Perfluoro-sulfonic acid (PFSA) polymers are used as the electrolyte and separator in most solid-state energy-conversion and storage devices. These materials must conduct the ions in a mechanically robust matrix that simultaneously inhibit reactant and product crossover. The performance and durability of the membranes are related to their properties governed by the physiochemical interactions between chemical structure, water uptake, and morphology. The transport of water and ions of these membranes is critical for their operation in various electrochemical technologies.  In this talk, the structure/function relationships of these membranes will be examined with a focus on the interface, which is thought to control the transport. Through a combination of time-resolved SAXS, dynamic vapor uptake, computed x-ray tomography, conductive AFM, and dynamic and steady-state transport-property measurements, a theory and data for the transport of ions and water have evolved. It will be shown how the interface and particularly its resistance is critical for controlling water movement into the polymer, which subsequently determines the membrane's structure.  For example, Figure 1 shows how the mass-transfer coefficient measured for steady-state transport correlates to the membrane surface's conductive-area fraction as measured by AFM.  This relationship shows that the interface and accessibility to water is what is causing the transport resistance (inverse of the mass-transport coefficient). In addition, ways to alter the nature of the interface including polymer moieties, environmental conditions, etc. will be discussed. Acknowledgement This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Fuel Cell Technologies, of the U. S. Department of Energy under contract number DE-AC02-05CH11231

Critical role of the interface in controlling transport in PFSA membranes

248th National Meeting (2014)

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