Thermodynamics and Kinetics of Redox Switching of Polyvinylferrocene Films in Perchlorate Solutions

Abstract

Cyclic voltammetry was used to create nonequilibrium populations of different solvation and configurational states of partially oxidized polyvinylferrocene (PVF). Oxidation levels were established by scanning either from a fully reduced state to the desired oxidation level or from a fully oxidized state to the desired level. Coulostatic conditions were then established by opening the external circuit, and subsequent mass and potential changes were followed. The film's approach to equilibrium involves configurational changes within the polymer and simultaneous and subsequent solvent transfer. At very short times (t ≤ 0.2 s) the approach to equilibrium is limited by both solvation and reconfiguration processes. For a short time afterward (0.2 < t/s < 1.0) reconfiguration alone is rate limiting. At intermediate times (1 < t/s < 5) both processes play comparable roles. At long times (t > 5 s) solvation is the controlling step. The electroactive polymer film does not completely reach equilibrium even after 1 h at open circuit as evidenced by continuous small mass changes. These mass changes are the result of water transfer between the polymer film and the bathing electrolyte. A scheme of cubes model rationalizes mechanistic pathways leading to equilibrium. In particular, the observed extrema in mass (solvent population) are predicted. The electrode potential, after 1 h at open circuit, shows nearly Nernstian dependence on the redox composition for film states produced by either anodic or cathodic cyclic voltammetric scans. These Nernst plots are displaced by only a few millivolts because only a weak Nernstian dependence on film water content exists. Films that are 50% oxidized exhibit a sub-Nernstian response with respect to the perchlorate concentration in the bathing solution under permselective conditions.