The corrosion behavior of pure aluminum in the dilute sodium fluoride and sodium sulfate solutions as simulating the PEMFC environment was studied through immersion process at different time. The corrosion was accelerated in the presence of fluoride ions whereas sulfate ions caused ver y less amount of corrosion. The electrochemical measurement, SEM and EDX analyses were carried out to examine the corrosion resistance, surface morphology and elemental compositions, respectively.

Corrosion behavior of pure aluminum in sodium fluoride and sulfuric acid media with different concentrations at 80°C for use as bipolar plate in PEMFC environment was investigated. Open circuit potential, single cell operation, amount of corrosion, and surface roughness of pure aluminum were measured to check the corrosion potential, corrosion resistance, corrosion amount, and surface roughness, respectively. Scanning electron microscopic images and energy-dispersive X-ray spectra were also taken to analyze the surface morphology, and elemental compositions.

Bipolar plate, aluminum is one of the most promising alternatives that can capable of uniform distribution of reactive gases over active areas and removal of the heat and exhaust water from the PEMFC. In this work, PEM fuel cell was operated using the bipolar plate constructed of two materials: a channel former made of glassy carbon and a gas isolation plate made of aluminum. Aluminum gas isolation plate at cathode side remained glossy after even1000 h cell operation. In contrast, an oxide layer with a thickness of about 1 μm was formed on the gas isolation plate at anode side. In the simulated corrosion test, a thick oxide layer was formed when aluminum was immersed in water while only a thin oxide layer was formed in the saturated water vapor. The microscopic images, scanning electron microscopy (SEM) images, energy dispersive X-ray (EDX) spectra and transmission electron microscopy (TEM) images were taken to evaluate the surface morphology, elemental composition and other analyses.

The global climate changes occurred because of the emissions of greenhouse gases such as CO2, NO x and SO x that are ongoing throughout the world pose a gradually higher demand for replacing today's fossil fuel based energy system by less pollutant technologies.[1-2] Among the alternative energies available, proton exchange membrane fuel cells (PEFCs) have been considered to power transportation vehicles such as automobiles and buses due to their high power density, relatively quick start-up and low operating temperatures.[3] The bipolar plate is one of the most important parts that impacts the manufacturing cost of the PEFC.[4] The high electrical conductivity, high gas impermeability, good mechanical performance and low cost are some of the desired properties of bipolar plate materials. Also, mainly two types of materials are used as bipolar plates in PEFC as carbon and metals. Carbon materials show a very good corrosion resistance but lower performance in separating reaction gases compare with metals. Among the many candidate materials, aluminum (Al) is considered as better bipolar plate applications in PEFC due to its low cost and low density.[5] Moreover, bipolar plate is required to have sufficient corrosion resistance even with a sulfuric acid solution having a pH of around 3.0, but it is considered that Al cannot tolerate this condition without its well surface modification which is proposed by Yashiro et al.[6] Sulfuric acid solutions are widely used for aluminum anodizing operations. That’s why it was our interest to study the corrosion of Al material for using as a gas separating plate in PEFC environment. Also, the influence of sulfate and fluoride ions on the stability of films formed on Al surfaces in aqueous solutions of sulfuric acid and sodium fluoride particularly at low concentration levels through finding the amount of corrosion was examined. Scanning electron microscopic (SEM), energy dispersive x-ray (EDX) and cross-sectional analyses were also done to observe the surface morphology and elemental composition. In the present investigation, pure Al having 6 cm2 of area were used as the test pieces for immersion in 2.5×10-6M H2SO4 (pH 5.41), 5×10-6M H2SO4 (pH 5.14), 2.5×10-6M H2SO4 + 2ppm F- (pH 5.57) and 5×10-6M H2SO4 + 2ppm F- (pH 5.45) solutions using reagent grade of H2SO4 and NaF. Test pieces were polished by emery paper 4/0, 6/0, 1200 and 2000, rinsed and immersed in around 100 cm3 aqueous solutions for a period of 24h, 48h, 72h, 96h and 120h at 80°C. A potentiodynamic polarization test was performed to check the corrosion resistance of Al in the simulated PEFC environment using 0.05M SO4 2- and 2ppm F- solutions (pH 3.3 and 5.8) and high current density of Al was found. Moreover, a 500h single cell operation was also performed to investigate the corrosion of Al as the gas separating plates. A glassy carbon was used as the channel former and aluminum foil was used as the gas separating plate. The single cell results showed that the corrosion of the gas separation plate was much milder than the corrosion expected in polarization test which showed the possibility of using Al as a gas separating plate in PEFC. Additionally, the oxide film was formed in the Al surfaces because of the oxidized aluminum at the time of immersion. As a result, the corrosion products were formed and graphed as amount of corrosion versus immersion time. Firstly, the amount of corrosion was increased with increasing the concentration of sulfuric acid and increased more after the addition of 2ppmF- ions in the acid solutions. From the results found in this study, it is concluded that pure as-polished Al undergoes severe corrosion in sodium fluoride whereas sulfate ions form much lower corrosion as well as act as inhibitor when Al is used as bipolar plate in the PEFC environment. Acknowledgements This work was supported by the Ministry of Education, Sports, Science and Technology (MEXT), Japan. References Carrette, K. A. Friedrich, U. Stimming, Fuel Cells, 1 (1), 5-39 (2001). O. Collantes, Technol Forecasting Soc. Change, 74(3), 267-280 (2007). Schäfer, J. B. Heywood, M. A. Weiss, Energy, 31 (12), 2064-2087 (2006). Cunningham, D. Baird, J. Mater. Chem, 16, 4385-4388 (2006). A. A. El-Enim, O. E. Abdel-Salam, H. El-Abd, A. M. Amin, J. Power Sources, 177 (1), 131-136 (2008). Yashiro, T. Ichikawa, S. -T. Myung, M. Kumagai, S. Kozutsumi, Zairyo-to-Kankyo, 60 (10), 432-434 (2011).

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. References Y. C. Park, S. H. Lee, S. K. Kim, S. Lim, D. H. Jung, S. Y. Choi, Int. J. Hydrogen Energy, 38, 10567–10576 (2013). S. F. Husby H, O. E. Kongstein, A. Oedegaard, Int. J. Hydrogen Energy, 2, 951–957 (2014). S. H. Lee, V. E. Pukha, V. E. Vinogradov, N. Kakati, S. H. Jee, S. B. Cho, Int. J. Hydrogen Energy, 38, 14284–14294 (2013). C.-H. Lee, Y.-B. Lee, K.-M. Kim, M.-G. Jeong, and D.-S. Lim, Renewable Energy, 54, 46–50 (2013). H. Yashiro, T. Ichikawa, S. -T, Myung, M. Kumagai and S. Kozutsumi, Zairyo-to-Kankyo, 60, 432–434, (2011). S.-T. Myung, M. Kumagai, R. Asaishi, Y.-K. Sun, H. Yashiro, Electrochem. Comm., 10, 480–484 (2008).