Research Papers

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

[+] Author and Article Information
Tomoaki Uchiyama

e-mail: tomoaki@uchiyama.tec.toyota.co.jp

Toshihiko Yoshida

Toyota Motor Corporation,
Fuel Cell System Development Div.,
R&D Group 1,
1200, Mishuku, Susono,
Shizuoka 410-1193, Japan

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received July 30, 2012; final manuscript received September 28, 2012; published online November 16, 2012. Editor: Nigel M. Sammes.

J. Fuel Cell Sci. Technol 9(6), 061005 (Nov 16, 2012) (8 pages) doi:10.1115/1.4007814 History: Received July 30, 2012; Revised September 28, 2012

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, equivalent weight (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 five 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 polymer electrolyte membrane (PEM) under deswelling and the CL cracks propagate into the PEM from the CL, which results in pinholes in the PEM.

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Fig. 1

Buckling test of NR211 and NR211-CL under humidity cycles

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Fig. 2

Elastoplastic parameters for materials

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Fig. 3

FEM analysis model for buckling test

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Fig. 4

Humidity change of NR211 in FEM analysis

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Fig. 5

Dimensional changes of NR211 and NR211-CL with relative humidity change

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Fig. 6

Stress-strain curves of NR211 and NR211-CL for (a) 5% RH, (b) 40% RH, (c) 80% RH, (d) 100% RH

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Fig. 7

Bulge deformations of NR211 and NR211-CL by laser microscope observations. (a) NR211 after five cycles and (b) NR211-CL after five cycles.

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Fig. 8

Height of bulge after humidity cycles. Error bar indicates maximum and minimum values.

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Fig. 9

Laser microscope observations for bulge deformations of NR211-CL. (a) After five cycles (height of bulge 3.6 μm), (b) after 1000 cycles (height of bulge 15.6 μm), (c) after 4000 cycles (height of bulge 28.8 μm).

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Fig. 10

Mises stress distributions in NR211 simulation. Comments indicate portion, dominant stress, and Mises stress value. (a) At 50% RH (step 1), (b) at 100% RH (step 2), (c) at 50% RH (step 3), and (d) at 5% RH (step 4).

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Fig. 11

Plastic equivalent strain distribution in NR211 simulation (step 5)

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Fig. 12

Mises stress distributions in NR211-CL simulation. Comments indicate portion, dominant stress, and mises stress value. (a) At 50% RH (step 1), (b) at 100% RH (step 2), (c) at 50% RH (step 3), and (d) at 5% RH (step 4).

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Fig. 13

Plastic equivalent strain distribution in NR211-CL simulation (step 5)

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Fig. 14

Strain behaviors with repeating humidity cycles. (a) True strain in 0–0.4, (b) true strain in 0–0.02, (c) relative humidity change.




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