Single-Layer Graphene Incorporation in PEMFCs for Enhancing Membrane Durability and Its Impact on Catalyst Layer Properties

Abstract

In the realm of heavy-duty vehicle (HDV) applications, the primary technological hurdle facing Proton Exchange Membrane Fuel Cells (PEMFCs) is durability. HDVs necessitate an exceptional lifespan of 1 million miles, 6.5 times longer than their light-duty vehicle (LDV) counterparts1. Two pivotal components, the polymer electrolyte membrane and the catalyst layer, have been identified as crucial for achieving durability.Chemical degradation of perfluorinated polymer electrolyte membranes, e.g., Nafion, is attributed to the small but finite H2 and O2 gas crossover through the polymer electrolyte from the anode and cathode, respectively. These crossover gases generate active oxygen radical species, such as -OH and -OOH, which attack the polymer backbone or side chains. Additionally, Pt particles have been observed migrating into the membrane, creating another site for radical formation2. This process results in pinhole formation, membrane thinning, carbon support, and platinum catalyst degradation3. Analyzing effluent water for fluoride ion release rate is the primary method for evaluating the membrane degradation rate.Recent approaches involve the utilization of cerium and manganese additives to mitigate radical formation and chemical degradation induced by radicals. However, a limitation associated with metal and metal oxide additives is their propensity for migration and agglomeration, causing alterations in membrane morphology and subsequently affecting proton conductivity. As an alternative to metal oxide additives, defect-free monolayer graphene, which is chemically stable, has been explored. It has been discovered that defect-free monolayer graphene is impermeable to all atoms and molecules but permeable to protons4. The incorporation of monolayer graphene with small defects may significantly reduce gas crossover while allowing the transport of water and protons. We also hypothesize that the graphene layer will serve as a barrier to catalyst ion transport (Pt2+ or Co2+) into the membrane, which can have an interesting impact on catalyst electrochemically active surface area (ECSA) and catalyst layer ionic resistance.In this study, we showcase the enhanced chemical durability of the Nafion membrane by introducing an atomically thin chemical vapour deposition (CVD) single graphene layer onto the PEM. Accelerated stress tests (AST), such as open circuit voltage (OCV) in H2 /Air at 30 %RH and 80oC, are used to evaluate the chemical durability of membranes within a relatively short time. Water collected from both the anode and cathode side were analyzed for fluoride emission rate and sulphate content. Catalyst layer ECSA, H2 crossover, membrane and catalyst layer ionic resistance were monitored every 24-28 hours. The insights gained from these results will be invaluable in assessing the promise and the challenges of employing 2D-material-modified PEM for HDV applications. Reference: (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. New Roads and Challenges for Fuel Cells in Heavy-Duty Transportation. Nat. Energy 2021, 6 (5), 462–474. https://doi.org/10.1038/s41560-021-00775-z.(2) Kim, T.; Lee, H.; Sim, W.; Lee, J.; Kim, S.; Lim, T.; Park, K. Degradation of Proton Exchange Membrane by Pt Dissolved/Deposited in Fuel Cells. Korean J. Chem. Eng. 2009, 26 (5), 1265–1271. https://doi.org/10.1007/s11814-009-0212-9.(3) Inaba, M.; Kinumoto, T.; Kiriake, M.; Umebayashi, R.; Tasaka, A.; Ogumi, Z. Gas Crossover and Membrane Degradation in Polymer Electrolyte Fuel Cells. Electrochim. Acta 2006, 51 (26), 5746–5753. https://doi.org/10.1016/j.electacta.2006.03.008.(4) Hu, S.; Lozada-Hidalgo, M.; Wang, F. C.; Mishchenko, A.; Schedin, F.; Nair, R. R.; Hill, E. W.; Boukhvalov, D. W.; Katsnelson, M. I.; Dryfe, R. A. W.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K. Proton Transport through One-Atom-Thick Crystals. Nature 2014, 516 (7530), 227–230. https://doi.org/10.1038/nature14015.