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

The single-ion diffusion coefficients of H+ and MePEG(n)SO(3)(-) ions have been electrochemically measured in a series of anhydrous proton-conducting electrolytes composed of mixtures of MePEG(n)SO(3)H acid (where n = 3, 7, 12, or 16) dissolved in our MePEG(7) polymer. The electrochemically measured H+ single-ion diffusion coefficients in this electrolyte are dependent on temperature such that, at low temperatures (25 degrees 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 MePEG(7)SO(3)(-) and MePEG(12)SO(3)(-) anions are larger than the corresponding H+ diffusion values, yielding a smaller fraction of charge carried by H+ (t(H+) values of 0.29 and 0.41, for the MePEG(7)SO(3)H and MePEG(12)SO(3)H mixtures at 85 degrees C). While the diffusion coefficient of the large MePEG(16)SO(3)(-) anion does not overtake DH+ at 85 degrees 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. (c) 2006 The Electrochemical Society.