Self-diffusion micromechanism in Nafion studied by ²H NMR relaxation dispersion

Area of Science:

• Materials Science
• Physical Chemistry
• Polymer Science

Background:

• Nafion is a perfluorinated sulfonic acid polymer crucial for proton exchange membranes.
• Understanding water dynamics within Nafion is key to optimizing its performance in electrochemical applications.
• Previous studies have explored water diffusion and proton conductivity, but the precise coupling mechanisms remain under investigation.

Purpose of the Study:

• To investigate the dynamics of water confined within Nafion NR 212 using advanced NMR techniques.
• To elucidate the relationship between molecular rotations and long-range mass transport of water in Nafion.
• To explore the structural similarities between confined water in Nafion and ice.

Main Methods:

• Field Cycling (FC) ²H nuclear magnetic resonance (NMR) relaxometry was employed over a temperature range of 300 K to 190 K.
• The temperature-frequency superposition principle and master curve were used to extend the sensitive time window of FC NMR.
• ²H NMR diffusometry was used to measure translational correlation times.

Main Results:

• Rotational correlation times from ²H FC NMR were found to coincide with translational correlation times from ²H NMR diffusometry.
• This agreement indicates that long-range mass transport in Nafion is coupled to molecular rotations.
• The dynamics of confined water in Nafion suggest short-range ordering similar to ice, potentially involving D and L defects.

Conclusions:

• A strong coupling exists between molecular rotations and water diffusion in Nafion.
• The behavior of confined water in Nafion can be described using concepts derived from ice structures.
• The findings provide insights into the temperature and humidity dependence of self-diffusion and proton conductivity in Nafion.