Electrical Conductivity Modeling of Polymer Composite Bipolar Plates

Abstract

Polymer composites offer the potential for manufacturing lightweight, low-cost, fuel cell bipolar plates [1]. In this work, we develop a micromechanical model to predict the electrical conductivity of these composites. Using the Fiber Contact Model, conductivity was predicted based upon direction in the material, fiber alignment, fiber length and diameter, fiber concentration, and fiber conductivity [2]. Fiber alignment was determined experimentally by imaging cross sections of injection-molded nylon/carbon fiber composites. In addition to modeling electrical properties, the Halpin-Tsai equations are used to model the elastic modulus and ultimate strain of the composite. Conductivity and elastic modulus are predicted to increase in the injection direction of the composite with higher fiber loadings as well as with improved alignment of the fibers [3]. Development of an objective function for optimizing cost, ultimate strain, and conductivity, leads to optimal fiber volume fractions in the longitudinal and transverse directions of the composite.In addition to modeling efforts, electrical conductivity measurements were also performed on injection-molded nylon/carbon fiber composites. Modeling predictions are compared to experimental conductivity data for nylon composites with carbon fiber loadings of 10, 20, 30, and 40 wt%. The analytical predictions show good correlation within 10% of the experimental results. For 20, 30, and 40 wt% samples, conductivities were above the United States DOE technical target for bipolar plate conductivity (> 100 S/cm), reaching 250 S/cm. While higher carbon fiber loadings would further increase conductivity, beyond 50 wt%, the increased viscosity of the polymer blends can inhibit proper injection molding. Mighri, F.; Huneault, M. A.; Champagne, M. F., Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application. Polymer Engineering and Science 2004, 44 (9), 1755-1765.Weber, M.; Kamal, M. R., Estimation of the volume resistivity of electrically conductive composites. Polymer Composites 1997, 18 (6), 711-725.Zameroski, R.; Kypta, C. J.; Young, B. A.; Sanei, S. H. R.; Hollinger, A. S., Mechanical and Electrical Properties of Injection-Molded MWCNT-Reinforced Polyamide 66 Hybrid Composites. Journal of Composites Science 2020, 4 (4), 14. Figure 1