Water in Perfluorinated Sulfonic Acid Nafion Membranes

Abstract

We have studied water contained in the perfluorinated sulfonic acid (denoted as RSO3H) membrane by FT-IR spectroscopy. The IR spectra of the membrane, which absorbed water quickly even from the air, changed so drastically during dehydration that they showed clearly two isosbestic points in the 4000−1500 cm-1 region. Analysis of the IR spectral changes made it clear that water molecules exist mostly as H3O+ in equilibrium with RSO3- and RSO3H in the membrane. The IR change accompanying dehydration was found to result from the reaction as RSO3- + H3O+ → RSO3H + H2O(evaporating out). The IR spectrum of H3O+ was clearly separated, which shows a broad OH stretching absorption with two unseparated peaks around 3370 and 3120 cm-1 and an OH deformation absorption at about 1720 cm-1. The spectrum is distinctively different from that of water separated, coexisting with H3O+, RSO3-, and RSO3H in the membrane, which shows OH stretching and deformation absorptions at about 3480 and 1628 cm-1, respectively.