(Invited) Structure-Property Relationships in Highly Permeable Dioxolane-Based Perfluorinated Ionomers

Abstract

Ion conducting polymers used as membranes and catalyst binders in membrane electrode assemblies (MEAs) have driven continuous improvements in fuel cell performance. Commercially available membrane materials (e.g., Nafion®) impart low gas permeability, high chemical and mechanical stability thanks to their semi-crystalline polytetrafluoroethylene (PTFE) matrix. This dense matrix also slows permeation of gases through the polymer, introducing significant mass-transport losses in the catalyst layers and limits fuel cell performance. In this presentation, I will discuss a new family of perfluorinated ionomers that incorporates 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 matrix free volume, and significantly improves its gas permeability.1 The dioxolane backbone also increases the glass transition temperature of the matrix, impacting its mobility, limiting the ionomer domain swelling and reducing its proton conductivity. I will describe a flexible synthesis method for PFMMD-based ionomers which allowed us to study ionomers with tunable sulfonic acid content and derive structure-property relationships through 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 incorporation of dioxolane monomers with bulky, asymmetric side-chains results in a stiff ionomer matrix with distinct chemical and mechanical properties. 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 elucidated the impact of the mass fraction and matrix chemical structure on water sorption rates. The 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.