Towards Conformal and Fluorine-Free Coatings on Carbon Fiber Substrates for Polymer Electrolyte Fuel Cells

Abstract

Decarbonizing the automotive sector has become a priority in the last decades. Polymer electrolyte fuel cells (PEFCs) are a promising technology for this purpose, thanks to their high power, high efficiency, fast refilling and potentially zero emissions. In particular, PEFCs are well-suited for heavy-duty applications, as they show unique scalability in terms of power and efficiency, and require a limited amount of infrastructure, as fewer refuelling stations are needed due to dedicated and more predictable routes1. However, to achieve reliable and cost-effective systems, existing challenges related to performance and durability need to be overcome. A powerful approach to improve the competitiveness of the system is to optimize the water management, as it is a complex but performance-defining issue2. Although the water generated during operation is beneficial to the membrane and the ionomer for proton conduction, it causes mass transport losses in the gas diffusion media by blocking the transport pathways for the reactant gases, limiting the overall performance of the system. State-of-the-art gas diffusion media are made by dip-coating carbon fiber-based substrates in a polytetrafluoroethylene (PTFE) dispersion. The treatment provides the material with the required hydrophobicity, but suffers from two major drawbacks. First, the PTFE coverage is heterogenous, which results in uneven distribution of the wetting properties3. Second, the weak physical interactions between the substrate and the coating cause PTFE loss under operating conditions, negatively impacting the overall system performance and durability4. In addition, efforts are being made to replace fluorine-containing materials, which are very prominent in PEFCs, because of the fluorine toxicity that might result in a ban of per- and polyfluoroalkyl substances in 20245.Our goal is to overcome the existing challenges through an alternative coating approach leveraging principles of liquid phase electrochemistry, electrografting. The proposed technique enables the formation of tunable and conformal hydrophobic layers covalently bonded to the substrate6,7, which we hypothesize would result in more stable and homogeneous coatings. Furthermore, given the large variety of molecules that can be grafted using this method, we foresee a pathway for the development of fluorine-free coatings. In this talk, I will first discuss our alternative synthetic approach to hydrophobize fibrous substrates using fluorine-free molecules, such as silanes or siloxanes. Secondly, I will present our efforts to elucidate the relationship between the fibrous substrate composition and their properties. To assess the chemical nature and the morphology of the surfaces, we perform X-ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy, confirming the presence of Si element on the surfaces. To evaluate the wetting properties, we measure external contact angles of water and lower surface tension mixtures on diffusion media surfaces, solid surface energies, and electrochemical double layer capacitance as an indirect metric of the wetted area. Thanks to these techniques, we find comparable water repellency between the new coating materials and the traditional PTFE-based ones. Finally, I will discuss the performance of the novel gas diffusion layers in single-cell polymer electrolyte fuel cells. The ultimate goal of our research is to synthesize advanced fluorine-free gas diffusion media with improved performance and durability. Acknowledgments This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (JU) under grant agreement No 101007170. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Hydrogen Europe and Hydrogen Europe Research. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 899987 and the Eindhoven Institute for Renewable Energy Systems. References (1) Cullen, D. A.; Neyerlin, K. C.; Ahluwalia, R. K.; Mukundan, R.; More, K. L.; Borup, R. L.; Weber, A. Z.; Myers, D. J.; Kusoglu, A. Nat Energy 2021, 6 (5), 462–474.(2) Yue, L.; Wang, S.; Araki, T.; Utaka, Y.; Wang, Y. Int J Hydrogen Energy 2021, 46 (3), 2969–2977.(3) Rofaiel, A.; Ellis, J. S.; Challa, P. R.; Bazylak, A. J Power Sources 2012, 201, 219–225.(4) Pan, Y.; Wang, H.; Brandon, N. P. J Power Sources 2021, 230560.(5) ECHA (European Chemical Agency), Restriction on the manufacture, placing on the market and use of PFASs , 2023, https://echa.europa.eu/-/echa-publishes-pfas-restriction-proposal.(6) Bélanger, D.; Pinson, J. Chem Soc Rev 2011, 40 (7), 3995–4048.(7) Thomas, Y. R. J.; Benayad, A.; Schroder, M.; Morin, A.; Pauchet, J. ACS Appl Mater Interfaces 2015, 7 (27), 15068–15077.