(413f) Molecular Dynamics Simulations of Nanoscale Hydrophilic Domain Structure in Hydrated Nafion

Experimental elucidation of the nanoscale structure of hydrated Nafion, the most popular polymer electrolyte membrane (PEM) to date, and its influence on macroscopic proton conductance is particularly challenging. While it is generally agreed that hydrated Nafion is organized into distinct hydrophilic domains within a hydrophobic matrix, the geometry and length scale of these domains continues to be debated. For example, at least half a dozen different domain shapes, ranging from spheres to cylinders, have been proposed based on experimental SAXS and SANS studies. Since the characteristic length scale of these domains is believed to be ~2 to 5 nm, which is approximately equal to the box size of a typical ~5,000 atom PEM system, large atomistic or coarse-grained molecular dynamics (MD) simulations are needed to accurately probe the structure and morphology of these domains, especially their connectivity and percolation phenomena at varying water content. Using classical, all-atom MD with explicit hydronium ions, we have simulated very large hydrated Nafion systems (~2.3 million atoms in a cube with a side length of ~30 nm) to directly observe several hydrophilic domains at the molecular level. The first system was built using a Monte Carlo algorithm without directly assuming any particular geometry or morphological model. For comparison, two more systems were built based on popular morphological models in the literature: the older cluster-network model of Gierke and the newer parallel cylinder model of Schmidt-Rohr and Chen. This talk will explore the results of these simulations, which may help provide guidance for the determination of the morphology of Nafion and the design of new alternative membranes with superior proton conductance that will improve overall PEM fuel cell performance.