Impact of chemical treatments on the behavior of water in Nafion® NRE-212 by 1H NMR: Self-diffusion measurements and proton quantization

In this article, we have studied by 1H NMR the behavior of water in Nafion under acid, sodium and potassium forms which arises from different chemical treatments with or without EDTA. Self-diffusion coefficients were measured by PGSTE (Pulsed Gradient STimulated Echo) whereas integration of the proton spectrum provides a quantitative information about water content. Examination of the self-diffusion coefficients of water in Nafion shows clearly that: (i) in the acid form, the use of a chelating agent does not affect translational diffusion of water, (ii) regardless of sample conditioning, water within Nafion in acid form diffuses more rapidly than in sodium and potassium forms, in particular at low water content (at λ = 3.4, D = 5.4 × 10−7 cm2 s−1 vs. 7.2 × 10−8 cm2 s−1), and (iii) for a given water amount, water diffusion is identical in sodium and potassium forms. Point (ii) is due to the presence of oxonium ions within Nafion in acid form, favoring both vehicle and Grotthuss mechanisms. It is also due to a higher number of protons available for diffusion (we find an average value of 1.19 in acid form vs. 0.91 in sodium form). Our results suggest in the case of Nafion in acid form that, at low water content, diffusion occurs by jumps between different sites. Moreover, plotting self-diffusion coefficients vs. relative humidity leads to a clear differentiation between sodium and potassium cations, related to their respective solvation number.