NMR Relaxometry Study of the Interaction of Water with a Nafion Membrane under Acid, Sodium, and Potassium Forms. Evidence of Two Types of Bound Water

Through H-1 NMR relaxometry techniques (determination of the spin-lattice relaxation time as a function of the NMR measurement frequency), we have investigated, on a molecular scale, the water behavior in Nafion NRE 2.12 under acid, sodium, and potassium forms, the latter arising from different chemical treatments (with and without EDTA). Quantitatively, it turns out that (i) EDTA removes unwanted cations that may affect water mobility and (ii) the natural countercations (sodium and potassium) also affect water mobility according to their size. In order to go further, we have developed a new methodology that rests on the comparison between samples prepared with H2O and D2O. For the latter, residual protons allow us to exclusively access intermolecular contribution to proton relaxation and, thus, enable us to deduce the intramolecular contribution of proton relaxation in H2O. The analysis of this contribution reveals, for the first time, two types of bound water in Nafion.