(Invited) Rethinking Diffusion Media for Polymer Electrolyte Fuel Cells: Replacing Fluoropolymers and Engineering Microstructures
Authors: Forner-Cuenca Antoni , Sinisgalli Irene , Rijsdijk Meike , van Gorp Rik , Leth Nicolaas , Jacquemond Rémy , Horst Rens Jan 
Published: December 2024

Diffusion media are performance-defining components in polymer electrolyte fuel cells, where they are responsible for multiple critical functions including providing an open porous structure for multiphase mass transport, a solid scaffold for electron and heat transport, and cushioning mechanical compression of the cell1. State-of-the-art materials are composed of an arrangement of two layers, namely a carbon fiber substrate and a microporous layer that have been engineered to sustain the complex performance requirements of the electrochemical cell. However, to date, the microstructure (e.g. pore size distribution) and surface chemistry (e.g. coating distribution, local wettability) of standard materials have not been precisely engineered which has motivated intensive research to optimize existing material sets. To improve water management, new methods aimed to introduce low capillary pressure pathways for the water to leave the cell. A non-exhaustive list of examples include the use of laser perforation2, plasma modifications3, or grafting with patterned wettability4. While these approaches have shown promising results, the addition of multiple steps onto the existing manufacturing pipelines results in higher manufacturing costs. Thus, developing methodologies that can readily fabricate precisely controlled microstructures and wetting properties from the bottom-up, while using low-cost and scalable manufacturing approaches, is an appealing avenue to further advance the technology.
In this lecture, I will first review the state-of-the-art in diffusion media research and highlight key challenges. Second, I will introduce the pressing need to find alternative hydrophobic coatings to prevalent per- and polyfluoroalkyl molecules such as polytetrafluoroethylene and fluorinated ethylene propylene, as they bioaccumulate and their derivatives are damaging to living organisms5, resulting in a prospective ban6. Our group investigates the application of uniformly-distributed, thin-film coatings based on siloxanes, silanes and hydrocarbons. I will discuss the correlation between the chemical composition of these coatings, their intrinsic wetting properties, and ultimately their impact on single cell performance. Additionally, I will discuss strategies to replace fluoropolymers in conventional microporous layer inks and pastes. Third and finally, I will introduce a new approach based on non-solvent induced phase separation (NIPS) to manufacture diffusion media with controlled three-dimensional structures7. Using NIPS, we can synthesize microstructurally diverse porous media, including isoporous, pore size gradients, and structures with bimodal pore size distributions8,9. I will discuss the correlation between the diffusion media microstructure prepared with NIPS and the resulting performance in single cells. Using these methods, we aim to integrate the combined functions of carbon fiber substrates and microporous layers in a single material concept, thereby decreasing complexity and costs.

References

Mathias, M. F., Roth, J., Fleming, J. & Lehnert, W. Diffusion media materials and characterisation. in Handbook of Fuel Cells (John Wiley & Sons, Ltd, 2010). doi:10.1002/9780470974001.f303046.

Csoklich, C., Xu, H., Marone, F., Schmidt, T. J. & Büchi, F. N. Laser Structured Gas Diffusion Layers for Improved Water Transport and Fuel Cell Performance. ACS Appl. Energy Mater.
4, 12808–12818 (2021).

Zahiri, B. et al. Through-plane wettability tuning of fibrous carbon layers via O2 plasma treatment for enhanced water management. Appl. Surf. Sci.
458, 32–42 (2018).

Forner-Cuenca, A. et al. Engineered Water Highways in Fuel Cells: Radiation Grafting of Gas Diffusion Layers. Adv. Mater.
27, 6317–6322 (2015).

Ackerman Grunfeld, D. et al. Underestimated burden of per- and polyfluoroalkyl substances in global surface waters and groundwaters. Nat. Geosci.
17, 340–346 (2024).

Tyrrell, N. D. A Proposal That Would Ban Manufacture, Supply, and Use of All Fluoropolymers and Most Fluorinated Reagents within the Entire EU. Org. Process Res. Dev.
27, 1422–1426 (2023).

Non‐Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes - Wan - 2021 - Advanced Materials - Wiley Online Library. https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202006716.

Jacquemond, R. R. et al. Microstructural engineering of high-power redox flow battery electrodes via non-solvent induced phase separation. Cell Rep. Phys. Sci.
3, 100943 (2022).

Wan, C. T.-C., Jacquemond, R. R., Chiang, Y.-M., Forner-Cuenca, A. & Brushett, F. R. Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non-Solvent-Induced Phase Separation. Energy Technol.
11, 2300137 (2023).