Understanding the Mechanism of Ionic Conductivity in an Anhydrous Proton-Conducting Electrolyte Through Measurements of Single-Ion Diffusion Coefficients

Abstract

The single-ion diffusion coefficients of H+ and MePEGn SO3- ions have been electrochemically measured in a series of anhydrous proton-conducting electrolytes composed of mixtures of MePEGn SO3 H acid (where n=3, 7, 12, or 16) dissolved in our MePEG7 polymer. The electrochemically measured H+ single-ion diffusion coefficients in this electrolyte are dependent on temperature such that, at low temperatures (25°C), the H+ cation is the primary charge carrier in the four acid/polymer solutions tested. However at high temperatures, the diffusion coefficients of the MePEG7 SO3- and MePEG12 SO3- anions are larger than the corresponding H+ diffusion values, yielding a smaller fraction of charge carried by H+ (tH+ values of 0.29 and 0.41, for the MePEG7 SO3 H and MePEG12 SO3 H mixtures at 85 C). While the diffusion coefficient of the large MePEG16 SO3- anion does not overtake DH+ at 85 C, its transference number nonetheless increases with increasing temperature. We conclude that our electrolyte is dominated by Grotthus conductivity at low to moderate temperatures, while the contribution of the vehicle mechanism of H+ conductivity increases at high temperatures. © 2006 The Electrochemical Society.