Tomoaki Uchiyama

Crack Formation in Membrane Electrode Assembly Under Static and Cyclic Loadings

The mechanical reliability of membrane electrode assemblies (MEAs) in polymer electrolyte fuel cells (PEFCs) is a major concern for fuel cell vehicles. Hygrothermal cyclic conditions induce mechanical stress in MEAs and cracks form under operating conditions. This paper investigates the failure mechanism of MEAs under several mechanical and environmental conditions with the aim of designing durable PEFCs. We performed static tensile tests and low-cycle fatigue tests on MEAs. During the tensile tests, the temperature and humidity of the test chamber were controlled and surface crack formation of MEAs was observed in situ by a video microscope. Low-cycle fatigue tests were performed at ambient conditions and the number of cycles to crack formation was measured. The results reveal that the temperature and the humidity affect the mechanical properties of MEA. Observations of MEAs during tensile tests reveal that cracks form on the surface of catalyst layers immediately after the MEAs yield. These results indicate that reducing the deformation mismatch between the catalyst layer and the proton exchange membrane is important for suppressing crack formation in MEAs. The results of low-cycle fatigue tests reveal that the fatigue strength of a MEA follows the Coffin–Manson law so that fatigue design of MEAs based on the Coffin–Manson law is possible. This result is valuable for designing durable PEFCs.

Mechanical Degradation Mechanism of Membrane Electrode Assemblies in Buckling Test Under Humidity Cycles

Membrane electrode assembly (MEA) buckling tests in microscopic clearances under humidity cycles and numerical analyses by finite element method (FEM) were conducted. The NR211 (Dupont, 25 μm thickness, EW = 1100) sandwiched between catalyst layers (CLs) was used as the MEA. Based on tensile tests of the NR211 and NR211-CL and FEM simulation of tensile tests, the Young’s modulus and yield point of CL were estimated. While the CL had a higher Young’s modulus than the NR211 in water vapor, the CL indicated a lower Young’s modulus than the NR211 in liquid water at 80 °C.The buckling tests in microscopic diameter of 200 μm in polyimide film were carried out. The heights of bulge in the NR211 and NR211-CL after 5 humidity cycles were measured with a laser microscope. The height of the NR211-CL was lower than that of the NR211 due to the stiffer CL and the lower swelling ratio of the NR211-CL. Moreover, when the humidity cycles were repeated less than 1000 times, cracks were formed in the CL.The stress-strain behaviors of the NR211-CL buckling test under a humidity cycle were investigated by using the FEM. When the NR211-CL swelled, higher stress was developed at the topside of bulge and topside of bulge round. These portions corresponded to the CL crack formed portions in the buckling test. When the NR211-CL deswelled, the tensile stress was induced in the entire NR211. The mechanical degradation mechanisms were considered as follows. Firstly, cracks initiate and propagate in the CL when the MEA swells in repeating humidity cycles. Moreover, the tensile stress is induced in the PEM under deswelling, and the CL cracks propagate into the PEM from CL, which results in the pinhole in the PEM.Copyright

In Situ Observation of Deformation Behavior of Membrane Electrode Assembly Under Humidity Cycles

The mechanical reliability of the membrane electrode assembly (MEA) in polymer electrolyte fuel cells (PEFCs) is a major concern with respect to fuel cell vehicles. When PEFCs generate power, water is generated. The proton exchange membrane (PEM) swells in wet conditions and shrinks in dry conditions. These cyclic conditions induce mechanical stress in the MEA, and cracks are formed. Failure of the MEA can result in leaking of fuel gases and reduced output power. Therefore, it is necessary to determine the mechanical reliability of the MEA under various mechanical and environmental conditions. The purpose of the present paper is to observe the deformation behavior of the MEA under humidity cycles. We have developed a device in which the constrained condition of the GDL is modeled by carbon bars of 100 to 500 μm in diameter. The carbon bars are placed side by side and are pressed against the MEA. The device was placed in a temperature and humidity controlled chamber, and humidity cycles were applied to the specimen. During the tests, cross sections of the specimen were observed by microscope, and the strain was calculated based on the curvature of the specimen. The temperature in the test chamber was varied from 25 to 80 °C, and the relative humidity was varied from 50 to 100%RH, and the wet condition was also investigated. The results revealed that the MEA deformed significantly by swelling and residual deformation was observed under the dry condition, even for one humidity cycle. The crack formation criteria for one humidity cycle corresponded approximately with those of the static tensile tests. The results of the humidity cycle tests followed Coffin–Manson law, and the number of cycles until crack formation corresponded approximately with the results of the mechanical fatigue tests. These results will be valuable in the critical design of durable PEFCs.