Mesoscale Simulation of Morphology in Hydrated Perfluorosulfonic Acid Membranes
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Mesoscale Simulation of Morphology in Hydrated Perfluorosulfonic Acid Membranes James T. Wescott,a) Yue Qi,b)* Lalitha Subramanian,c) and T. Weston Capehart b) a) Accelrys Ltd., 334 Cambridge Science Park, Cambridge, CB4 0WN, U.K. b) GM R&D Center, MC:480-106-224, 30500 Mound Rd., Warren, MI 48090-9055 c) Accelrys Inc., 10188 Telesis Court, Suite 100, San Diego, CA 92121 ABSTRACT Current fuel cell proton exchange membranes (PEM) 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 (PFSA) (equivalent weight of 1100) as a model 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. For λ8; 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. INTRODUCTION The interest in fully perfluorinated perflurosulfonic acid (PFSA) polymer membranes, like Nafion has been prompted by their wide application in polymer electrolyte fuel cells as proton exchange membranes (PEM). The PFSA structure consists of a highly hydrophobic polytetrafluoroethylene (PTFE) backbone with a fully perfluorinated ether side chain terminated by the strongly hydrophilic –SO3H group. This leads to spontaneous phase segregation at the nano-structural level. For fully hydrated PFSA, the sulfonic groups and water develop an interconnected proton conducting network while the fluorocarbon backbone forms a semicrystalline hydrophobic phase. Further improvements in PEM performance for fuel cell applications require increasing the proton conductivity at low relative humidity (RH) and high temperature[1]. A detailed knowledge of the nano-scale morphology would be useful in reaching this goal since ionomer morphology determines the network connectivity of the membrane that strongly influences its conductivity. A thorough review by Mauritz and Moore[2] summarizes the efforts in the last two decades to understand the morphology of Nafion. Based on variety of scattering experiments, various models have been suggested, such as cluster-channel model [3], core-shell model[4], lamellar model[5], sandwich-like model[6], channel model[7], and rod-like model.[8]. The common feature of all these models is the nano-phase segregation into hydrophilic and hydrophobic domains, but the detailed shape and structure of the ionic clusters and semicrystalline polymer
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