Incorporation of 1,1 '-dibenzyl-4,4 '-bipyridinium cation into Nafion films and charge transport in loaded film electrodes

This paper describes the partitioning equilibrium of 1, 1 ' -dibenzyl-4,4 ' -bipyridinium cation (BV2+) between aqueous electrolytes and Nafion films, and the influence of BV2+ -Nafion interaction on the rate of charge transp ort in loaded films. The voltammetric charge measured at BV2+ -loaded film electrodes demonstrated that the redox ion was highly concentrated in the p olyanionic film from its dilute aqueous solution. An electrochemically poor response of highly loaded polymers resulted from the deactivation of the i ncorporated BV2+, which was due to the dehydration of the polymer. Although hydrophobic interactions contributed to some extent to bind the redox spec ies to the polymer, the reduced positive charge of the redox species weaken ed the binding. Thus, an electrostatic interaction between them is a major factor determining the incorporation of BV2+ into the polymer. The impedanc e response of the loaded film electrodes was consistent with the behavior e xpected for the diffusion of charge carriers through a layer of finite thic kness. When the surface concentration of BV2+ increased from 2 to 40 nmol c m (-2), the apparent diffusion coefficient for charge transport decreased b y a factor of 14. This dependence indicates a primary role of the physical diffusion of BV2+ in charge transport through the film. A decrease in the B V2+ mobility with increased loading arises probably from the ionic associat ion between incorporated redox ions and polymer sulfonate groups, which are caused by a reduction in the film permittivity induced by cation exchange. (C) 2001 Elsevier Science Ltd. All rights reserved.