Beyond Nafion: Structure-Property Relationships in Novel Perfluorinated Ionomers with Tunable Transport Properties

Abstract

Rapid improvements in fuel cell performance have been driven by the development of commercially available ionomers used as membranes and catalyst binders in membrane electrode assemblies (MEAs). Commercially available ionomers share a semi-crystalline polytetrafluoroethylene (PTFE) matrix, which imparts low gas permeability and high mechanical and makes them attractive membrane materials. At the same time, the slow permeation of gases (i.e., oxygen and hydrogen) through the ionomers introduces significant mass-transport losses in the catalyst layers, ultimately limiting fuel cell performance. In this study, we present a new family of perfluorinated ionomers that incorporate an amorphous matrix based on a perfluoro(2-methylene 4-methyl-1,3-dioxolane) (PFMMD) backbone. The introduction of a PFMMD backbone disrupts the crystallinity of the matrix, increases the ionomers’ free volume, and significantly improves their gas permeability (permeability >3x that of Nafion). On the other hand, the dioxolane backbone increases the glass transition temperature of the matrix, restricting its mobility, which in turn limits the ionomer domain swelling and reduces its proton conductivity. In this presentation, I will present a facile and flexible synthesis method for PFMMD-based ionomers with tunable sulfonic acid content. Furthermore, I will describe structure-property relationships in these materials derived from a combination of transport measurements (i.e., water uptake, proton conductivity, and permeability) and morphological characterization through Small- and Wide-angle X-ray scattering. Lastly, I will discuss the implication and potential utilization of these new materials for improved fuel cell performance. The results presented here underscore the significance of tailoring materials chemistry to specific requirements of fuel cell or electrolysis MEAs.