Effects of Electron Exchange and Single-File Diffusion on Charge Propagation in Nafion Films Containing Redox Couples

The concentration dependence of the diffusion coefficients of redox ions that undergo rapid electron exchange predicted by Dahms and Ruff and co-workers has been observed and measured for cobalt(2,2′-bipyridine)32+, Co(bpy)32+, incorporated in Nafion coatings applied to graphite electrodes. The diffusion coefficient of Co(bpy)32+ was measured both by reducing it to Co(bpy)3+ and by oxidizing it to Co(bpy)33+. The resulting diffusion coefficients differed considerably. That measured by reduction of Co(bpy)32+ increased linearily with its concentration while that measured by oxidation of the complex was much smaller and exhibited a concentration dependence of the opposite sign. The latter is attributed to the effects of single-file diffusion which involves competition between the diffusing species for residence sites within the coating. The surprisingly wide range of diffusion coefficients that have been reported for structurally similar cations incorporated in Nafion coatings is explained in terms of a model for the interior of Nafion coatings that includes both hydrophilic and hydrophobic phases between which incorporated reactants are partitioned. Two general classes of diffusional behavior for redox reactants in Nafion are identified and examples of each class are provided.