Phenomenological description of Clark redox reactions at electrodes coated with conducting polymer films. Part II. Mechanism of tetracyanoquinodimethane and iodine electroreduction at polythiophene

Studies of the voltammetric behavior of polythiophene (PT) film electrodes in solutions containing various redox forms have established that the mechanism and kinetics of electrochemical reactions depend on the nature and concentration of mobile charge carriers within the polymer. As has been shown in Part 1 (J. Electroanal. Chem., 271 (1989) 193), at potential values close to the ferrocene/ferrocenium (Fc/Fe⁺) standard equilibrium potential in acetonitrile, the concentration of mobile charge carriers in PT is relatively large so that the Fc oxidation process at the polymer electrode can be described by the quasi-reversible diffusion wave equation, similar to this reaction at the uncoated platinum electrode. At the same time, the reduction process of tetracyanoquinodimethane (TCNQ) at PT is characterized, to a first approximation, by the equation for redox reactions of species adsorbed on the electrode surface (whereas at the platinum electrode this process occurs according to the diffusion mechanism). The dependence of the voltammetric peak height on the upper potential sweep limit is explained by the relatively low value of the TCNQ/TCNQ^- redox couple standard potential, at which PT displays the properties of an organic semiconductor. Molecular iodine, which is a stronger oxidizer than TCNQ, is reduced at PT by the mechanism of direct injection of polarons, similar to the injection of holes in inorganic semiconductors.