Abstract
Bipolar plates are commonly fabricated from graphite, however the associated machining costs are a major limitation to fuel cell commercialization. In addition, graphitic bipolar plates comprise up to 80% of the weight of a fuel cell. Here we propose an alternative approach to bipolar plate manufacturing: injection molding of electrically-conductive polymer composites. Injection molding is well-suited for mass production and polymeric materials can significantly reduce the weight of the bipolar plate. Prior efforts to injection mold polymer composites for fuel cell bipolar plates have resulted in electrical conductivities that do not meet the United States DOE technical targets for bipolar plate conductivity (> 100 S/cm) [1-3]. In efforts to achieve higher conductivities, Hwang et al. compression molded carbon fiber epoxy composites [4]. 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. For this reason, we will be investigating the suitability of injection molding polymer composites with sufficient electrical conductivities.
In this work, carbon fiber and carbon nanotubes were added to nylon at varying weight percentages. The conductivity of carbon fiber-filled composites ranged from 10 – 20 S/cm. High carbon fiber loadings increased conductivity, however beyond 50 wt%, the increased viscosity of the polymer blends can inhibit proper injection molding. Carbon nanotubes were then added to these composites to investigate the synergistic effects of adding conductive fillers with different aspect ratios. These samples reached conductivities up to 65 S/cm, rising closer to the US DOE technical target. In efforts to achieve higher conductivities, nickel-coated carbon fibers were added to nylon. The conductivities of these samples were above the US DOE target, reaching 150 S/cm. Interestingly, the addition of carbon nanotubes to these composites did not increase conductivity. If successful in a fuel cell, these polymer composites may provide a pathway to cheaper, lighter fuel cells with higher specific energies.
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