Hydration and Water Molecules Mobility in Acid Form of Nafion Membrane Studied by 1H NMR Techniques

Abstract

Hydration of perfluorinated sulfonic cation-exchange membrane Nafion 117 in acidic ionic form was investigated. It was shown that cation H+ is coupled with two water molecules and hydroxonium ion H5O2+ at low water content is formed. The main feature of the temperature dependence of the self-diffusion coefficients is the increasing curve slopes in low-temperature regions. At high humidity activation energies do not depend on the water content above 0 °C and are close to the bulk water activation energy. The changes in slopes of self-diffusion coefficients and proton conductivity temperature dependencies are usually explained by the freezing of free water at temperatures below 0 °C. This explanation contradicts the temperature dependence of the amount of mobile water. Water does not freeze till λ = 9–10 (λ is the number of water molecules per sulfonate group), but, anyway, ice-like phase DSC peak is observed at λ ≤ 9. From analysis of the 1H NMR relaxation time temperature dependences, it was proposed that at the temperatures below 0 °C water molecules are associated, but are not able to form ice phase because part of their hydrogen bonds are occupied by sulfonate group oxygen and H+ cation. The mobility of these associates is rather high at negative temperatures; however, these associates show a DSC peak.