Effects of the secondary counterions in the electrochemistry of polypyrrole

Thin polypyrrole layers (thickness: 0.2 to 0.3 mu m) on gold were studied in aqueous solutions of sodium hexafluoroaluminate, sodium hexafluorosilicate, sodium hexafluorophosphate, and sodium chloride. Polypyrrole was prepared by the electrochemical oxidation of pyrrole in the chloride solution. The systems were examined by cyclic voltammetry and impedance spectroscopy. Transport/redox capacitances, effective charge diffusion coefficients, and charge transfer resistances were determined at several d.c. potentials of the polymer electrodes in a region of the reversible change of the redox/conductivity state of polypyrrole: the redox capacitance values of the oxidized polypyrrole are in the range of (0.7 to 0.9) X 10(3) Fcm(-3), and the diffusion coefficients are in the range of 10(-8) to 10(-9) cm(2) s(-1); the charge transfer resistances are higher for the secondary counterion systems than for the primary counterion system. Contributions from the bulk and interfacial phenomena of the polymer electrode to the electrochemical characteristics were separated and the overall mechanisms of the redox process of polypyrrole in studied electrolyte solutions are postulated. The thin layer polypyrrole electrode was found to respond more slowly to the electrical perturbations when working in the secondary counterion systems. The significance of the polycation-counterion-co-ion interactions in the mechanism of the transport and transfer of charges in the polymer electrode is pointed out A contribution of sodium ions to the charge transport in the polypyrrole-hexafluoroaluminate system is postulated. (C) 1997 Elsevier Science S.A.