Electron transport in quaternized poly(4-vinylpyridine) films containing pentacyanoferrate(II/III) on electrodes. The influence of the binding type of the electroactive complex

Abstract

This study is an attempt to investigate how charge transport in thin redox polymer films spin-coated onto glassy carbon electrode surfaces is affected by the way in which the electroactive group is bound to the polymeric matrix. Two different redox polymers based on methylated and cross-linked poly(4-vinylpyridine) and pentacyanoferrate(II/III) have been synthesized. In the first case, an anionic complex formed by coordinating pyridine to pentacyanoferrate(II) was electrostatically bound to the methylated pyridine groups in the polymer matrix. In the other case, pentacyanoferroate(II) was coordinated directly to unmethylated pyridine groups in the partly methylated polymer film. The film thicknesses were estimated with ellipsometric measurements. Their electrochemical characteristics were investigated using cyclic voltammetry, chronocoulometry and impedance spectroscopy. The apparent charge-transport diffusion coefficient D app was almost two orders of magnitude greater in the ion-exchange polymer than in the redox polymer with coordinatively bound electroactive groups. The temperature dependence of D app was evaluated using Arrhenius plots. The total amounts of electro-active groups in the polymer films were determined from both the low frequency capacity measured by impedance spectroscopy (IS) and the integration of cyclic voltammetric peaks (CV). For low concentrations of redox sites there was good agreement between the two methods, but at high concentrations the results from IS were systematically lower than those from CV. This phenomenon has been observed earlier in this laboratory for other systems [34,35] and implies that under certain conditions a smaller fraction of the electroactive complexes is electro-active in IS than would be expected. No satisfactory explanation for this behaviour has been reported in the literature to our knowledge, but similar observations have been made earlier [36]. We speculate that the reasons for the reduced electro-activity in IS may include the following: (i) the existence of two parallel pathways for electron transport, one of which is very slow and does not contribute to the response in IS; (ii) aggregation of the electro-active complexes at higher loading, where the aggregates are electro-active only when a major potential perturbation is applied to the polymer film. Charge transport in the two systems was determined with two different methods: chronocoulometry (CC) and IS. Since there is a discrepancy in concentration determination depending on the method used, it is necessary to assess which determination is the more correct. In IS the low frequency capacitance was used to determine the concentration needed to calculate the diffusion coefficient from impedance measurements at medium frequencies in the Warburg region. It is thus assumed that the same amount of complex is electro-active in the two frequency ranges. When the diffusion coefficient is to be estimated from CC, we suggest that integration of voltammetric peaks should be exploited in the concentration estimation because both these methods involve a complete transformation of the redox state of the film. No significant differences were obtained in the diffusion coefficients determined using the two methods. This indicates that the charge transport is not affected by the method with which it is measured, at least for the systems under study. The apparent diffusion coefficient is about two orders of magnitude greater in the ion exchange polymer film than in the film with coordinatively bound electro-active groups. This is explained by the possibility of physical translation of the redox complex within the ion-exchange polymer matrix which augments charge transport. In the coordinated polymer, where the complex is firmly bound to the polymer chain, electron hopping alone is responsible for charge propagation.