Structure-Property Relationships in Highly Permeable Dioxolane-Based Perfluorinated Ionomers with Tunable Transport Properties

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 (e.g., Nafion^®) share a semi-crystalline polytetrafluoroethylene (PTFE) matrix, which imparts low gas permeability and high mechanical stability. 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 by a factor of >3x compared to Nafion).¹ 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 describe 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. We will show how the implementation of dioxolane monomers with bulky, asymmetric side-chains leads to a stiff ionomer matrix with chemical and mechanical properties that diverge significantly from those of common PFSA ionomers that contain polytetrafluoroethylene (PTFE) matrixes. By varying the dioxolane matrix mass fraction, we gained fundamental insights into the role of the matrix chemical structure on the dynamics of structural and transport processes in ion-conducting polymers.² Through in situ water uptake measurements, we demonstrate decoupled effects of the mass fraction and matrix chemical structure on water sorption rates. The Fickian mass-transport swelling rate of Nafion was around 50% higher than in dioxolane-containing ionomers and was independent of the matrix mass fraction. Non-Fickian polymer relaxation rate was sensitive to both type and fraction of the matrix, increasing by 22% across the range of dioxolane fractions studied and by another 100% in Nafion. These effects are attributed to reduction of segmental mobility of the hydrophobic matrix upon incorporation of dioxolane groups. The polymer relaxation is shown to correlate to changes in ionomer conductivity and nanostructure, reveled through in situ Grazing-Incidence Small Angle X-ray Scattering (GISAXS) measurements, showing two distinct physical processes that enable (i) rapid water sorption and ionomer domain swelling and (ii) polymer relaxation and ordering of ionomer domain structure. 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.

1. Katzenberg, A.; Chowdhury, A.; Fang, M.; Weber, A. Z.; Okamoto, Y.; Kusoglu, A.; Modestino, M. A., Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure–Property Relationships. Journal of the American Chemical Society 2020, 142 (8), 3742-3752.
2. Katzenberg, A.; Mukherjee, D.; Dudenas, P. J.; Okamoto, Y.; Kusoglu, A.; Modestino, M. A., Dynamic Emergence of Nanostructure and Transport Properties in Perfluorinated Sulfonic Acid Ionomers. Macromolecules 2020, 53 (19), 8519-8528.