Use of the flux ratio method for mechanistic diagnosis in electroactive polymer film redox switching

The redox switching of poly(vinylferrocene) (PVF) films was investigated using the electrochemical quartz crystal microbalance in conjunction with cyclic voltammetry (at different voltage scan rates) and reverse potential steps. The mechanism of the redox process was determined using PVF films supported on Au electrodes and exposed to aqueous bathing solutions of 0.1 M sodium hexafluorophosphate. PVF electro-oxidation proceeds via coupled oxidation of uncharged ferrocene sites and entry of counterion and is followed by the entry of water into the film. Structural changes within the polymer may also accompany the latter two steps. Any of these three steps may be the slowest for particular redox conditions. The controlling kinetic step depends on the film's instantaneous water content, its instantaneous oxidation state, the electrochemical control function, the direction of redox switching, and the associated time scale of the experiment. We describe a new general quantitative approach based upon comparison of the instantaneous fluxes of solvent (water) and counterion during the redox cycle to characterize the rate-controlling process as a function of the extent of film oxidation. This new methodology has the capability to resolve time scale- and potential- (charge-) dependent mechanistic shifts and film relaxation phenomena as they are reflected through the ratio of fluxes of solvent and counterions.