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

The mechanism of charge transport through Nafion polymers coated on glassy carbon electrodes 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 polymer 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 2 0 , 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" (electrodes 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 electrodes and has been the subject of numerous recent inve~tigations.~,~.~-" Charge can be transported 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 polymer coatings. The relative contributions of electron transfer (electronic conduction) and diffusion to charge transport in electrochemical (1) (a) Merz, A.; Bard, A. J. J. Am. Chem. SOC. 1978,100, 3222-3223. (b) Itaya, K.; Bard, A. J. Anal. Chem. 1978,50, 1487-1489. (2) (a) Van De Mark, M. R.; Miller, L. L. J. Am. Chem. SOC. 1978, 100, 3223-3224. (b) Kerr, J . B.; Miller, L. L. J. Electroanal. Chem., Interfacial Electrochem. 1979, 101, 263-267. (c) Miller, L. L.; Van De Mark, M. R. Ibid. 1978.88, 437-440. (d) J. Am. Chem. SOC. 1978, 100, 639-640. (3) (a) Kaufman, F. B.; Engler, E. M. J. Am. Chem. SOC. 1979, 101, 547-549. (b) Kaufman, F. B.; Schroeder, A . H.; Engler, E. M.; Kramer, S. R.; Chambers, J. B. Ibid. 1980, 102, 483-488. (4) (a) Abruiia, H. D.; Denisevich, P.; Umaiia, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. SOC. 1981, 103, 1-5. (b) Denisevich, P.; Willman, K. W.; Murray, R. W. Ibid. 1981, 103,4727-4737. (c) Nowak, R.; Schultz, F. A.; Umaiia, M.; Abruiia, H. D.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1978, 94, 219. (d) Nowak, R.; Schultz, F. A,; UmaAa, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980,52, 315-321. (e) Facci, J.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1981, 124, 339-342. ( f ) Lenhard, J . R.; Murray, R. W. J. Am. Chem. SOC. 1978,100, 787C-7875. (9) Daum, P.; Murray, R. W. J. Electroanal. Chem. Interfacial Electrochem. 1979, 103, 289-294. (h) Daum, P.; Murray, R. W. J. Phys. Chem. 1981, 85, 389-396. (i) Daum, P.; Lenhard, J. R.; Rolism, D. R.; Murray, R. W. J. Am. Chem. SOC. 1980, 102, 4649-4653. (5) (a) Oyama, N.; Anson, F. C. J. Electroanal. Chem. Interfacial Electrochem. 1980,127,247-250. (b) J . Am. Chem. SOC. 1979,101,3450-3456. (c) Anal. Chem. 1980, 52, 1192-1198. (d) Oyama, N . ; Shimomura, T.; Shigehara, K.; Anson, F. C. J. Electroanal. Chem. Interfacial Electrochem. 1980, 127, 640-647. (e) Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 20, 518-522. (f) J . Am. Chem. SOC. 1981, 103, 2552-2558. (6) Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J. Am. Chem. SOC. (7) Wrighton, M. S.; Palazzotto, M. C.; Bocarsly, A. B.; Bolts, J . M.; Fisher, A. B.; Nadjo, L. J. Am. Chem. SOC. 1978, 100, 7264-7271. (8) Diaz, A. F.; Kanazawa, K. K.; Gardin, G. P. J. Chem. SOC., Chem. Commun. 1979, 635. (9) Lanron, E. J. Electroanal. Chem. Interfacial Electrochem. 1980, 112, 1-9. (b) Laviron, E.; Roullier, L.; Degrand, C. Ibid. 1980, 112, 11-23. (10) Andrieux, D. C.; Saveant, J . M. J. Electroanal. Chem. Interfacial Electrochem. 1978, 93, 163-168. (b) Ibid. 1980, 111, 377-381. (c) Andrieux, C. P.; Dumas-Bouchait, J. M.; Saveant, J . M. Ibid. 1980, 114, 159-163. (11) Anson, F. C. J. Phys. Chem. 1980, 84, 3336-3338. (12) Peerce, P. J.; Bard, A . J . J. Electroanal. Chem. Interfacial Electrochem. 1980, 112, 97-115. 1977, 99, 3 154-3 158. systems were first considered for solution processes by DahmsI3 and Ruff.I4 Similar (and equivalent) representations were later given for theoretical models of polymer e l e c t r o d e ~ . ~ J ~ The basic idea is that during the electrochemical reduction of a species, represented by A, at an electrode (A + eA-), A can be brought to the electrode by diffusion of A-, driven by its concentration gradient, and also by the electron-transfer reaction