Molecular Modeling of Structural Transformations in Ionomer Solutions and Membranes

Different functions are expected from the polymer electrolyte membranes used in fuel cells. They work as a proton conduction medium, as a separator, and as an electronic insulator. The current membrane materials of choice are perfluorosulfonic acid (PFSA) ionomers such as Nafion. The two main challenges that PFSAs still face, after three decades of extensive research, are a limited lifetime and a lack of basic structural understanding. To investigate the chemical degradation phenomena, we devised a kinetic model of radical formation and attack to PFSA ionomers. Analytical relations are derived to obtain the content of aggressive radicals as a function of iron ion content and hydrogen peroxide. The mean-field type, coarse-grained ionomer model distinguishes ionomer headgroups, side chains, and ionomer backbone. The model is used to study the impact of different degradation mechanisms and ionomer chemistries on PEM degradation. Application of the model to degradation data of various PFSAs highlights the important role of radical attack to the ionomer headgroups. The insufficient understanding of the membrane structure thwarts further forays in degradation modeling. To this end, we undertook molecular dynamics simulations of the conformation of single chain ionomers as a function of different structural parameters. This study revealed the nonmonotonic effect of the side chain length and density on the conformational behaviour and rigidity of ionomer backbones. We discuss how the changes in these architectural parameters change the ionomer affinity to counterions and the corresponding ion mobility. Studying the aggregation of ionomer chains revealed their spontaneous aggregation in dilute solution. We explored the effect of various parameters such as ionomer hydrophobicity and side chain content on ionomer bundle formation. Minimization of the surface free energy of hydrophobic backbones is the driving force of ionomer aggregation, while the repulsion of anionic headgroups opposes the aggregation. The results rationalize the experimental studies and highlight the role ionomer bundles as the prevailing structural motif in PFSA materials.