Impedance analysis of poly(tetracyanoquinodimethane) electrodes: effect of electrolyte concentration and temperature

Abstract

Impedance measurements are reported for poly(tetracyanoquinodimethane) coated on Pt electrodes and immersed in aqueous solutions of LiCl. Three distinct regions are observed in the complex impedance plots for these polymer film electrodes: kinetic control at high frequencies, diffusional control at intermediate and charge saturation at low frequencies. The variation of ohmic resistance, double layer capacity, charge transport diffusion coefficient and exchange current density has been examined as a function of temperature at different concentrations of supporting electrolyte. The variation of the ohmic resistance measured as a function of temperature and electrolyte concentration follows the variation of the solution resistance which indicates that the film resistance plays practically no role at the formal potentials of the redox couples of the polymer. The temperature dependence of the double layer capacitance was interpreted in terms of the classical double layer models, but the variation of the effective charged area was also taken into account. Similar values were obtained for the activation energies of the charge transfer and charge transport processes. This result is explained in terms of the dominant role of the co-operative chain and segmental motions of the polymer in both processes. The rate of these processes is strongly influenced by the temperature and by the solvent swelling of the polymer film determined by the electrolyte concentration and temperature. The increase of the low frequency capacitance with decreasing electrolyte concentration and increasing temperature is assigned to the hindered polymer chain motion, as well as to the restricted motion of counterions bound to the charged sites of the polymer network.