Replacing Fluoropolymers with Hydrocarbons Alternatives in Gas Diffusion Media for Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) hold significant potential to decarbonize segments of the transportation sector thanks to their high power density, high efficiency, rapid refuelling, and potentially zero emissions. In particular, PEFCs are well-suited for heavy-duty applications, as they require a limited number of refilling stations due to dedicated and predictable routes 1 . The current generation of PEFC materials heavily relies on fluorinated polymers: only considering the hydrophobic ones, we estimate that approximately 0.4 kg of polytetrafluoroethylene (PTFE) is present in a 168 kW net fuel cell stack. This fluoropolymer, together with fluorinated ethylene propylene and perfluoropolyether, is most commonly used for the hydrophobic treatment of the gas diffusion media (GDM), and plays a key role in water management within the fuel cell 2 . Typically, a GDM consists of a microporous layer (MPL) and a gas diffusion layer (GDL). The MPL achieves water repellency through the use of PTFE (or other previously mentioned fluoropolymers) as a binder in a carbon paste, while the GDL is typically dip-coated with the same fluoropolymer. These treatments impart the required hydrophobicity to the components, but come with significant drawbacks. First, the PTFE coverage on the GDL is heterogeneous, meaning that the wetting properties are unevenly distributed 3 . Second, the weak physical interactions between the substrate and the coating cause gradual PTFE loss over time 4 , hampering system durability and resulting in a release of “forever chemicals” in the environment. Furthermore, per- and polyfluoroalkyl substances have been proposed for a ban by the European Chemicals Agency due to their environmental and public health hazards 5 . With this imminent regulatory shift, there is a strong need for fluorine-free replacements. To address this challenge, our goal is to develop hydrocarbon-based GDMs that match or exceed the performance of their fluorinated counterparts. By ensuring comparable or superior properties in terms of water management, durability, and cell overpotentials, these alternatives can enhance system competitiveness while aligning with regulatory requirements. For the hydrophobization of the MPL, we investigate the utilization of waxes as alternative fluorine-free binders. For the treatment of the GDL, we investigate a method that enables the formation of tuneable and conformal coatings covalently bonded to the substrate, namely chemical grafting. The covalent nature of these layers is hypothesized to result in more homogeneous and long-lasting coatings, addressing the limitations of the traditional coatings obtained via dipping. Moreover, a large variety of functionalities can be introduced onto the substrate via this method, paving the way for the development of highly tailored fluorine-free GDMs. In this talk, I will first discuss our synthetic approach to obtain fluorine-free GDMs with different degrees of hydrophobicity, based on alkyl chains with different lengths. Second, I will present our efforts to elucidate the relationship between the different modifications and their resulting physicochemical properties. Using X-ray photoelectron spectroscopy, we assess the chemical nature of the modified surfaces and we confirm the successful modifications. With external water contact angles and solid surface energy measurements we determine their degree of liquid-repellency. Through electrochemical techniques we assess the uniformity (i.e. coverage) of the coatings on the carbon GDL substrates. To conclude, I will discuss the performance of the novel GDMs in single-cell PEFCs evaluated at different conditions. Through our research, we demonstrate that hydrocarbon-based materials can achieve comparable hydrophobicity to fluorinated ones, and that the uniformity of the layers offers a potential advantage for performance. 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. 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) Athanasaki, G.; Jayakumar, A.; Kannan, A. M.; Int. J. Hydrogen Energy 2023 , 48 (6), 2294–2313. (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