Electrochemical impedance study of the polymerization of pyrrole on high surface area carbon electrodes

Polymerization of pyrrole on a Vulcan XC-72 carbon black coated glassy carbon electrode was performed using potentiodynamic and galvanostatic regimes in an aqueous solution of 0.5 M pyrrole and 0.5 M NaClO4. Responses of the electrodes prepared under different conditions were recorded using cyclic voltammetry and electrochemical impedance spectroscopy. It was found that voltammograms of electrodes prepared galvanostatically showed a reversible oxidation–reduction of the polypyrrole. The redox waves became less reversible, characterized by greater peak separations, when the electrodes were prepared potentiodynamically. These observations were investigated by electrochemical impedance spectroscopy, which revealed significant charge transfer resistances for the electrodes prepared potentiodynamically. The bulk resistances of the electrodes prepared by the two methods also differed, both in magnitude and their dependence on the polypyrrole loading and potential. These differences, in combination with scanning electron microscopy and voltammograms recorded during potentiodynamic polymerization, were used to elucidate differences in the structures of the polypyrrole–carbon black composites. The polymerization voltammograms showed a two-step nucleation on the carbon black coated electrodes rather than the one-step polymerization on a bare electrode. This was attributed to initial polymerization on the carbon particles followed by sustained polymerization on top of the carbon black layer. More uniform deposition of polypyrrole on the full area of the carbon particles was achieved by slow galvanostatic polymerization.