Uncovering New Ionomer Membrane Morphology Features with Resonant X-Ray Scattering

Ion-conducting polymer, or ionomer, membranes play a key role as the ion-conducting electrolyte in electrochemical devices, including fuel cells and electrolyzers. Perfluorinated sulfonic acid (PFSA) ionomers are the most common, and they are cast into membranes and thin films from alcohol-rich dispersions. We examine the connections between dispersion and membrane properties. Controlling the phase separated morphology of the ionomer and understanding its relationship to transport properties is crucial for improving ion transport and device performance. It is challenging for hard x-ray scattering to resolve all the morphological details in chemically heterogeneous ionomers. We use tender resonant x-ray scattering (TReXS) near the sulfur K-edge to improve scattering contrast and enhance chemical specificity to the sulfonate groups in PFSA ionomers. We find that TReXS reveals unique energy-dependent morphological features not apparent in hard x-ray scattering that depend on dispersion composition and ionomer chemistry. These structure-property relationships can be used to guide design of next-generation ionomers for energy applications. Furthermore, this work highlights TReXS as an emerging technique for structural characterization of ionomers and chemically heterogeneous polymers in general.