(325a) Elucidating Structure-Property Relationships in Highly Permeable Perfluorinated Sulfonic Acid Ionomers
Rapid improvements in fuel cell performance have been driven by perfluorinated sulfonic acid ionomers (PFSIs), utilized as polymer electrolytes in membrane electrode assemblies (MEAs). PFSIs have a phase separated nanostructure characterized by a hydrophobic polytetrafluoroethylene (PTFE) matrix and a strongly acidic sulfonated side-chain. The semicrystalline PTFE matrix imparts mechanical integrity and low gas permeability, making these materials attractive as membranes. However, their widespread implementation in electrode catalyst layers results in significant mass-transport resistance that limits oxygen reduction at the cathode and is a significant barrier towards low catalyst loading fuel cells. In this study, we present the synthesis of a PFSI incorporating perfluoro(2-methylene 4-methyl-1,3-dioxolane) (PFMMD) in the backbone, resulting in an amorphous and highly permeable matrix. This impacted the material nanostructure at multiple length scales, simultaneously increasing gas permeability (>3x oxygen permeability of Nafion) via increased fractional free volume while reducing proton conductivity due to changes in matrix physical properties which limit phase separation of ionomer domains. This trade-off yielded significant improvements in a fuel cell MEA; current density per cm2 platinum was increased by up to 22% by substituting the PFMMD based ionomer for Nafion in the cathode binder. In this presentation, I will discuss the facile synthesis of PFMMD based ionomers with tunable sulfonic acid content. Next, I will discuss structure-property relationships resolved from transport measurements and morphological characterization of these materials. Finally, I will discuss the implementation of these materials in fuel cells and the implications for future ionomer design. These results demonstrate the value of rational ionomer design toward better performing electrochemical devices.