Short Communication
Chemical durability of thin pore-filling membrane in open-circuit voltage hold test

https://doi.org/10.1016/j.ijhydene.2019.09.143Get rights and content

Highlights

  • Chemical durability of the thin pore-filling membrane was evaluated as MEA.

  • The thin pore-filling membrane showed durability comparable to Nafion 211.

  • Membrane thinning was not observed in pore-filling membrane contrary to Nafion 211.

  • Little amounts of fluorine leakages were observed in the pore-filling membrane.

Abstract

Long-term chemical stability of proton exchange membranes in polymer electrolyte fuel cells (PEFCs) is an important issue for widespread commercialization. Here, we report on the chemical stability of a membrane-electrode assembly with a 7 μm thick pore-filling membrane (porous substrate filled with high ion exchange capacity perfluorosulfonic acid (PFSA) polymer) using an open-circuit voltage hold test. The very thin pore-filling membrane shows comparable chemical durability to Nafion 211. Interestingly, the pore-filling membrane shows a different degradation behavior from Nafion 211 due to the use of chemically and mechanically stable porous substrate, with no thickness change and little amounts of fluorine leakages are observed in the pore-filling membrane compared to membrane thinning and large amounts of fluorine leakage in Nafion 211. The thin pore-filling membrane shows promise for application in PEFCs, as it balances high fuel cell performance at high temperature and low relative humidity with high chemical durability.

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.

References (35)

Cited by (10)

  • Assessing the degradation pattern and mechanism of membranes in polymer electrolyte membrane fuel cells using open-circuit voltage hold and humidity cycle test protocols

    2022, Materials Science for Energy Technologies
    Citation Excerpt :

    However, a small peak around 1753.0 cm−1 emerged for PEM tested under conditions of OCV holding for 540 h. This peak was attributed to the attack from radical chemical species in PFSA-based PEM [55]. In addition, SO peak at 1058.

  • Development of low-cost process for pore generation in cellulose acetate by utilizing calcium salts

    2021, Journal of Industrial and Engineering Chemistry
    Citation Excerpt :

    Porous membranes are attracting attention in various fields such as batteries, filtration and water treatment, biochemistry and gas separation, regardless of pore size [1–7].

  • Simulation of membrane chemical degradation in a proton exchange membrane fuel cell by computational fluid dynamics

    2021, International Journal of Hydrogen Energy
    Citation Excerpt :

    H2O2 then decomposes into •OH or •OOH radicals in the presence of transition metals, such as iron, and these highly reactive species attack any H-containing end-groups present in PFSA polymers. This attack causes membrane thinning and the release of fluoride, which have been frequently reported in the literature [4–19]. Membrane thinning increases gas crossover that further intensifies degradation.

View all citing articles on Scopus
View full text