(431b) Molecular Dynamics Simulations Of Nanoscale Hydrophilic/hydrophobic 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/hydrophobic domains, 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. Using classical, atomistic MD with explicit hydronium ions and a flexible water molecule, we have simulated a very large hydrated Nafion system (~2.3 million atoms in a cube with a side length of ~30 nm) at varying water contents to directly observe several hydrophilic domains at the molecular level and to probe the morphology and lifetimes of the connecting ?structures? between them. This talk will explore the results of these simulations, which may help provide guidance for the design of new alternative membranes with superior proton conductance that will improve overall PEM fuel cell performance.