Transport of uncharged redox species through an organic thin film analyzed under pseudo-impermeable boundary conditions

Abstract

The mathematical models that are typically employed for describing the responses of chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) of guest species that diffuse into an impermeable host material have been adapted for the case of an electrode covered with an organic thin film (TF). The mathematical solution has been used for simulating and fitting the electrochemical responses of ferrocene-methanol (FcMeOH) on the surface of a working electrode modified with a TF of highly plasticized polyvinyl chloride. The analysis of chronoamperometric profiles allows us to determine the diffusion coefficient and the concentration of the electroactive species in the TF. Also, from those values is possible to estimate values for the partition coefficient of FcMeOH between an aqueous solution and an organic film, as well as for the film thickness. The EIS responses exhibited remarkable consistency with the CA results. In the high frequency range, the TF system displayed a Warburg-type behavior, while at low frequencies, Nyquist's plot revealed a pronounced capacitive behavior due to the limited insertion of FcMeOH. The fit of CA and EIS responses yielded a set of resistance and capacitance values from which it is possible to infer that the diffusion resistance is significantly larger than the charge transfer resistance and that there are no trapping sites that could be associated with imperfections in the lattice of PVC. To our knowledge, this is the first instance where a model developed for the electrochemistry of insertion has been adapted to elucidate the partitioning of redox species between two immiscible liquids, followed by a charge transfer electrode process.