The Development of Compression Moldable Polymer Composite Bipolar Plates for Fuel Cells
Cunningham, Brent David
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The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement. The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability. The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors.
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