Transport Mechanism in Nafion Revealed by Detailed Comparison of 1 H and 17 O Nuclear Magnetic Resonance Diffusion Coefficients.

We use 1 H and 17 O 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 id 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 + (H 2 O) m ions with neighboring SO 3 - groups lead to slower water dynamics in the vicinity of the polymer framework than in the center of the water nanochannels.