ELECTROCHEMICAL INVESTIGATIONS OF ELECTRONICALLY CONDUCTIVE POLYMERS .7. CHARGE TRANSPORT IN LIGHTLY DOPED POLYPYRROLE

We, and other researchers, have shown that the mechanism and rate of charge transport in polypyrrole films (on electrode surfaces) depend on whether the polymer is in its electronically conductive or electronically insulating state. This paper focuses on the mechanism and rate of charge transport in the electronically insulating state, i.e., we describe results of electrochemical investigations of polypyrrole films which were equilibrated at initial potentials negative of -0.30 V versus SCE. With regard to the mechanism of charge transport, we show that this lightly doped material behaves like a redox polymer. In particular, we show that, like other redox polymers, redox reactions for lightly doped polypyrrole begin at the polymer/electrode interface and propagate to the polymer/solution interface. With regard to the rate of charge transport, we show that apparent diffusion coefficients for polypyrrole synthesized from acetonitrile solutions containing 2% added water are significantly lower than charge transport rates in films synthesized in rigorously dried acetonitrile. Finally, we report exchange current densities associated with the oxidation of lightly doped polypyrrole. These exchange current densities are on the order of 4 mA/cm2, indicating relatively facile electron transfer kinetics.