases with an increase in the ac-frequency of the applied voltage, obeying a power law of the frequency. This variation necessarily yields an electric resistance component at the interfaces. The DL resistance does not really exist but is involved inevitably in the measured current as a result of the time-dependent capacitance. It cannot be discriminated against the heterogeneous kinetics with the Butler-Volmer type. The frequency-dependence agrees with the behavior of the CPE. The DL capacitance is brought about by orientation of solvent molecules, independent of concentrations of salts, kinds of salts, and applied dc-potential. The orientation is modeled by combination of the external electric field, the image force of polarized charge of solvent on the electrode, and the interaction of nearest neighboring solvent molecules. The combination belongs to cooperative phenomena because the orientation is competitive with the molecular interactions energetically. The competition generates two-dimensional local phases, prolonging the orientation much larger than flip rates of solvent dipoles.