Comparative Study of Redox Reactions of Polyaniline Films in Aqueous and Nonaqueous Solutions

Electrochemical redox reactions of polyaniline films were investigated in aqueous and nonaqueous solutions, and the mechanism was discussed on the basis of comparison of those reactions in aqueous and nonaqueous media. The polyaniline films were prepared under two different conditions, at a constant current density in an aqueous solution (0.1 M aniline-0.2 M HC1O₄aqueous solution) and at a constant potential in a nonaqueous solution (0.1 M aniline-0.1 M LiC10₄acetonitrile solution). Cyclic voltammograms, in situ absorption spectra, conductivity, and spin density were measured for three different cases: redox reactions (i) in aqueous media using polyaniline films prepared in aqueous solution, (ii) in nonaqueous media using polyaniline films prepared in aqueous solution, and (iii) in nonaqueous media using polyaniline films prepared in nonaqueous solution. Cyclic voltammograms showed two pairs of redox peaks in all cases. Even in aprotic solvent, the redox reaction at a lower potential region was quite reproducible on repeated cycles. In situ absorption spectra and ESR spectra indicated the formation of radical cations at the first anodic peak for all cases. The conductivity of polyaniline reached a maximum at the first oxidation peak and decreased by the second oxidation. The conductivity change was correlated with that of the spin density calculated from ESR spectra. The results were similar in both aqueous and nonaqueous solutions. The potential dependence of in situ absorption spectra and conductivity was discussed in terms of a polaron model.