COULOMBIC IRREVERSIBILITY AT POLYANILINE-COATED ELECTRODES BY ELECTROCHEMICAL SWITCHING

The electrochemical conversion of the conducting polymers between the conducting (C) and the insulating (1) state is associated with growth of the C zone in the film. In order to visually observe the conversion, the growth perpendicular to the film was simulated to that along the film. The I --> C conversion exhibited a well-defined growing boundary between the C and the I zone. In contrast, the C --> I conversion occurred rapidly and uniformly over the film. The latter conversion was not completed and hence the C species was left behind in the film. The subsequent I --> C conversions showed a vague boundary. These variations were examined semi-quantitatively by the time-dependence of the light absorption of the film responding to the switching. As a more quantitative measurement, amounts of the faradaic charge consumed by the C --> I conversion were evaluated from cyclic voltammograms. The amount of the anodic charge was larger than that at the cathodic one. The ratio of the charge was independent of the potential sweep rate v for v > 0.6 V s-1. A quarter of the C species had no experience of the I --> C conversion owing to the self-cutoff of the electric path from the C zones toward the electrode. This may be equivalent to the threshold of the random percolation, and hence is regarded as a thermodynamically significant quantity.