Effect of Charge Transport in Electrode-Confined N,N'-Dialkyl-4,4'-bipyridinium Polymers on the Current-Potential Response for Mediated, Outer-Sphere Electron-Transfer Reactions

Article abstract of DOI:10.1021/j150654a017

Mediated outer-sphere redox processes have been examined at rotating disk Pt/<(PQ^2+/+)n>_surf electrodes.The <(PQ^2+/+)n>_surf is a redox polymer anchored to the surface and is formed from N,N'-bis<(trimethoxysilyl)propyl>-4,4'-bipyridinium, I.The polymer coverages for the electrodes selected for study are sufficiently great that Fe(phen)3^3+/2+, E^0' = +1.03 V vs.SCE, shows no electrochemial response near its E^0'.The mediated reduction of Fe(phen)3³⁺ and a number of other outer-sphere oxidants is mass-transport limited when the Pt/<(PQ^2+/+)n>_surf electrode is held ca. 100 mV more negative than E^0'<(PQ^2+/+)n>_surf = -0.45 v vs.SCE in CH3CN/0.1 M ClO4.However, contrary to theoretical expectations based only on the rate constant for reaction of Fe(phen)3³⁺ with a surface PQ⁺, the onset of current for the mediated reduction is at the onset for <(PQ²⁺)n>_surf -> <(PQ⁺)n>_surf reduction; in fact, the mediated reduction current in the onset region is directly proportional to the concentration of PQ⁺ in the surface-confined polymer.Data for Pt/<(PQ²⁺*χFe(CN)6^3-/4-)n>_surf electrodes show directly that charge transport in the polymer can be a limitation to the maximum steady-state mediation current in aqueous electrolyte solution at the coverages of <(PQ^2+/+)n>_surf that have been employed.The charge-transport properties of the polymer are concluded to control the current-potential profile, as has been reported previously for other surface-modified electrodes, for the large polymer coverages employed in these studies.