Short CommunicationChemical durability of thin pore-filling membrane in open-circuit voltage hold test
Introduction
Clean emission and high efficiency of polymer electrolyte fuel cells (PEFCs) display them as promising energy devices in both automobile and stationary applications. Recent studies of PEFCs have centered around high temperature and low relative humidity (RH) operation as advantages such as higher reaction rates, easier water management and unnecessary external humidifiers have already been established [[1], [2], [3], [4], [5]]. For widespread commercialization of such PEFC systems, their long-term stability has been an important factor to be taken into consideration.
The degradation of the proton exchange membrane has been one of the main causes of lower durability in PEFCs [[6], [7], [8]] and their durabilities have been previously evaluated [[9], [10], [11], [12]]. Chemical degradation in particular crucially affects membrane durability [8,13,14]. During fuel cell operation, generated hydrogen peroxide (H2O2) reacts with contaminated metal cations such as Fe2+ and Cu2+, to form hydroxyl and hydroperoxyl radicals that attack and accelerate membrane degradation [8,15,16].
A thin pore-filling membrane has been recently developed using low equivalent weight (EW, reciprocal value of ion exchange capacity) perfluorosulfonic acid (PFSA) polymer and a 6 μm ultra-high molecular weight polyethylene (UHMWPE) porous substrate [17]. As a result, higher fuel cell performance was observed in a membrane-electrode assembly (MEA) with pore-filling membrane compared commercial Nafion 211 at a high temperature of 100 °C and low RH of 30% under maximum power density of over 1,000 mWcm−2 due to better water transport and reduced IR loss. Given the suppressed swelling of low EW PFSA polymer using the UHMWPE substrate, the amount of hydrogen (H2) crossover in the 7 μm thick pore-filling membrane was also comparable to that in the 25 μm thick Nafion 211 at various RH regions. Although the superiority of this thin pore-filling membrane over Nafion 211 in terms of membrane properties and fuel cell performances under high temperature and low RH condition has been demonstrated, the chemical stability of MEA is still to be studied experimentally.
Herein, the chemical stability of MEA with a thin pore-filling membrane was evaluated. An accelerated open-circuit voltage (OCV) hold test was used to examine the long-term chemical stability of the membrane. Under OCV conditions at high temperature and low RH, crossover of reactants such as hydrogen and oxygen across the membrane is higher and the formed radicals promote and accelerate the chemical degradation of PFSA polymer [[18], [19], [20], [21]]. Following the OCV hold test, H2 crossover current density and fluoride ion emission rate (FER) were assessed. In addition, the prepared MEAs were analyzed by cross-sectional environmental scanning electron microscope (E-SEM) or electron probe microanalyzer (EPMA) subsequent to the durability tests. Also, the MEA with Nafion 211 was evaluated as a comparison.
Section snippets
Preparation of the membrane-electrode assembly
A thin pore-filling membrane was prepared using UHMWPE substrate (thickness = 6 μm, porosity = 66%, Teijin Limited) and EW580 PFSA polymer in accordance with previously reported procedures [17]. It is possible that the hot pressing in the decal method adopted in previous report influences the thin membrane durability evaluation. Hence, in this study, the catalyst layers were prepared via the spray method. In brief, a catalyst ink containing 0.1 g of TEC10E50E (Pt 46.8 wt%), 3.9 g of RO water,
Results and discussion
The I–V curves of MEAs using 25 μm thick Nafion 211 and 7 μm thick pore-filling membrane prepared via the spray method with different RHs are shown in Fig. 1a and b. The maximum power density of the MEA with pore-filling membrane showed around 1,200 mWcm−2 at 80 °C and 20% RH, of which this value was almost the same as that of our previous MEA prepared by the decal method [17]. Repeatability of high fuel cell performance of MEAs using the spray method was also confirmed. Therefore, the
Conclusions
In summary, the chemical stability of fabricated MEA with 7 μm thick pore-filling membrane (UHMWPE substrate filled with low EW PFSA polymer) was evaluated via an accelerated OCV hold test. The pore-filling membrane showed comparable chemical durability of approximately four times thick the commercial Nafion 211. The degradation behavior of Nafion 211 and the pore-filling membrane was interestingly seen to be different. Although drastic membrane thinning and large amount of fluorine leakages
Acknowledgments
We gratefully acknowledge the financial support by Kanagawa Institute of Industrial Science and Technology (KISTEC). We thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for the ion chromatography and E-SEM analysis. Also, a great thanks to the Toray Research Center, Inc. for the EPMA analysis.
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