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

Abstract

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+), E⁰' = +1.03 V vs. SCE, shows no electrochemical response near its E⁰'. 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 E⁰'[(PQ^(2+/+))ₙ]_(surf) = -0.45 V vs. SCE in CH₃CN/0.1 M [n-Bu₁N]C1O₄. 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.

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