Applicability of Phosponated Poly(pentafluorostyrene) Fiber Reinforced Nafion Composite Membranes in Low-Temperature Fuel Cells

Abstract

Reducing the fuel crossover of proton exchange membranes (PEMs) is an important measure to improve the fuel efficiency and performance of fuel cells and electrolyzers 1, 2. Additionally, the long-term stability and safety of PEM-based systems increase with a reduction in the gas crossover 1, 3, 4. A possible approach for reducing the gas crossover of a PEM is the implementation of fiber meshes to form a composite membrane 5. Furthermore, fiber-reinforced composite membranes can also increase the mechanical properties of PEMs and therefore enable the utilization of thinner membranes without sacrificing the mechanical integrity of the membrane or causing safety risks 6.In this work, we demonstrate the successful fabrication of electrospun fibers of a partially phosophonated, conductive polymer (phosponated poly(pentafluorostyrene); PWN70) and its non-phosphonated and, therefore, non-conductive equivalent (poly(pentafluorostyrene); PPFSt). Two different loadings of both fiber meshes were successfully infiltrated with Nafion, analyzed mechanically and electrochemically, and compared to a non-reinforced Nafion reference. We show that the mechanical properties of both fiber-reinforced composite membranes are superior to the reference membrane. The electrochemical analysis of the membrane electrode assemblies (MEAs) shows a reduction in performance with the incorporation of the non-conductive PPFSt fibers. An increase in the high-frequency resistance (HFR) of the MEAs containing the non-conductive PPFSt fibers explains the reduced performance and scales with fiber loading. In contrast, the MEAs with the proton conductive PWN70 fiber reinforcement maintained the same HFR and performance as the reference, independently of the fiber loading. Additionally, hydrogen crossover measurements revealed a significantly reduced hydrogen crossover of the MEAs composed of the conductive PWN70 fiber-reinforced membranes (between 30 and 40 % less than the reference). We hypothesize that PWN70 shows a decreased diffusivity for hydrogen compared to Nafion, and therefore, the fibers in the Nafion matrix increase the tortuosity of the membrane for fuel crossover.In summary, we developed a fiber-reinforced Nafion membrane with improved mechanical and gas barrier properties while maintaining the membrane's proton conductivity compared to the same membrane without reinforcement.References Q. Tang, B. Li, D. Yang, P. Ming, C. Zhang and Y. Wang, Int. J. Hydrog. Energy, 46(42), 22040–22061 (2021).M. Schalenbach, M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, Int. J. Hydrog. Energy, 38(35), 14921–14933 (2013).B. Wu, M. Zhao, W. Shi, W. Liu, J. Liu, D. Xing, Y. Yao, Z. Hou, P. Ming, J. Gu and Z. Zou, Int. J. Hydrog. Energy, 39(26), 14381–14390 (2014).C. Klose, P. Trinke, T. Böhm, B. Bensmann, S. Vierrath, R. Hanke-Rauschenbach and S. Thiele, J. Electrochem. Soc., 165(16), F1271-F1277 (2018).J. Choi, K. M. Lee, R. Wycisk, P. N. Pintauro and P. T. Mather, Macromolecules, 41(13), 4569–4572 (2008).J. B. Ballengee and P. N. Pintauro, Macromolecules, 44(18), 7307–7314 (2011).