Transport Mechanism in Nafion Revealed by Detailed Comparison of¹H and¹⁷O Nuclear Magnetic Resonance Diffusion Coefficients

Abstract

We use 1H and 17O NMR static field gradient diffusometry to measure self-diffusion coefficients of protons (D H) and oxygens (D O) in Nafion 212 with various hydration levels (λ = 4–18). For all samples and both nuclei, we obtain activation energies (E a) of ≈0.19 eV. Analyzing the hydration-level dependence of D H and D O, we find D O/D H ≈ 1 at λ ≈ 18, resembling the situation in bulk water, while oxygen diffusion becomes faster than proton diffusion when the water content is decreased, leading to D O/D H ≈ 1.2 at λ ≈ 4. A comparison with literature data for acidic bulk solutions implies that faster oxygen than proton diffusion results from the existence of the polymer framework. To rationalize the observed ratios D O/D H ≥ 1, we consider a bimodal dynamical model in which the interactions of H+(H2O) m ions with neighboring SO3 – groups lead to slower water dynamics in the vicinity of the polymer framework than in the center of the water nanochannels.