Electron transport dynamics in partially quaternized poly(4-vinylpyridine) thin films containing ferri/ferrocyanide

Abstract

Electron transport dynamics were studied for the Fe(CN) 6 3− 4− couple incorporated in partially-quaternized polyvinylpyridine (QPVP) modified electrodes. The goal was to identify the major factors controlling electron diffusion in the electroactive polymer films. Variable-temperature (0–30° C) chronocoulometry was employed to measure the electron diffusion coefficients ( D E) and activation energies ( E a) for the electron transport. The E a values were in the range of 24–80 kJ/mol (6–20 kcal/mol); E a increased steadily as the concentration of the ferri/ferrocyanide couple was increased in the films; E a was also influenced strongly by the anionic species (nitrate, p-toluenesulfonate, and perchlorate) in the aqueous electrolyte adjacent to the film. The D E values in the presence of nitrate were compared to those measured by a form of steady-state voltammetry. The electron diffusion coefficients were essentially equal by both methods. The permeability of Fe(dmbpy) 3 2+ ion was measured in QPVP films loaded with Fe(CN) 6 3− 4− and exposed to nitrate; the observed permeation rates ( D S · κ) were normally much larger than the electron diffusion coefficients. The results suggest that counterion motion is not a factor limiting the electron motion. The polymer lattice clearly becomes more crosslinked upon taking up the redox ions, an effect that induces a loss in short-range ion mobility within the film and causes a decline in D E as the film is more heavily loaded. The anion dependence of E a was explained on the basis of changes in internal structure reflecting interactions between the cationic polymer lattice, the anionic species in the background electrolyte, and the solvent. Perchlorate produces nearly dehydrated, rather impermeable films. The anion dependence is shown to be a structural, not a dynamic, effect.