Charge effectiveness of sulfonated polymer membranes under low-water-content condition

In this study, the effective charge density of a sulfonated polymer membrane was investigated under low-water-content condition in order to develop novel polyelectrolyte membrane for the fuel cell which can be operated without humidification. At first, the sulfonated polymer membranes, which have various water contents and an inhomogeneous fixed-charge distribution, were prepared, and then the membrane potentials across them were measured. The experimental results were analyzed on the basis of the Donnan equilibrium theory and the Nernst–Planck transport equation considering the fixed-charge inhomogeneities in the membrane. It was revealed that the charge effectiveness depended on the water content, the ionic radii of the counterions and the valence of the counterion. In order to interpret the variation of the charge effectiveness depending on those values, the ion-pair theory based on the Fuoss formula was introduced. This approach showed that the ion-pairing effect causes the difference between the effective charge density and the fixed-charge density especially for low-water-content condition.