Metal bipolar plates

Bipolar plate Metal-coated alloy Incoloy 825, 310S, or316 steel 0.2-0.5... 

Graphite Composite Bipolar Plates Metal Bipolar Plates... 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The elec trodes contain veiy finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite or metal. 

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

H. Tawfik, Y. Hung, and D. Mahajan. Metal bipolar plates for PEM fuel cell—A review. Journal of Power Sources 163 (2007) 755-767. 

T. Matsuura, M. Kato, and M. Hori. Study on metallic bipolar plate for proton exchange membrane fuel cell. Journal of Power Sources 161 (2006) 74-78. 

Section 5.2.2 include composites and metals. From a cost reduction point of view, it is estimated, according to the cost model, that the cost percentage of the plate in a stack can be reduced from -60 to 15-29% if the graphite plate were replaced by the composite plate or metal plate [15]. However, many uncertain factors are involved in the estimation. The progress and major challenges in development of bipolar plates fabricated by these candidate materials will be introduced in the following parf of this section. 

Wind, J., A. LaCroix, A. Braeuninger, et al. 2003. Metal bipolar plates and coatings. In Handbook of fuel cells—Fundamentals, technologies, and applications, ed. W. Vielstich, H. A. Gasteiger, and A. Lamm. Vol. 3 Fuel cell technologies and applications, 294-307. New York John Wiley Sons. 

Brady, M. R, B. Yang, J. A. Wang, et al. 2006. Formation of protective nitride surface for PEM fuel cell bipolar plates. Journal of the Minerals, Metals and Materials Society 8 50-57. 

Brady, M. R, P. R Tortorelli, J. Rihl, et al. 2007. Nitrided metallic bipolar plates. Annual progress report, U.S. DoE Hydrogen Rrogram. http //www.hydrogen. energy.gov/pdfs/progress07/v b 2 brady.pdf (accessed 2008). 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

The bipolar plate design is illustrated in Fig. 47. It consists of a cross-flow arrangement where the gas-tight separation is achieved by dense ceramic or metallic plates with grooves for air and fuel supply to the appropriate electrodes. A porous cathode, a dense and thin electrolyte and a porous anode form a composite flat layer placed at the top of the interconnected grooves. The deposition of the porous electrodes can be achieved by mass production methods. Moreover, the bipolar plate configuration technology makes it possible to check for defaults, independently and prior to assembly of the interconnection plate and the anode-electrolyte-cathode structure. 

For the HTE process, the electrochemical cell consists of a tri-layer ceramic, well known for its brittleness, which limits applied loads. In addition, the relatively low ionic conduction properties of the electrolyte materials (3% yttrium-stabilised zirconia) requires an operating temperature above 700°C to reduce ohmic losses. This creates difficulties for the involved metallic materials, including bipolar plates and seals. 

Volume resistance of a graphite plate is about 17 mfl cm [ 1 ], therefore from an underside is formed metal contact (a position 5). However the most important is minimization of electric contact of a bipolar plate with gas diffusion layer. It is achieved with the help of clamping contact, which is created, as a rule, with the help of connection by bolts. Thus contact resistance 30 mQ cm2 is reached at pressure of compression not less than 120 H cm2 [ 1 ]. 

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