Characterization of polypyrrole films by impedance spectroscopy

Abstract

Impedance spectroscopy was used to investigate the electrochemical redox behavior of polypyrrole films in LiDS, NaDS and TBADS aqueous solution, respectively and all the films were electrochemically polymerized in LiDS aqueous solution. Nyquist plots of PPy films showed that the charge transfer process was a rate-controlling step at reduction potential while the diffusion process was a rate-controlling step at oxidation potential. The charge transfer control at reduction state resulted from the release of some of DS- in PPy film irrespective of its bulkiness. Also, the impedance at reduction potential was much smaller than that at oxidation potential in the same frequency range and the smaller impedance was due to the ionic conductivity of salts which were formed by the small cations and DS- remained in PPy film during the reduction process since the mobility of DS- is slow. This behavior was confirmed by electrochemical quartz crystal microbalance (EQCM) results : The decrease in mass during reduction process was obviously due to the partial loss of DS- from PPy on dedoping and the fact that the mass decrease at the reduction state is less than the mass increase at the oxidation state is due to formation of salts by cations, The impedance spectra of PPy films at reduction state were well interpreted by Randles equivalent circuit and the modified Warburg impedance was introduced to obtain more accurate diffusion coefficients at oxidation state where ion diffusion controls the whole process.