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Electrochemistry of polypyrrole films

Electrochemically prepared films of polypyrrole have been studied as electrode coatings in an electrochemical cell with potassium ferricyanide as the electroactive species in solution. The electrode-solution interface was studied by AC impedance measurements over the frequency range 0.003-105 Hz. A novel model circuit description of the electrode-electrolyte interface was devised, which consisted of seven elements (including a frequency-dependent Warburg impedance) for the bare platinum electrode and ten elements (including an additional Warburg impedance concerned with film oxidation and reduction) for the polypyrrole-coated electrodes. The values of the circuit element were calculated by computer fitting and excellent agreement was found between measured impedances and those predicted from the calculated circuit element values. Where parameters could be calculated from known physical properties of the system (e.g., the Warburg parameters) these agreed well with the fitted values. Model circuit element values were calculated for the bare platinum electrode and for polymer-covered electrodes for polypyrrole films of four different thicknesses. The most striking result is the large drop in the charge transfer resistance for the ferricyanide reaction when the electrode is covered with the polypyrrole film. The large drop in this value is partly explained by the increase in surface area (which from other parameters can be estimated to be 40-fold), but might additionally reflect electrocatalytic properties of the polypyrrole surface.