Peering into Phase-Separated Perfluorinated Sulfonic-Acid Ionomers with Energy-Tunable X-rays

Ion-containing polymers such as perfluorinated sulfonic-acid (PFSA) ionomers play a critical role as the ion-conducting electrolyte in various electrochemical energy devices such as polymer-electrolyte fuel cells, redox flow batteries, and solar fuel generators. The ion transport properties in PFSAs are controlled by the nanoscale phase-separation of hydrophilic and hydrophobic domains. Next-generation membranes leverage new chemistries that enable good conductivity without requiring high relative humidity. As with well-studied PFSAs like Nafion, understanding the structure-property relationships in these new materials is critical to optimizing performance. Here, synchrotron X-rays with energies tuned to the sulfonic-acid functional group provide enhanced contrast to reveal the phase separation in dry and wet PFSA membranes with various chemistries. These studies elucidate connections among molecular architecture, morphology and transport properties that can be used as design rules for ionomer membranes and highlight the capabilities of X-rays to understand structure and chemistry in ion-containing polymers in general.