Bipolar plates are one of the primary components of polymer electrolyte membrane fuel cells. These plates deliver reactants to electrodes, remove reaction byproducts, transfer heat, provide mechanical stability, and conduct electrical current. While graphite is the most commonly used material for fabrication of bipolar plates, the machining costs associated with graphite are a major drawback to mass production. In addition, graphite-based plates make the greatest contribution to the weight of the fuel cell stack. For this reason, electrically-conductive polymer composites have been explored as an alternative to the traditional metal alloys commonly used for fuel cell bipolar plates [1]. Polymer composites can reduce both the weight and cost of the bipolar plate, while still providing sufficient electrical properties.

In prior efforts, compression-molded, carbon fiber epoxy composites have shown promise to achieve the electrical conductivities needed for fuel cell bipolar plates [2]. While compression molding can be used for production of high volumes, injection molding is most ideal for the levels of mass production required for full scale commercialization of fuel cells. The main objective of this study is to investigate the suitability of injection molding polymer composites with sufficient electrical conductivities. Polymer composites based on nylon were developed for injection molding of bipolar plates. Carbon fiber was added to nylon at weight percentages ranging from 10 to 50%. The highest conductivity was obtained by direct injection molding. Results show that the percolation threshold for carbon fiber in nylon occurs around 25 wt%. Multi-walled carbon nanotubes were added to the direct injection-molded samples at weight percentages ranging from 2 to 4% to investigate the synergistic effects of adding multiple conductive fillers [3]. The addition of MWCNTs increases conductivities to United States Department of Energy technical targets for bipolar plate electrical conductivity (> 100 S/cm). Samples fabricated with MWCNTs and carbon fiber are shown to achieve up to 400% improvement in conductivity over samples with carbon fiber alone. The addition of carbon black as a third conductive filler, as well as the use of a polymer blend, may further increase electrical conductivity.

[1] Mighri F, Huneault MA and Champagne MF. (2004) Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application. Polymer Engineering & Science 44: 1755-1765.
[2] Hwang IU, Yu HN, Kim SS, et al. (2008) Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells. Journal of Power Sources 184: 90-94.
[3] Shen L, Wang FQ, Yang H, et al. (2011) The combined effects of carbon black and carbon fiber on the electrical properties of composites based on polyethylene or polyethylene/polypropylene blend. Polymer Testing 30: 442-448.