Modulating Perfluorinated Ionomer Functionality via Sidechain Chemistry

Ionomers are important to electrode function in energy conversion devices, such as fuel cells and electrolyzers, as the catalyst-binding nanometer-thick films under confinement, which compromises mass transport of species. Mitigating confinement effects is necessary for improved performance of polymer electrolyte fuel cells. Studies on perfluorosulfonic acid (PFSA) ionomers provided insights into origins of transport resistances, but limited improvements by current strategies necessitate a need for alternative chemistries to enhance film function. This work investigates new ionomers based on sidechain modifications of PFSA to tune local acidity and structure. Their application-relevant properties (hydration, thermal-transition) and nanomorphology are systematically characterized using environmental ellipsometry and grazing-incidence X-ray scattering, respectively, to develop chemistry-structure-property relationships in thin-film motif (≤100 nm). Introducing acid sites to sidechains improves hydration and nano-phase-separation but reduce chain mobility. Replacing the hydrophilic end-groups with hydrophobic perfluoro-groups enhances mobility but disrupts phase-separation. Although modifying sidechain functional groups alters ionomer-substrate interactions (Pt/C), chemistry has greater influence on structure-properties. Proton conductivity correlates with hydration, regardless of chemistry. This work improves the fundamental understanding of chemistry-function relationships of ionomers by providing a novel means to independently control their side-chain vs. end-group and observe resulting effects on interfacial properties to modulate electrode function in electrochemical technologies.