Modulating Perfluorinated Ionomer Functionality via Sidechain Chemistry

Abstract

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.

Revealing Mesoscale Ionomer Membrane Structure by Tender Resonant X-ray Scattering

“…Furthermore, they revealed through transport simulations that the transport pathways for water and protons may not always be the same, which opens possibilities for controlling transport functionality or selective ion transport in ionomer systems through their mesoscale features. While such effects are inferred in structure–property investigations of different ionomers supported by hard X-ray scattering, ,,,− the lack of complete structural information on mesoscales, especially regarding information on the distribution of sulfur in ionic domains, created a gap in understanding and implementing of such design rules. The technique presented here begins to fill this gap by providing missing information on the mesoscale.…”

Does Confinement Mediate Cerium Exchange and Its Impact on Ionomer Properties?

“…Studies show that Nafion suffers significant changes in the morphological, thermodynamic, mechanical, and transport properties under confinement. ,− Confinement occurs when the ionomer film thickness approaches the characteristic length scales for nanostructures, thereby resulting in a deviation in the properties from bulk response. Moreover, the much-reduced thickness of the thin films allows substrate effects to dominate their properties. − For PFSA ionomers, the confinement regime begins starting from a few 100 nm and intensifies as the ionomer approaches the thicknesses in fuel-cell CLs, thereby impacting the transport function and electrode performance. These differences can significantly vary the properties of thin films from those of their more well-studied bulk counterparts.…”

“…It is, therefore, highly unlikely that cerium migration in fuel cells has a thermodynamically driven contribution from an ionomer perspective. Although thin-film ionomers have a complex morphology, often influenced by its supporting substrate, , a macroscopic thermodynamic model can be a useful tool to analyze the average thermodynamic properties such as total ion and water uptake. An adjustment to the material properties in the model, concurrent with confinement-related changes such as increased stiffness with decreasing thickness, was shown to fit the decrease in the experimental water uptake by the thin film.…”