(161s) In Situ Characterization of the Dynamic Emergence of Nanostructure and Transport Properties in Perfluorinated Sulfonic Acid Ionomer Thin Films
The need to increase power density in low-catalyst loading fuel cells has motivated the development of novel polymeric ionomers with high gas permeability for electrochemical catalyst layers, where the ionomer forms a nanometer-thick film around the catalyst and electrode material. An emerging class of ionomer incorporates high fractional free volume (FFV) monomers copolymerized with the perfluorinated sulfonic acid (PFSA) side chain common to traditional ionomers. Such ionomers phase separate into a hydrophobic and highly gas-permeable matrix and hydrophilic acid-rich domains, which provide pathways for ion transport. However, the bulky, asymmetric monomers which have shown promise for improving gas permeability also result in a stiff matrix with chemical and mechanical properties that diverge significantly from those of common PFSA ionomers (e.g., Nafionâ¢) with a rubbery and semi-crystalline polytetrafluoroethylene (PTFE) matrix. In this study, we compare dynamic humidity-driven processes in Nafion and perfluorodioxolane-based ionomer thin-films with varied matrix mass fractions to gain fundamental insights into the dynamic emergence of transport processes and nanostructure in ionomers. Through in situ water uptake measurements, we demonstrate decoupled effects of the mass fraction and chemical structure of the matrix 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, revealed through in situ Grazing-Incidence Small Angle X-ray Scattering (GISAXS) measurements, showing that initial water sorption is associated with rapid ionomer domain swelling while the polymer relaxation corresponds to a slower ordering these domains. These unique structure-performance relationships revealed by polymer dynamics studies can help guide the design of novel high-performing ionomers.