Nanometer-Scale Water Dynamics in Nafion Polymer Electrolyte Membranes: Influence of Molecular Hydrophobicity and Water Content Revisited

Abstract

The ionic conductivity of polymer electrolyte membranes (PEMs) is an essential parameter for their device applications. In water-swollen PEMs, protons and other ions are transferred through hydrophilic channels of a few nanometers in diameter at most. Thus, optimizing the chemical and physical properties of the channels can enhance the conductivity of PEMs. However, the factors controlling the conductivity have not been completely clarified. Here, we report that measurements taken near the channel walls by a special nuclear magnetic resonance technique with ≤1 nm spatial resolution showed the largest water diffusivity when ∼80% of hydrophilic sulfonic acid groups were blocked, but the proton conductivity was low. The water diffusivity was much less affected by differences in water content. Our results provide a concept for changing the properties of PEMs and a challenge to implement the improved diffusivity in a way that enhances net ion conductivity.

Pulsed Field Gradient Nuclear Magnetic Resonance and Diffusion Analysis in Battery Research

“…Here, we introduce and briefly discuss Overhauser dynamic nuclear polarization (ODNP) relaxometry for diffusion measurements. As Saun et al point out, ODNP-NMR can measure the dynamics of water/proton systems with time and length scales of 0.01–1 ns and ≤1 nm, respectively, in contrast to PFG-NMR which typically probes ranges covering 0.001–1 s and ∼1 μm. As an example, these shorter length and faster time scales in ODNP-NMR made it possible to discriminate the 4 orders of magnitude faster diffusion of water/protons in the vicinity of the sulfonyl (−SO 3 – ) acid groups and the fluorocarbon surfaces compared with water/proton diffusion through the core of water-channels in Nafion membranes .…”