Correlation between the Swelling Characteristic and Humidity Cycle Durability of a Polymer Electrolyte Membrane
Authors: Hashimasa Yoshiyuki , Daitoku Hiroshi , Numata Tomoaki , Matsuda Yoshiyuki 
Published: February 2020

Introduction

Polymer electrolyte fuel cells (PEFCs) for automotive applications must operate over a wide range of operation conditions. Humidity cycle tests that simulate the operation of fuel cell under dry and wet conditions are widely used to test the mechanical durability of polymer electrolyte membrane of PEFCs in the form of membrane electrode assembly (MEA). The degradation phenomenon of membrane under conditions of cycling humidity has been studied by many researchers[1-5], but the mechanism is not clear enough. In this study, correlation between the humidity cycle durability of MEAs equipped with different type of membranes and their mechanical properties obtained by dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA) was investigated.

Experimental

MEAs of different catalyst layer thickness were prepared using NR-211(25μm) membranes or sulfonated polyethersulfone (SPES) membranes (32μm). Humidity cycle tests using a rectangular waveform of 0-150% RH (4min per cycle) were carried out on a JARI standard single cell (25cm2 electrode area, one serpentine flow channel). The cell temperature was set at 80°C. Linear sweep voltammetry was used to measure the hydrogen crossover rate through membrane as a diagnostic of the degradation status of the membrane in the humidity cycle tests. The cycle tests were finished when the hydrogen crossover rate through the membrane reached 10 times the initial value. To consider the reason of difference of the cycle life of MEAs, swelling tests in water was conducted at 80°C．Moreover, dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA) of MEAs were carried out by changing the humidity of atmospheric gas.

Result and Discussion

The cycle life, which was defined as the terminated cycle, by humidity cycle tests were shown in Fig.1 The cycle life of SPES MEA was greatly shorter than that of NR-211 MEA at the same catalyst layer thickness (10μm). To consider the difference of humidity cycle durability of both membranes, DMA of membrane in dry nitrogen gas was carried out. Fig.2 shows the relationship between temperature and storage elastic modulus of membranes measured under the condition of 0%RH of relative humidity. Storage elastic modulus of SPES membrane was higher than that of NR-211 membrane. This result shows that the membrane stiffness of SPES was higher than that of NR-211 membrane at 0%RH. The difference of humidity cycle durability of both membranes was not explained by the result of DMA.
Swelling tests of membranes were conducted to consider the reason of difference of cycle life of both MEAs. Swelling ratio of NR-211 and SPES were shown in Fig.1. Swelling ratio of SPES were greatly larger than that of NR-211. Our previous research revealed that the mechanical durability in humidity cycle test was influenced by the catalyst layer thickness of MEA[6]. In the case of this study, the thickness of both MEAs were same(10μm). The difference of cycle life of both MEAs was thought to be influenced by the difference of swelling ratio of both MEAs.These results indicate that it is important to prevent membrane swelling in order to improve the mechanical durability under humidity cycle .

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CKNOWLEDGMENTS

This work was supported by the New Energy and Industrial Technology Organization (NEDO).

REFERENCES

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Figure 1