Molecular Insightsof Hydrated Perfluoro QuaternaryAmmonium Anion-Exchange Membranes: Ionomer Structure and HydroxideIon Transport

Perfluoro membranes are recognized electrolytes for fuel cell applications due to their outstanding electrochemical properties. Perfluoro membrane precursors are used to synthesize anion-exchange membranes with certain modifications. Quaternary ammonium-modified perfluoro membranes successfully demonstrated their suitability and performance for prolonged fuel cell operation. This work aims to probe the characteristics of a hydrated perfluoro membrane (Nafion) consisting of trimethylammonium cationic head using molecular dynamics (MD) simulations. The atomic interactions of the membrane with hydroxyl ions in the hydrated matrix were analyzed by using radial distribution functions at varying hydration. The time-averaged pendant chain length and radius of gyration of the polymer chain revealed the extended form of the pendant. Distinct interchain/intrachain interactions were observed for the quaternary ammonium pendant tail. Spatial distribution maps depict specific distribution patterns of hydroxyl ions and water molecules. The water cluster around the trimethylammonium group of pendants becomes more uniform with increased hydration. The mobility of hydroxyl ions and water molecules is found to be higher in quaternary ammonium-modified perfluoro membranes than in poly(vinylbenzyltrimethylammonium) anion-exchange membranes. The results explore the future possibility of quaternary ammonium-modified perfluoro membranes for alkaline fuel cell applications.