Impact of Solvent on PFSA Properties from Dispersion to Cast Film

Perfluorosulfonic-acid (PFSA) ionomers are a critical part of proton-exchange-membrane (PEM) fuel-cell catalyst layers (CLs). They serve both to bind the catalyst particles together, lending mechanical support, and provide pathways for ion transport. The CL fabrication process has garnered significant attention in recent years, with research efforts focused on understanding the properties of the CL ink (colloidal dispersions of solvent(s), catalyst particles, and ionomer), and how those properties translate to the cast CL structure.1-2 The PFSA/solvent interaction is a crucial and complex piece of the larger multi-component interaction phenomena. Recent work has shown the strong impact of solvent composition on dispersion properties3 and structure,4 and eventual CL performance,1 but there is little understanding of how these initial (dispersion) and final (dried film) states are connected. How do different dispersion properties as a result of varying ionomer/solvent interactions control cast structure and behavior? Do these interactions persist upon drying? This information is vital to understand, predict, and control PFSA film and CL structure and properties, instead of relying on current empirical approaches to the formation process. In this presentation, we investigate these questions by studying the impact of solvent on PFSA properties over the course of film preparation. In-situ grazing incidence x-ray scattering (GISAXS)5 experiments on ionomer dispersions are systematically performed over a range of water to n-propanol composition to explore the evolution of film structure from dispersion to dried state. Dispersion properties (particle size, viscosity, structure) and film properties (structure, water uptake, conductivity, etc.) are evaluated within the context of the GISAXS data, connecting properties across the range of PFSA states, and lending insight into interaction/property relationships. These results provide important understanding of the impact of solvent during film and CL manufacturing processes, and reveal that final properties are a strong function of ionomer/solvent interactions in the dispersion state that persist long past formation and annealing. Acknowledgements Scattering work was conducted at the Advanced Light Source (ALS), beamline 7.3.3, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (Contract No. DE-AC02-05CH11231). This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Hydrogen and Fuel Cell Technologies Office (HFTO), of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy under contract number DE-AC02-05CH11231. S.B. acknowledges support from the Graduate Research Fellowship Program by the National Science Foundation under Grant No. DGE 1752814. References Van Cleve, T.; Khandavalli, S.; Chowdhury, A.; Medina, S.; Pylypenko, S.; Wang, M.; More, K. L.; Kariuki, N.; Myers, D. J.; Weber, A. Z.; Mauger, S. A.; Ulsh, M.; Neyerlin, K. C. ACS Appl. Mater. Interfaces 2019, 11 (50), 46953-46964. Hatzell, K. B.; Dixit, M. B.; Berlinger, S. A.; Weber, A. Z. J. Mater. Chem. A 2017, 5 (39), 20527-20533. Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z. J. Phys. Chem. B. 2018, 122 (31), 7790-7796. Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S. ACS Macro. Lett. 2012, 1 (12), 1403-1407. Dudenas, P. J.; Kusoglu, A. Macromolecules 2019, 52 (20), 7779-7785.