Effect of annealing on two different niobium-clad stainless steel PEMFC bipolar plate materials Sung-Tae HONG 1 , Dae-Wook KIM 2 , Yong-Joo YOU 2 , K. Scott WEIL 3 1. School of Mechanical and Automotive Engineering, University of Ulsan, Ulsan, Korea; 2. Department of Material Science and Engineering, University of Ulsan, Ulsan, Korea; 3. Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA, USA Received 2 March 2009; accepted 30 May 2009 Abstract: Niobium (Nb)-clad stainless steels(SS) produced via roll bonding are being considered for use in the bipolar plates of polymer electrolyte membrane fuel cell(PEMFC) stacks. Because the roll bonding process induces substantial work hardening in the constituent materials, thermal annealing is used to restore ductility to the clad sheet so that it can be subsequently blanked, stamped and dimpled in forming the final plate component. Two roll bonded materials, niobium clad 340L stainless steel (Nb/340L SS) and niobium clad 434 stainless steel (Nb/434 SS) were annealed under optimized conditions prescribed by the cladding manufacturer. Comparative mechanical testing conducted on each material before and after annealing shows significant improvement in ductility in both cases. However, corresponding microstructural analyses indicate an obvious difference between the two heat treated materials. During annealing, an interlayer with thick less than 1 μm forms between the constituent layers in the Nb/340L SS, whereas no interlayer is found in the annealed Nb/434 SS material. Prior work suggests that internal defects potentially can be generated in such an interlayer during metal forming operations. Thus, Nb/434 SS may be the preferred candidate material for this application. Key words: clad sheet; bipolar plate; proton exchange membrane fuel cell; annealing 1 Introduction Despite significant technical progress made in recent years toward developing a commercially viable polymer electrolyte membrane fuel cell(PEMFC) system, potential application of this device in the automobiles remains limited to prototypes[1]. The reasons for this are the high cost of PEMFC stack manufacture, the steady loss in power output during long-term continuous operation, and the larger than desired size and mass of the latest generation PEMFC systems[2−3]. The latter factor limits the specific power that can be generated, which then necessitates mass reduction in other parts of the vehicle (likely at a further increase in cost)[3]. For this technology is competitive in the automotive industry, cost reduction and performance improvement must come from all aspects of PEMFC system design and manufacture. One of the most bulky components in the stack with respect to both mass and volume is the bipolar plate. It is also one of the most expensive to manufacture, accounting for about 45% of the total cost and 80% of the mass of the stack. This component not only serves as the electrical junction between serially connected cells in the stack, but also performs several other key functions in the overall device including uniform distribution of fuel and oxidant over the active areas of the cells[4], proper humidification of the electrochemical membrane while minimizing the potential for flooding, prevention of fuel and oxidant mixing (i.e. a chemically resistant, leak-proof barrier to both gas streams), structural support for the stack and heat removal from the cells. While various materials were considered for use in this component[5−10], metals offer a number of intriguing design advantages particularly for transportation applications, including low-cost, mass-production via stamping or embossing of sheet product, fabrication in thin form (＜150 μm), to reduce mass and volume in the overall stack, impermeability to fuel, oxidant and water vapor, excellent thermal conduction properties and good mechanical robustness. Despite this material exhibits a higher density than alternative carbon-based bipolar plate materials, the reduction in thickness afforded by the strength of a typical candidate metal leads to an overall decrease in the mass of the plates required for the stack. Corresponding author: Sung-Tae HONG; Tel: +82-52-2592129; E-mail: sthong@ulsan.ac.kr