Synchrotron-Based Characterization of Perfluorinated Sulfonic-Acid Ionomers

Abstract

Perfluorinated sulfonic-acid (PFSA) ion-conducting polymers (ionomers) serve as both a binder and active ion-transport material in electrochemical energy-conversion electrodes including fuel cells, electrolyzers, and redox-flow batteries. Within the catalyst layer of these devices, ionomer shuttles ionic and liquid or gaseous species to/from catalytic sites.1-2 Optimizing the multi-functional role that PFSA ionomers play is crucial to developing next generation devices, and, to do so, the underlying fundamental structure-property relationships of these materials must first be established. Reducing the complex heterogeneous environment of catalyst layers down to representative model systems enables probing these relationships in a systematic and fundamental manner. To accomplish this effort, we spin-cast thin films of Nafion and 3M ionomer chemistries, including multi-acid side chains, across a range of thicknesses and processing conditions. Synchrotron-based x-ray scattering and spectroscopy, and advanced analysis of the collected data, is employed to characterize the phase-separated nano-structure of PFSA thin films. Results show the universality of confinement effects across chemistries, and how processing conditions modify the chemistry-confinement interplay and impact structure-function relationships. X-ray characterization is carried out in both ambient and humidified environment, in-situ, to determine the nanostructural changes in ionomer film in response to environment. These structural investigations are correlated to thermodynamic swelling properties using ellipsometry to identify factors controlling ionomer transport functionality in electrode structure of energy conversion devices. AcknowledgmentsWe thank Mike Yandrasits and Andrew Haug of 3M for providing ionomers and helpful discussions. This research used beamline 7.3.3 of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.3 The study was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02- 05CH11231. References Mauritz, K. A.; Moore, R. B., State of understanding of Nafion. Chemical reviews 2004, 104 (10), 4535-4586.Kusoglu, A.; Weber, A. Z., New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chemical Reviews 2017, 117 (3), 987-1104.Alexander, H.; Wim, B.; James, G.; Eric, S.; Eliot, G.; Rick, K.; Alastair, M.; Matthew, C.; Bruce, R.; Howard, P., A SAXS/WAXS/GISAXS Beamline with Multilayer Monochromator. Journal of Physics: Conference Series 2010, 247 (1), 012007.