(Digital Presentation) Corrosion Behavior of Aluminum-Carbon Composite Bipolar Plates in Polymer Electrolyte Membrane Fuel Cells
Authors: Jahan Aklima , Alam Md. Ashraful , Yonezawa Sekai , Suzuki Eiichi , Yashiro Hitoshi 
Published: July 2022

ABSTRACT

Bipolar plate (BP) is an important multifunctional component in polymer electrolyte membrane fuel cells (PEMFCs) system (1,2) which should possess some specifications such as superior electrical and thermal conductivity, high corrosion resistance, good mechanical performance, and low cost (3). BP candidate materials are roughly classified into carbon-based and metal-based materials. Carbon-based materials are lightweight and have excellent corrosion resistance, but are inferior to metal-based materials in terms of gas shielding properties and mechanical strength. Metallic materials have the opposite characteristics, and corrosion resistance is a particular and important issue. Aluminum (Al) is considered as a promising BP material because of having some important characteristics such as low density and low cost (4). Earlier, it was proposed that the PEMFC BP can be divided into two parts: gas isolation plate and flow path forming material (5). Normally, metallic BPs are corroded exclusively at the rib part where BP contacts with the gas diffusion layer (GDL), while there is not much corrosion at the bottom of the flow path. Therefore, the corrosion resistance required for the reaction gas isolation plate would not be as high as required for the flow path forming material (5). In this study, a composite BP using Al as the reaction gas isolation plate and a carbon-based material as the flow path forming material was fabricated to investigate the corrosion behavior of Al through power generation test.
A single cell was assembled using a BP consisting of a 1 mm thick glassy carbon flow path forming material and an Al reaction gas isolation plate, and the power generation tests were performed for 500-1000 h to investigate the corrosion of Al. Separately, Al plates were subjected to exposure tests in the expected environment during the cell operation. After the exposure and power generation tests, the Al bipolar plates were analyzed by SEM and TEM. Furthermore, in order to improve the contact resistance, a power generation test using a bipolar plate coated with TiN-SBR on both sides of Al isolation plate (6) was also conducted.
When Al plate was immersed in water at 80 °C, a thick oxide layer of about 1.3 µm was formed with whitish appearance. On the contrary, Al maintained its gloss after exposer in the saturated steam. After 1000 h power generation test using the Al-carbon BP, the surface of Al diaphragm plate maintained its gloss at cathode side. On the other hand, a thick oxide layer of about 1 µm was formed from center part to outlet part along with the flow field on the anodic plate. It suggests that the water drops were generated on the anodic flow field although the corrosion products were slight enough for safe use of Al as BPs with the channel former made of carbon.
Thus, the following findings were obtained as a result of conducting the PEMFC power generation test using a composite BP with Al as the reaction gas isolation plate and glassy carbon as the flow path forming material.

No significant corrosion was observed on the Al isolation plate on the cathode side after the 1000 h power generation test, but the anode side turned into white from the center of the flow path to the gas outlet side, and about 1 µm of a thick oxide film was formed.

When a power generation test was conducted using Al BP coated with TiN-SBR, the cell voltage was increased significantly which was approached as similar as the performance of graphite. It indicates that Al can be used as bipolar plates without any problem by performing surface treatment in combination with a carbon channel forming material.

Acknowledgements

The authors thankfully acknowledge the financial support obtained from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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