Kinetics of redox switching of electroactive polymers using the electrochemical quartz crystal microbalance. Part I. Identifying the rate limiting step in the presence of coupled electron/ion and solvent transfer

The oxidation of poly(vinylferrocene) (PVF) films in aqueous bathing solutions proceeds via the oxidation of the uncharged PVF followed by entry of water into the film. The change in mechanism between rate controlling coupled electron/ion and solvent transfer was studied in 0.1 M sodium perchlorate bathing solution using the electrochemical quartz crystal microbalance (EQCM). The film's redox switching was investigated using cyclic voltammetry at voltage scan rates between 0.004 and 0.080 V s(-1). The controlling kinetic step depends on the instantaneous oxidation state of the film, the voltage scan rate and its direction. These quantities determine the relative fluxes of water and counter ion during the redox cycle; the flux ratio at any charge level is diagnostic of the instantaneous rate determining step. We give these diagnostic relations, and show how to use EQCM data to obtain these flux ratios. The flux ratio that maintains the (pseudo) equilibrium population ratio in the PVF film is the dividing line between kinetic control by water transfer or counter ion transfer. High charge fluxes are likely to be associated with kinetic control by solvent transfer. Low charge fluxes lead to kinetic control by coupled electron/counter ion transfers. Both kinetic situations are observed with PVF under the conditions that were used here.