(Invited) Transport-Stability Relationships in Composite Polymer-Electrolyte Membranes

Research activities and interests in electrochemical conversion devices (e.g., fuel cells, flow batteries, and electrolyzers, etc.) have been continuously increasing due to their great potential to provide clean and renewable energy technologies for stationary, transportation, and solid-state applications. At the heart of these devices is the polymer-electrolyte membrane (PEM) that functions as an electrolyte and a separator. Ion-conductive polymers (ionomers) are the ubiquitous choice for PEMs as they inherently conduct ions and therefore could provide key roles such as ion/solvent transport and gas/reactant separation.1, 2 Nevertheless, diverse operating environment and performance metrics in these electrochemical devices require optimized ionomer multi-functionalities with a mechanically robust matrix.2-4 An improvement in transport functionality in these ionomers, however, usually undermines their mechanical stability and separator functionality.1 Thus, a challenge in these devices is achieving sustainable performance in their solid-state electrolyte membranes, which requires a synergistic effort to meet the performance and durability demands, i.e., to improve transport without compromising mechanical integrity.2 Accomplishing this requires, first, an understanding of how the ionomer's transport and mechanical properties are interrelated through the morphology, and then, altering this structure-transport relationship by incorporating performance additives and mechanical support in ionomers. Next-generation polymer-electrolyte membranes contain not only an ionomer, but also additional materials that provide complementary functionalities,5-7 which create an even more complex structure with varying heterogeneities across the lengthscales, i.e. from microscopic composite morphology to changes in local interactions at nanoscales. In such a structure, it becomes challenging to establish a structure-property relationships for these hybrid ionomers and explore the balance between their mechanical and chemical properties. In this talk, an overview of the structure/property relationship of perfluorosulfonic acid (PFSA)-based ionomers and their composites will be presented with a focus on comparison of their conductivity-hydration correlations and transport-mechanical properties. In particular, we will examine properties of Nafion XL composite membrane with a porous reinforcement layer,6 as well as an electro-spun Nafion/PVDF polymer membrane.5 Water uptake, conductivity and nanostructural features of these membranes will be analyzed to provide insights into how the presence of distinct secondary materials affects the membrane's "composite" response, compared to the baseline PFSA membrane.2 In addition, the concept of newly formed phases in these membranes that already contain a phase-separated ionomer will be discussed along with the critical lengthscales for ionomers that could influence their structure-functionality. It will also be explored how hydration-conductivity correlations observed in PFSA ionomers could be extended to various composite structures by accounting for the macroscopic and microscopic changes in morphology. Lastly, the interplay between the transport properties and mechanical response will be elucidated to identify the key factors in this new paradigm for hybrid ionomers, which could help development of robust high-performance membranes that can meet the design requirements for various electrochemical energy devices. AcknowledgementsWe acknowledge Peter Pintauro of Vanderbilt University for providing us with the electrospun membranes. AK acknowledges the ECS-Toyota Young Investigator Fellowship. This work made use of facilities at the Advanced Light Source, which is a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy. Part of this work was funded through the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References Kusoglu, A.; Weber, A. Z. Chem. Rev. 2017, 117, (3), 987-1104.Kusoglu, A.; Weber, A. Z. J Phys Chem Lett 2015, 6, (22), 4547-52.Mukundan, R.; Baker, A. M.; Kusoglu, A.; Beattie, P.; Knights, S.; Weber, A. Z.; Borup, R. L. J. Electrochem. Soc. 2018, 165, (6), F3085-F3093.Tang, Y. L.; Kusoglu, A.; Karlsson, A. M.; Santare, M. H.; Cleghorn, S.; Johnson, W. B. J. Power Sources 2008, 175, (2), 817-825.Woo Park, J.; Wycisk, R.; Lin, G.; Ying Chong, P.; Powers, D.; Van Nguyen, T.; Dowd Jr, R. P.; Pintauro, P. N. J. Membr. Sci. 2017, 541, (Supplement C), 85-92.Shi, S. W.; Weber, A. Z.; Kusoglu, A. J. Membr. Sci. 2016, 516, 123-134.Ballengee, J. B.; Pintauro, P. N. J. Electrochem. Soc. 2011, 158, (5), B568-B572.