The Studies of Interfacial Property and Water Dynamic Process within Self-Humidifying Confined PFSA-Zeolite Composite Membrane

Abstract

Self-humidifying proton exchange membrane fuel cell (PEMFC) is important in the development of portable fuel cell devices as it avoids the need for bulky eternal humidification equipment. The use of inorganic additives and confinement of the perfluorosulfonic acid (PFSA) are shown to provide advantages in membrane hydration. More recently, we demonstrated that confinement of subnanoliter volumes of PFSA within zeolite coated walls enables fuel cell to operate at high temperatures and dry conditions. The resulting PEMFC delivered higher performances than those of fuel cells made with traditional PFSA membrane. It is believed that the narrow confinement of PFSA and its surface interaction with the zeolite influence the polymer chain conformation and therefore the transport pathway through the PFSA volume. Using focused ion beam under cryogenic conditions, the membrane dissected microscopically to reveal the structure and composition at the interfacial region between the PFSA and zeolite. Exfoliated sections were examined under high resolution electron microscopy. Water dynamics include water diffusion in PFSA matrix, its transport across the interface and reversible adsorption and retention within the zeolite. Real-time monitoring of water dynamics was carried out using quartz crystal microbalance at different water activities. The results indicate that the anchorage of PFSA polymer on zeolite intercrystalline pores and the oriented PFSA chain distribution within confined space increase mechanical and thermal stabilities of composite membrane, and accelerate water adsorption, exchange and transport processes.