Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Science

Background:

• Nafion membranes are crucial for ion transport in energy devices due to their water absorption and ion conductivity.
• The performance of Nafion membranes is highly dependent on their hydrated morphology and composition.
• Understanding water uptake kinetics is essential for optimizing membrane performance under varying conditions.

Purpose of the Study:

• To investigate the influence of interfacial interactions on Nafion membrane water uptake kinetics.
• To compare water absorption rates and morphological changes when exposed to liquid water versus water vapor.
• To provide insights into Schroeder's paradox concerning ionomer water content.

Main Methods:

• In situ small-angle X-ray scattering (SAXS) experiments were used to observe real-time membrane swelling.
• Nafion membranes were exposed to both liquid water and water vapor to study uptake differences.
• Analysis of nanostructure rearrangement and morphological changes during water absorption.

Main Results:

• Water uptake from liquid water is rapid (seconds) with significant nanostructure rearrangement.
• Water uptake from vapor is four orders of magnitude slower, indicating interfacial resistance.
• Liquid water absorption induces a shift from spherical/cylindrical domains to exclusively cylindrical domains.

Conclusions:

• Interfacial resistance significantly impacts water penetration into Nafion membranes from vapor.
• Differences in water uptake kinetics and resulting morphology challenge existing theories like Schroeder's paradox.
• These findings offer critical insights for designing robust energy conversion devices that handle dynamic environmental conditions.