Manufacturing of composite bipolar plates for PEM fuel cell applications

The bipolar plate (BP) material should possess contradictory properties. They should also manufacture from low-cost methods and materials. In the current investigation, thermoset-based composite materials reinforced with conductive fillers, and the compression molding process is implemented. In addition to fabricating the bipolar plates (BPs) with and without the flowing channels, alleviating the defects during the molding process is performed. The channels are perfectly formed on the plates with the designed depth of ۰.۶۵ mm and ۰.۵ mm of width. Regarding the physical properties, the interfacial contact resistance measured to be ۱۸-۲۴ mΩ cm۲. The mean value of the flexural strength of the produced samples was equal to ۴۷ MPa, which is almost twice the DOE target (>۲۵ MPa).Proton exchange membrane fuel cell (PEMFC) features high power density, low emission, and relatively low operating temperatures, which make them a favorable candidate for a wide range of transportation and stationary applications [۱]. Bipolar plate (BP) is among the most critical components of a fuel cell. BPs should have high thermal conductivity. They should also have an excellent mechanical strength to withstand the compressive load in a cell or stack and provide structural support for the cell. Graphite was shown to be the most popular material for the manufacturing of conventional BPs due to its high electrical conductivity and corrosion resistance. Nevertheless, brittle nature of its microstructure, and lack of mechanical strength limited its applicability and the ability to reduce the thickness and, subsequently, the weight of the bipolar plates [۲]. Regarding the cost reduction, it was stated that, by replacing the graphite plates with composite or metallic ones, the percentage of the plate cost in a stack could be decreased from approximately ۶۰ to ۱۵-۲۹% [۳].To the best knowledge of the authors, a large number of publications only investigated the fabrication and characterization of flat composites using different materials and methods. Additionally, the production of the flat plates necessitates additional machining steps such as grinding and CNC milling of flow field channels. These steps can significantly increase production time and cost. Hence, in the present paper, the compression molding process is used to manufacture thin composites. Using the proposed approach in this research, we aim to integrate the molding, grinding, and machining steps into a single short press cycle to fabricate thin BPs containing gas flow fields. Then, we will characterize the molded composites in terms of their mechanical and electrical properties, which are fundamental in PEMFC applications.As a comparison for the properties and molding defects, two types of commercial production are used. A flat-plate BMC۹۴۰-۸۶۴۹ and a BMC۹۴۰-۱۵۲۵۲A black bulk were purchased from Bulk Moulding Compound Inc., USA. The compression mold halves were made using P۲۰ (DIN ۱.۲۳۱۱) mold steel. The walls and surfaces of the halved were polished for better surface quality. We implemented two cartridge heating elements, one Pt۱۰۰ thermocouple inside each mold halves, and two thermostats to heat the molds and control the temperature. For the BMC material, we tried various values of pressure, time, and temperature to show the effects of processing methods. Flexural strength of the fabricated composites is measured using a three-point bending test on a universal testing machine (Santam STM-۲۰, Iran) following the ASTM D۷۹۰ standard [۴]. Contact resistance between GDL and BP interfaces is an essential parameter that can cause significant power loss in PEMFCs. The interfacial contact resistance between the samples and GDL was measured using two different under pressure configurations of cell components based on Ohm’s law Our goal was to reach ۱.۸ mm of thickness in the short cycle time. To overcome the problem ofnonuniform temperature distribution, we implemented a press machine with heating platens for the BMC۹۴۰-۱۵۲۵۲ black bulk material. The mold halves were also polished to reach an excellent surface finish,and the same cycle provided in Table ۲ is used. The flat surface and the surface with gas flow channels ofthe molded sample are shown in Fig. ۱ The proposed cycle by the material producer is used for themolding process The thickness of the molded sample is ۱.۸ mm and has a tolerance of ±۰.۰۳ mm over theentire area of ۶ cm×۶ cm. The flat side of the sample in Fig. ۱a shows an excellent surface appearance.The flowing channels are also molded in high quality without any defects, as shown in Fig. ۱b The depthof channels on the cavity mold was ۰.۶۵ mm, and the molded sample had channel depth between ۰.۶۳ and۰.۶۵ mm.US DOE requirement suggested values higher than ۲۵ MPa for the flexural strength. Commerciallypurchased flat plates (BMC ۹۴۰-۸۶۴۹) exhibited a mean value of ۴۴ MPa as their flexural strength. Thisvalue is in good accordance with the material datasheet to be ۴۰ MPa. BMC molded samples showedmechanical strength much higher than the US DOE criterion, and the BMC-P had the best flexuralstrength of ۴۷ MPa, which is more closer to ۵۶ MPa measured by the BMC company. BMC #۲ showedthe best contact resistance behavior (۱۸ mohm.cm۲) among the molded composites. However, a slightdeviation also exists between the two BMC materials. The discrepancy can be ascribed to the differentbulk conductivity of these two compositions. Another reason is the grinding process, which has done onthe commercially purchased BMC ۹۴۰-۸۶۴۹ samples.We concluded that utilizing a proper press machine along with a uniform temperature distribution on themold surfaces, could drastically enhance the properties of the parts. We observed that using the BMCcomposition, high-quality bipolar plates containing desired flow field channels (with ۰.۵mmof width and۰.۶۵mmof depth) can be produced in one press cycle in a short time. The average measured flexuralstrength of BMC plates was ۴۸ MPa, which is almost twice the US DOE requirement (۲۵ MPa).Concerning the ICR between the molded samples and the GDL, on the other hand, the molded BMCcomposites showed competitive properties in comparison to commercially available BMC plates