Mesoscale simulation of morphology in hydrated perfluorosulfonic acid membranes

Abstract

Current fuel cell proton exchange membranes rely on a random network of conducting hydrophilic domains to transport protons across the membrane. Despite extensive investigation, details of the structure of the hydrophilic domains in these membranes remain unresolved. In this study a dynamic self-consistent mean field theory has been applied to obtain the morphologies of hydrated perfluorosulfonic acid membranes (equivalent weight of 1100) as a model system for Nafion at several water contents. A coarse-grained mesoscale model was developed by dividing the system into three components: backbone, side chain, and water. The interaction parameters for this model were generated using classical molecular dynamics. The simulated morphology shows phase separated micelles filled with water, surrounded by side chains containing sulfonic groups, and embedded in the fluorocarbon matrix. The size distribution and connectivity of the hydrophilic domains were analyzed and the small angle neutron scattering (SANS) pattern was calculated. At low water content (lambda<6, where lambda is the number of water molecules per sulfonic group) the isolated domains obtained from simulation are nearly spherical with a domain size smaller than that fitted to experimental SANS data. At higher water content (lambda>8), the domains deform into elliptical and barbell shapes as they merge. The simulated morphology, hydrophilic domain size and shape are generally consistent with some experimental observations.

Modelling of morphology and proton transport in PFSA membranes

“…Nevertheless, Wescott et al 100 found that, in common with the DPD simulations of Yamamoto and Hyodo 94 there was a reasonable agreement between the absolute cluster sizes predicted by SCMFT and those observed in scattering experiments. Significantly, they found a linear increase in characteristic size scale of the morphologies with increasing water content, which provides a counterexample to earlier suppositions that this must necessarily imply a lamellar morphology.…”

Phase Separation and Water Channel Formation in Sulfonated Block Copolyimide

“…The commercial force field COM-PASS was used, and their potential parameters were listed in the literature. 19,43,44 All of the molecular simulations were carried out using a Hewlett-Packard workstation and the Material Studio software package (Accelrys Inc., CA, U.S.A.).…”

“…For PEMs, extensive studies have been carried out to investigate their peculiar structures and proton transport behavior. 19,20 Paddison used ab initio strategy to study proton dissociation of the sulfonic acid group in triflic acid and p-toluene sulfonic acid, simulating Nafion and PEEKK models, respectively. 21 Later, he and Elliott performed ab initio calculations to investigate proton dissociation of the short side chain of a perfluorosulfonic acid membrane such as Nafion and the resulting proton transfer with solvent molecules.…”

Molecular Simulations of Hydrated Proton Exchange Membranes: the Structure

“…Wescott et al [45], for example, performed mesoscale simulations of hydrated Nafion using a self-consistent mean field theory approach and a coarse-grained model, deriving the interaction parameters from classical MD. In this study, four systems (with different water contents) with an initially homogeneous distribution of the components were simulated at room temperature.…”