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

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°′ = +1.03 V vs. SCE, shows no electrochemical response near its E°′. 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 ∼100 mV more negative than E°′[(PQ^2+/+)_n]_surf = -0.45 V vs. SCE in CH₃CN/0.1 M [n-Bu₁N]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⁺·xFe(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.