Polymer films on electrodes. 8. Investigation of charge-transport mechanisms in Nafion polymer modified electrodes

Abstract

The mechanism of charge transport through Nafion polymers coated on glassy carbon and containing Cp2FeTMA+, R~(bpy),~+, and O~(bpy),~+ (where Cp2FeTMA+ is ((trimethylammonio)methyl)ferrocene, bpy = 2,2'-bipyridine) is described. The apparent diffusion coefficients, Dam,, of modified electrodes, as measured by conventional electrochemical methods and which can include contributions from intermolecular electron transfer, are compared to the diffusion coefficients of actual mass transport of the electroactive species through the polymer, D,, as measured from the rate of permeation of the electroactive material. The results indicate that Dapp = D, for Cp2FeTMA+, Dapp 20, for Os(bpy),*+, and DaP4 = 15D, for Ru(bpy)t+. These results are discussed in terms of the Dahms-Ruff model, where the observed diffusional behavior is due to both physical diffusion of the electroactive species and an electron-transfer component that can contribute to the observed behavior. The contribution of the electron-exchange mechanism toward the value of Dapp decreases in the order of electroactive species, Ru(bpy),*+ >> O~(bpy),~+ 2 Cp2FeTMA+. A key question in the behavior of polymer electrodes (elec- trodes coated with a thin layer of polymer1-*) involves the mechanism of charge transport through the layers. The rate of charge transport frequently governs the rate of electrochemical and catalytic processes at such and has been the subject of numerous recent inve~tigations.~,~.~- Charge can be trans- ported through the layer by electron transfer between redox centers, counterion diffusion, and diffusion of the electroactive species; the relative contribution of these effects is probably different for different kinds of coatings.