Impedance analysis of galvanostatically synthesized polypyrrole films. Correlation of ionic diffusion and capacitance parameters with the electrode morphology

Impedance measurements of galvanostatically synthesized polypyrrole films using different polymerization charges, are reported at several stages of oxidation and analyzed in terms of distributed impedance models (transmission lines). Different physical mechanisms involved in the capacitive response observed at low frequencies are described in terms of Cole-Cole relaxation functions associated to ionic diffusion and charge storage processes with a distribution of diffusion lengths. The film thickness (polymerization charge) modifies the characteristic frequency of the observed relaxation, which varies heavily from 1 to 100 mHz, while the large specific capacitance (100-200 F cm(-3)) is found to be independent of the film size. A transition between two types of deposition mechanism from 3D nucleation and growth to 1D transversal growth after nuclei overlapping, explains the change of behavior of the relaxation characteristic frequency observed at Q approximate to 60 mC cm(-2). The value of this frequency at the transition point, where the diffusion length should be of the order of the film thickness, indicates that the diffusion coefficient D lies within the range 0.5-1.0 x 10(-12) cm(2) s(-1), which is a more realistic value than those obtained by previous analyses. (C) 2002 Elsevier Science Ltd. All rights reserved.