Characterizing Hydration of Ultrathin Film Anion Exchange Membranes Via in Situ Spectroscopic Ellipsometry

Anion exchange membranes (AEMs) play a critical role in optimizing the performance of various applications, including redox flow batteries and other ion separation technologies. This study investigates the balance between membrane selectivity and anion transport in ultrathin AEMs. Poly(4-vinylpyridine) was synthesized via initiated chemical vapor deposition (iCVD) and subsequently quaternized with bromoethane. Spectroscopic ellipsometry, combined with an in situ environmental cell, was employed to analyze film thickness, refractive index and volume % of water as a function of relative humidity. Results indicate that humidity results in increased membrane thickness (swelling) and a decrease in refractive index due to water absorption.

The extent of P4VP quaternization was found to increase water uptake, as the introduction of more fixed charges increases the thermodynamic driving force for swelling. Water content in the quaternized P4VP films was determined by modeling the optical properties of the hydrated films with the Bruggeman effective medium approximation, showing the direct correlation between water content and extent of quaternization. Additionally, experiments revealed that over prolonged exposure to high humidity, the maximum membrane thickness stabilized at approximately the same value, remaining consistent across subsequent tests. These findings highlight the important interactions between quaternization and hydration, providing insights essential for optimizing AEM performance in various ion separation applications.