Solvation of sulphonic acid groups in Nafion® membranes from accurate conductivity measurements

The wide degree of scatter of experimental data shows unambiguously that Nafion (R) 117 conductivity is very sensitive to climatic conditions, temperature and relative humidity. In order to allow equilibrium with the surrounding atmosphere to be reached rapidly, a conductivity cell, adapted to a small size membrane samples, has been designed. Conductivity measurements have been carried out by electrochemical impedance spectroscopy (EIS) between 10 degrees C and 95 degrees C in a broad domain of relative humidity, i.e., 10-98%. The measurement set up provides reproducible data with a total uncertainty of 30% in a large range of conductivity values. These accurate data enable a power relationship to be proposed at constant temperature between conductivity and relative humidity. This suggests that the solvation process of a sulphonic acid group involves four water molecules. Based on the assumption that a proton mobility depends weakly on temperature, a solvation enthalpy of a sulphonic acid group by water of -135 kJ mol(-1) is deduced from conductivity variations with the temperature. (c) 2006 Elsevier B.V. All rights reserved.