Modelling of morphology and proton transport in PFSA membranes

Abstract

Computational modelling studies of the structure of perfluorosulfonic acid (PFSA) ionomer membranes consistently exhibit a nanoscopic phase-separated morphology in which the ionic side chains and aqueous counterions segregate from the fluorocarbon backbone to form clusters or channels. Although these investigations do not unambiguously predict the size or shape of the clusters, and whether or not the channels percolate the matrix or if the connections between them are more transient, the sequence of co-monomers along the main chain appears strongly to influence the domain size of the ionic regions, with more blocky sequences giving rise to larger domain sizes. The fundamental insight that substantial rearrangement of the sulfonic acid terminated side chains and fluorocarbon backbone takes place during swelling or shrinkage is borne out by both molecular and mesoscale simulations of model PFSA polymers, along with ab initio electronic structure calculations of minimally hydrated oligomeric fragments. Molecular-level modelling of proton transport in PFSA membranes attests to the complexity of the underlying mechanisms and the need to examine the chemical and physical processes at several distinct time and length scales. These investigations have revealed that the conformation of the fluorocarbon backbone, flexibility of the sidechains, and degree of aggregation and association of the sulfonic acid groups under minimally hydrated conditions collectively control the dissociation of the protons and the formation of Zundel and Eigen cations. The former appear to be the dominant charge carriers when the limiting water content allows only for the formation of a contact ion pair with the tethered sulfonate anion. As the water content increases, solvent-separated Eigen ions begin to appear, indicating that the dominant mechanism for diffusion of protons occurs over a region approximately 4 Å away from the sulfonate groups. Finally, both the vehicular and Grotthuss shuttling mechanisms contribute to the mobility of the protons but, surprisingly, they are not always correlated, resulting in a lower overall diffusion coefficient. In summary, as the preceding observations indicate, the state of computational modelling of PFSA membranes has progressed sufficiently over the last decade to enable its use as a powerful predictive tool with which to guide the process of designing novel membrane materials for fuel cell applications.