Water-Transport Resistance of Perfluorosulfonic-Acid (PFSA) Membranes: Delineating the Role of Interface and Chemistry

The proton-exchange membrane (PEM) is an integral component in PEM fuel cells, water electrolyzers, and other electrochemical energy-conversion devices. In these devices, the PEM must conduct protons but inhibit reactant/product gas crossover; its operation depends heavily on its hydration level. Furthermore, PEMs exhibit complicated water management balanced between transport phenomena, interfacial resistance, and the electro-osmosis vs back-transport trade-off, all of which also depend on external environmental conditions. In this paper, we investigate interfacial and bulk water-transport resistance for perfluorosulfonic-acid (PFSA) membranes with long, medium, and short side-chain lengths (Nafion, 3M, and Aquivion, respectively) in controlled temperature-humidity environments. Using a factorial design of experiments based on 12 chemistry-thickness × 16 environmental conditions, a large systematic dataset is constructed. Transport resistance decreases with increasing temperature and RH, with the latter having an order-of-magnitude higher impact. Water content (λ), or water volume fraction (ϕ), is the key controlling parameter, where all resistances decrease with increasing water content. The interfacial resistance diminishes with increasing side-chain length, while the bulk resistance is the lowest for the medium side-chain length, indicating that the water-transport relationship with the internal polymer structure is more convoluted. Activation-energy analysis further reveals that interfacial resistance requires substantially more thermal energy to overcome than bulk resistance, confirming that the two are governed by distinct transport mechanisms that respond differently to side-chain chemistry and hydration conditions. These findings provide a mechanistic framework for tailoring PEM chemistry to balance water transport and proton conductivity in next-generation membrane design.