(452c) Toward a Predictive Understanding of Water and Charge Transport In Proton Exchange Membranes

   An analytical model for water and charge transport in highly acidic and highly confined systems such as proton exchange membranes of fuel cells is developed and compared to available experimental data. The model is based on observations from both experiment and multiscale simulation. This model has its basis in the molecular-level mechanisms of water transport but has been coarse-grained to the extent that it can be expressed in an analytical form. The model accounts for three factors in the system: (1) acidity, as characterized by the concentration of hydronium ions in the volume of the aqueous domain, (2) confinement, as characterized by the interfacial surface area per water molecule, and (3) connectivity, as characterized by the percolation theory. The effect of acidity is determined from bulk hydrochloric acid solutions. The effect of confinement is determined from water in carbon nanotubes. The effect of connectivity is determined by fitting percolation theory to the self-diffusivities water in Nafion from the classical MD simulations.

   Several important results were found. First, an integrated multiscale simulation approach including both molecular dynamics simulation and confined random walk theory is capable of quantitatively reproducing experimentally measured self-diffusivities of water in the perfluorinated sulfonic acid proton exchange membrane material, Nafion. The simulations, across a range of hydration conditions from minimally hydrated to fully saturated, have an average error for the self-diffusivity of water of 16% relative to experiment. Second, accounting for three factors—acidity, confinement, and connectivity—is necessary and sufficient to understand the self-diffusivity of water in proton exchange membranes. Third, an analytical model based on percolation theory is capable of quantitatively reproducing experimentally measured self-diffusivities of both water and charge in Nafion across a full range of hydration.