Multi-scale Modeling of Proton Transport, Water Distribution, and Methanol Permeability in Proton Exchange Membranes (PEMs); Final rept

摘要

We have undertaken our multiscale modeling of proton exchange membranes through three simultaneous but distinct studies of perfluorosulfonic acid (PFSA) systems at different time and length scales: (1) hydrated morphology of PFSA membranes with dissipative particle dynamics (DPD) simulations; (2) classical MD simulations of the hydration and hydronium ion diffusion of the short-side chain (SSC) PFSA membrane; and (3) ab initio density functional theory Car-Parrinello MD simulations of model PFSA systems. A course-grained study of the hydrated morphology of Nafion and the SSC ionomer as a function of water content and equivalent weight (EW) was undertaken using DPD simulations. The results were analyzed through water contour plots, radial distribution functions of the water, and scattering plots. This work provides the first insight into how EW and side chain chemistry of the ionomer effects morphology. In the second study a unique force field was derived for the fully atomistic MD simulations of the SSC ionomer. The EW was fixed but the water content (from minimal to intermediate) was varied. Structural data and proton diffusion coefficients were computed on a small system of the SSC ionomer and provide a base line for simulations that treat the atoms as classical particles only. Finally, we performed density functional theory Car-Parrinello MD simulations of the mono-, di- and tetra- hydrates of trifluoromethanesulfonic acid as model systems for minimally hydrated PFSA polymers. We developed a consistent set of dispersion corrected atom-centered potentials to account for van der Waals interactions in these solids in order to match structural and lattice parameters.