Alternating current impedance study of a polypyrrole-based anion-exchange polymer
The ac impedance of poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium]+(poly-MPMP+) as a function of potential has been investigated in acetonitrile solutions. When the polymer is reduced it acts merely as a barrier to the migration of ions to the double layer of the underlying electrode. Thus the ionic conductivity of the reduced polymer can be obtained. This conductivity is not dependent on the concentration of the electrolyte in the bulk solution, indicating that there is little contribution from non-exchange electrolyte in the film. Oxidized poly-MPMP+ behaves as a finite transmission line with two resistance rails. One rail corresponds to the film's ionic resistance, which is virtually independent of potential. The other rail represents the film's electronic resistance, which increases as the potential of the film is decreased. Analysis of Nyquist plots allows determination of both of these resistances as well as the film's Faradaic capacitance. Quantitative results are compared with results from chronoamperometry and rotating-disc voltammetry.