To overcome this issue, scientists have synthesized 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 unmodified membranes conventionally employed in fuel-cell operations. Unfortunately, none of the existing proton-conductive membranes have fulfilled the highly challenging technical target—passing an accelerated durability test or a combined chemical and mechanical test—set by the U.S. Department of Energy (DOE) to facilitate their use in automobile fuel cells by 2025.
Against this background, a group of researchers from Japan, led by Professor Kenji Miyatake from Waseda University and the University of Yamanashi, has recently synthesized novel proton-conductive membranes for PEMFCs. Their 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 synthesized proton-conductive membranes using a partially fluorinated aromatic ionomer (polymer material consisting of thermoplastic resins stabilized by ionic cross-linkages) called SPP–TFP-4.0 (SPP: sulfonated polyphenylene, TFP: bis(trifluoromethyl) terphenylene). They then utilized the push-coating method to reinforce the ionomer either with electrospun, nonwoven, and isotropic poly(vinylidene fluoride) (PVDF) nanofibers with high porosity (78%) or using porous expanded polytetrafluoroethylene (ePTFE). This resulted in composite membranes, SPP–TFP-4.0–PVDF and SPP–TFP-4.0–ePTFE, of thicknesses 14 and 16 µm, respectively.
The researchers performed a wide variety of tests on these proton-conductive membranes and demonstrated the one reinforced with PVDF to be superior. “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%,” highlights Miyatake.
The SPP–TFP-4.0–PVDF membrane demonstrated a long lifetime of 148,870 cycles or 703 hours—over seven times longer than that of the DOE target—in the accelerated durability test with frequent wet-dry cycling under open-circuit-voltage conditions. In addition, it exhibited high chemical stability with little 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 effect, the proposed aromatic polymer-based reinforced proton-conductive membrane meets the U.S. DOE’s target for future automobile fuel cells, providing a lucrative alternative. Thus, this study could pave the way for PEMFCs with high-temperature operability and durability. “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,” concludes an optimistic Miyatake.
We, too, hope such highly efficient electric vehicles with powerful, durable, and sustainable fuel cells become a reality soon!