Bipolar coated stainless-steel plates

Bipolar plates are currently made from milled graphite or gold-coated stainless steel. Ongoing research is aiming to replace these materials with polymers or low-cost steel alloys, which will allow the use of low-cost production techniques. Even today, bipolar plates can be produced at 200 /kW, if the production volume... 

Stainless steels, as well as A1-, Ni-, and Ti-based alloys have been studied extensively as possible candidates for bipolar plates. One of the most well-studied materials for bipolar plates is SS 316/316L (16-18% Cr, 10-14% Ni, 2% Mo, rest Fe) other candidates are 310,904L, 446, and 2205. Bare stainless steel plates form a passive 2-A nm chromium oxide surface layer under PEMFC conditiOTs that leads to unacceptably high ICRs. A similar trend is observed for the other alloys and therefore surface modification or surface coatings on selected substrate material has to be considered as a pathway to meet the technical targets of low ICR and high corrosion resistance. 

Fukutsuka, T, Yamaguchi, T., Miyano, S.-l. et al. 2007. Carbon-coated stainless steel as PEFC bipolar plate material. Journal of Power Sources 174 199-205. 

Joseph, S., McClure, J. C., Chianelli, R. et al. 2005. Conducting polymer-coated stainless steel bipolar plates for proton exchange membrane fuel cells (PEMFG). International Journal of Hydrogen Energy 30 1339-1344. 

A1 alloy, Ti, nitrided Ni and Fe alloys, coated stainless steel (nitride, proprietary), and stainless steel bipolar plate surfaces, decreasing electronic conductivity... 

The bipolar plates are made from either Type 310 or Type 316 stainless steel, which is coated on the fuel side with nickel and aluminized in the seal area around the edge of the plates. Both internally and externally manifolded stacks have been developed. 

Planar geometry Although there are variations in detail, the design of the planar or flat plate cell, and how cells are interconnected are essentially as illustrated in Fig. 4.27. The construction may involve a self-supporting, coated electrolyte or, if the operating temperature permits, the anode-electrolyte-cathode structure may be supported on a stainless steel bipolar plate. 

Ni-state-of-the-art anodes contain Cr to eliminate the problem of sintering. However, Ni-Cr anodes are susceptible to creep, while Cr can be lithiated by the electrolyte and consumes carbonate, leading to efforts to decrease Cr. State-of-the-art cathodes are made of lithiated-NiO. Dissolution of the cathode is probably the primary life-limiting constraint of MCFCs, particularly under pressurised operation. The present bipolar plate consists of the separator, the current collectors, and the seal. The bipolar plates are usually fabricated from thin sheets of a stainless steel alloy coated on one side by a Ni layer, which is stable in the reducing environment of the anode. On the cathode side, contact electrical resistance increases as an oxide layer builds up (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

Alternative materials for bipolar plates include graphite, stainless steels, titanium and aluminium, all with a developed fabrication technique, and coating technique if needed. Major competitors UTC Fuel Cells has an active fuel cell bus programme, but give sparse details of its flow plate and other technology. (See UTC web site.)... 

Wang, Y. and Northwood, D.O., An investigation into polypyrrole-coated 316L stainless steel as a bipolar plate material for PEM fuel cells, J. Power Sources, 163(1), 500, 2006. 

Abo El, Enin SA, Abdel-Salam OE, El-Abd H, Amin AM (2008) New electroplated aluminum bipolar plate for PEM fuel cells. J Power Sources 177 131-136 Feng K, Shen Y, Sun H, Liu D, An Q, Cai X, Chu PK (2009) Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells. Int J Hydrogen Energ 34 6771-6777... 

Finally, the stainless-steel bipolar plate consists of a separator and current collectors. The plate is exposed to the anodic environment on one side and the cathodic environment on the other. The low oxygen partial pressure on the anodic side of the bipolar plate prevents the formation of a protective oxide coating and, on the cathode side, the contact electrical resistance increases as an oxide scale builds up. Active research is focused on finding alloys for bipolar current-collector materials that function well in both anodic and cathodic environments, have a low cost and ohmic resistance, and have good corrosion resistance [15]. 

From the total costs for metal bipolar plates as stated in the DoE cost review report, the costs of the metal plates amount to 20 m active cell area, which will be at maximum 10% lower per m metal area. For the metal-based bipolar plates, the materials costs are significantly higher than the processing costs. As metal, coated 316L stainless steel is selected. 

Cho EA, Jeon US, Hong SA, Oh IH, Kang SG (2005) Performance of a 1-kW-class PEMFC stack using TiN-coated 316 stainless steel bipolar plates. J Power Sources 142 177-183... 

Coolers Coolers in UTC Power s PAFCs are made of stainless steel tubes embedded in molded material (mixture of FEP and Graphite) as shown in Fig. 12.6. These coolers use the same manufacturing process as that of the bipolar plates. A serpentine cooler tube is placed between two molded preforms and laminated together at high pressures and temperatures. The ends of the tubes coming out of the molded material are wrapped in PTFE film or coating to prevent attack of stainless... 

The bipolar plates used in MCFC stacks are usually fabricated from thin ( 15 mil) sheets of an alloy (e.g., Incoloy 825, 310S or 316L stainless steel) that are coated on one side (i.e., the side... 

Today, uncoated stainless-steel materials are still in focus of research, as bare substrates promise to be the most cost-effective solution for metal bipolar plates. Nevertheless, it becomes obvious that a surface modification method or protective coating applied to metal bipolar plates is essential to improve their corrosion resistance. 

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