ION ASSOCIATION AND ELECTRIC-FIELD EFFECTS ON ELECTRON HOPPING IN REDOX POLYMERS - APPLICATION TO THE OS(BPY)3(3+)/2+ COUPLE IN NAFION

The high ionic content and low dielectric constant that prevail in the interior of many redox polymers might be expected to accentuate the extent of ionic association within the polymers. The present study is an attempt to investigate the effects of ionic association on the dynamics of electron hopping within such materials. The general concepts of the treatment are exemplified by the experimental system chosen for study: the Os(bpy)3(3+/2+) redox couple incorporated in Nafion films on electrode surfaces. Extensive ion association between both halves of the redox couple and the fixed anionic groups in the Nafion is argued to be responsible for the irreversibility of the incorporation. The apparent diffusion coefficient, which reflects the dynamics of electron propagation in the films, exhibits a remarkably sudden increase as the quantity of the redox couple in the film approaches electrostatic saturation with respect to the fixed anionic sites present. The observed behavior is not satisfactorily accounted for by previous models which have been proposed, even those that take account of the presence of electric fields within the films. The introduction of ionic association into the model for electron propagation leads to predictions that are in accord with the observed behavior. A key ingredient in the successful model is the assumption that the predominant forms of the electroactive counterions incorporated in Nafion are neutral aggregates resulting from association with several of the fixed anionic sites. Dissociation of the fully associated, neutral, oxidized half of the redox couple into a singly charged species which is associated with the same number of fixed anionic sites as the predominant form of the reduced half of the redox couple prior to electron self-exchange appears to be the lowest energy pathways for this process. The presence of ionic association equilibria which control such prior dissociation provides a satisfactory explanation for the steep increase in the apparent diffusion coefficient, i.e., in the rate of electron self-exchange between pairs of the incorporated redox couple, as the concentration approaches the saturation value. The steep increase results from a shift in the ion association equilibrium to produce more of the partially dissociated, oxidized half of the redox couple, which is the best partner for accepting an electron from the fully associated, reduced half of the redox couple. The inevitable electric fields present within the polyelectrolyte films also affect the observed behavior, especially as the concentration of incorporated electroactive counterions is increased, but the simultaneous presence of ionic association is required to produce predicted behavior which matches that observed experimentally.