Thermodynamics of water vapor uptake in perfluorosulfonic acid membranes

A two-parameter equation describing water vapor uptake in perfluorosulfonic acid membranes is derived based on the Flory-Huggins model combined with a chemical equilibrium constant for proton transfer. Temperature-dependent e xpressions for the proton-transfer equilibrium constant K(T) = 0.0256 X exp [(22.4 kJ/mol)/RT] and the Flory interaction parameter chi(T) = 1.936 - (2. 18 kJ/mol)/RT art: discussed. Advantages of the new equation include accura te reduction of activity vs. water vapor uptake data for perfluorosulfonic acid membranes with few parameters and a well-behaved dependence of activit y on composition and temperature. Comparisons of model results with water u ptake measurements in the literature are given. An explanation for the diff erence in water uptake by membranes in contact with liquid water and with s aturated vapor is proposed based on an interpretation of the temperature de pendence of water uptake. (C) 1999 The Electrochemical Society. S0013-4651( 98)10-021-6. All rights reserved.