Electron transfer properties of a redox polyelectrolyte based on ferrocenated imidazolium

Abstract

We report the characterization of a redox polyelectrolyte based on ferrocenated imidazolium and imidazole repeating units to evaluate the impact of the charged structure on the electron transfer rates. N-vinyl-N′-(methylferrocene)imidazolium chloride monomer was polymerized in the presence of various amounts of vinylimidazole to obtain the electroactive heteropolymer, as the homopolymerization was unsuccessful due to electrostatic repulsion between the imidazolium monomers. NMR was used to determine the ratio of electroactive to neutral monomers in the polymer. The electrochemistry of a thin film of the polymer casted on the surface of glassy carbon electrode was studied in 1 M NaClO4 aqueous electrolyte. Cyclic voltammetry at low scan rates (i.e. ν < 0.1 V s−1) showed a response close to the expected behavior for a surface-confined process without any significant interactions between the ferrocene redox centers, in contrast with poly(vinylferrocene). At high scan rates (ν > 1 V s−1); the behavior was diffusion-controlled. The diffusion coefficient of the ferrocene centers in the imidazolium polymer is 1.7 × 10−9 cm2 s−1 which is one order of magnitude higher than poly(vinylferrocene), and the standard rate constant, determined by the Nicholson's approach, was k0 = 3.8 × 10−4 cm s−1. These high values are explained by favorable interactions of the electrolyte with the charged imidazolium, providing a fast ion diffusion and pairing with ferrocenium. The potential use of this redox polymer in electrochemical systems is demonstrated via the formation of a composite with electrochemically exfoliated graphite to increase polymer loading.