Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes

Abstract

An atomistic model for perfluorinated ionomer membranes (PIMs), in particular Nafion materials, is presented and used in conjunction with NVT molecular dynamics simulations to investigate the dynamic and configurational properties of these polymers. It is found that the electrostatic term in the force field is responsible for the formation of an apparently phase separated morphology which is selectively conductive, favouring the passage of cations. Specifically, the mobility of H₃O⁺ ions is found to be ∽3.2 times greater than that of OH^- ions, under the application of an external electric field. This phenomenon is shown to be consistent with a jump diffusion model of ion transport in PIMs. There is also evidence for the existence of water in two distinct environments in the simulations: both tightly bound to ion exchange groups, and more loosely associated with the fluorocarbon matrix.

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