Fuel cells are incredible energy conversion units that harness the power of clean sources like hydrogen and transform them into electricity through a series of oxidation–reduction reactions. One type of fuel cell, called proton exchange membrane fuel cells (PEMFCs), is crucial for electric vehicles as it relies on proton-conductive membranes to function. However, these membranes face a challenge – they must strike a balance between durability and ion conductivity, which directly impacts the performance and lifespan of PEMFCs.
Fortunately, scientists have developed chemically and physically modified perfluorosulfonic acid polymer membranes, such as Nafion HP, Nafion XL, and Gore-Select, which have proven to be much more durable than the unmodified membranes traditionally used in fuel-cell operations.
However, none of the existing proton-conductive membranes have met the highly demanding technical target set by the U.S. Department of Energy (DOE) to enable their use in automobile fuel cells by 2025. This target involves passing an accelerated durability test or a combined chemical and mechanical test.
Now, a team of researchers from Japan, led by Professor Kenji Miyatake from Waseda University and the University of Yamanashi, has developed novel proton-conductive membranes for PEMFCs. Their groundbreaking work, published in the journal Science Advances, is co-authored by Dr. Liu Fanghua from Waseda University and the University of Yamanashi, and Dr. Ick Soo Kim from Shinshu University.
The researchers created proton-conductive membranes using a partially fluorinated aromatic ionomer called SPP–TFP-4.0 (SPP: sulfonated polyphenylene, TFP: bis(trifluoromethyl) terphenylene).
They then reinforced the ionomer using the push-coating method, either with electrospun, nonwoven, and isotropic poly(vinylidene fluoride) (PVDF) nanofibers with high porosity (78%), or with porous expanded polytetrafluoroethylene (ePTFE). This resulted in composite membranes, namely SPP–TFP-4.0–PVDF and SPP–TFP-4.0–ePTFE, with thicknesses of 14 and 16 µm, respectively.
The researchers conducted a wide range of tests on these proton-conductive membranes and found that the one reinforced with PVDF outperformed the state-of-the-art chemically stabilized and physically reinforced perfluorinated Nafion XL membrane. It exhibited superior fuel-cell operation and in situ chemical stability at a high temperature of 120oC and a low relative humidity of 30%. Miyatake states, “It outperformed the state-of-the-art chemically stabilized and physically reinforced perfluorinated Nafion XL membrane in terms of fuel-cell operation and in situ chemical stability at a high temperature of 120oC and a low relative humidity of 30%.”
The SPP–TFP-4.0–PVDF membrane demonstrated an impressive lifetime of 148,870 cycles or 703 hours, which is over seven times longer than the DOE target, in the accelerated durability test with frequent wet-dry cycling under open-circuit-voltage conditions. Additionally, it exhibited high chemical stability with minimal degradation, stable rupture energy at various humidity levels, highly stable mechanical properties from zero to 60% relative humidity at 80oC, and excellent fuel-cell performance at high temperatures (100–120oC).
In essence, this innovative aromatic polymer-based reinforced proton-conductive membrane meets the U.S. DOE’s target for future automobile fuel cells, providing a promising alternative. This study could pave the way for PEMFCs with high-temperature operability and durability, making fuel cell-based electric vehicles more powerful and affordable. Ultimately, it contributes to the realization of a hydrogen-based, carbon-free society. Miyatake concludes, “As a result, fuel cell-based electric vehicles may become more powerful and affordable. This would also contribute towards realizing a hydrogen-based, carbon-free society.”
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