Investigation of electron transfer processes at open circuit polyvinylferrocene films using rotating ring-disk electrodes

Methodology for evaluation of electron transfer processes at unbiased polymer films using a rotating ring-disk electrode is described. The technique is based on the measurement of the time dependent ring shielding current which results from depletion of a solution species by its reaction at a polymer film confined to the disk at open circuit. A mathematical description of the shielding currents at open circuit which considers electron transfer at the disk and mass transfer of solution species to the polymer film allows measurement of kinetic parameters. Redox reactions between oxidized polyvinylferrocene (PVF) and Fe(CN)₆^4-, and PVF and I₃^- in aqueous media are described in this context. The ring shielding currents exhibit mass transfer limited behavior when the bimolecular reaction flux exceeds the mass transfer flux of the solution species to the electrode. This condition is most prevalent at early stages of the bimolecular reaction when the concentration of the electroactive component of the polymer film is at a maximum. As the reaction proceeds the bimolecular reaction flux decreases due to the decreasing concentration of the polymer confined electroactive material and the process becomes kinetically limited. For simple single electron transfer processes such as the PVF⁺/Fe(CN)₆^4- reaction, the bimolecular reaction rate constant can be determined. Under proper conditions, the mass transfer dependent pseudo-first order rate constant, reaction order and overall stoichiometry can be determined for reactions of fractional order such as the PVF/I₃^- reaction. It is determined that each monomer unit of PVF reacts with 1.5 I₃^- in two steps: a rate determining electron transfer followed by fast incorporation of I₃^- to form PVFI₃.