Computational modelling and structural studies of perfluorosulfonate ionomer membranes

摘要

Perfluorosulfonate ionomer membranes (PIMs) consist of a polytetrafluoroethylene, or "Teflon", backbone with sulfonic acid groups periodically substituted along the chain, and are of great commercial interest due to their peculiar ion transport properties. In particular, under certain conditions, these membranes are selectively conductive, passing cations preferentially to an-ions. This makes them ideal as efficient membrane separators in redox fuel cells. It has been known for some time that the interesting properties of PIMs derive from the microscopic phase separation of hydrated ionic material and the fluorocarbon matrix. However the precise nature of this remains controversial. In this paper, we present data from small angle X-ray scattering (SAXS) and molecular dynamics studies which provide convincing evidence that PIMs possess an ion-clustered morphology with a structural hierarchy. In particular, a model-independent instantiation of the segregation between polar and non-polar material can be obtained from a maximum entropy interpretation of the SAXS data. Such models are consistent with surface images of membranes taken with atomic force microscopy, and molecular dynamics simulations show that these structures demonstrate selective conductivity in the presence of an applied electric field.