Fabrication of multi-filler thermoset-based composite bipolar plates for PEMFCs applications: Molding defects and properties characterizations
Graphical abstract
Introduction
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 [1].
Characteristics and the manufacturing process of each fuel cell component have a significant effect on the final properties of PEMFCs. Moreover, there are still debates on the ways to reduce the cost and increase the durability of each fuel cell components for large-scale commercialization of PEMFCs [2,3].
Bipolar plate (BP) is among the most critical components of a fuel cell. The primary role of a BP is to connect the adjacent unit cells electrically. Also, to regulate thermal management inside the 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. Furthermore, BPs should be impermeable to the reaction gases in order to prevent the interactions between the gas species and to separate the cells, effectively [4]. Among different properties, high mechanical strength and low interfacial contact resistance (ICR) between the BP and gas diffusion layer (GDL) are essential properties of BPs to increase the cell durability and performance, respectively. Another characteristic that is of high importance in the fuel cell environment is corrosion resistance. High corrosion current density leads to ion leaching and contamination of the cell membrane [5].
There are not many materials that can fulfill these contradictory requirements for BPs [6]. 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, its poor machinability, 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 [7]. Although, by using metallic bipolar plates, thin plates with high conductivity are obtainable [8,9]. However, metallic BPs suffer from their low corrosion resistance in the fuel cell environment, which necessitates an additional coating process to obtain a corrosion-resistance surface [7,[10], [11], [12]].
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 60 to 15–29% [2]. Moreover, both material selection and manufacturing processes of the BPs should be cost-effective to reduce the overall cost of the cell. Non-metallic composites have drawn the attention of researchers and developers in recent years due to high corrosion resistance, low cost, light-weight, and ease of fabrication [[13], [14], [15], [16], [17]].
Various types of resins and fillers (or fibers) can be applied to the composition of the composite. Thermoset or thermoplastic polymers are employed in the literature [7,[18], [19], [20], [21], [22], [23], [24]]. Thermoplastics generally show a lower chemical resistance and lower creep strength in comparison to thermoset materials. Also, their low glass transition temperature limits their applications in many types of fuel cell applications [2,25]. Thermosets, on the other hand, are more suitable for high-temperature fuel cell applications and has a shorter cure time. They also provide higher strength and lower toughness. Their most important feature in composite manufacturing is that thermosets have a lower viscosity in higher temperatures compared to thermoplastic materials, which enables the developers to load them with higher amounts of fillers [26]. Hence, the properties can be improved by utilizing more percentages of filler (or fibers) to some extent. It is also proved that using multi-fillers leads to better properties compared to single filler composites [27]. The effects of different filler/fibers are investigated in the literature. Despite using graphite which is a popular filler in many researches related to BPs, carbon fiber (CF) [4,22,28,29], carbon black, carbon nanotubes [16,22,30], graphene [4,20,22,31], and expanded graphite (EG) [23,32,33] are the most intriguing fillers in this field.
Different methods are used in the literature to fabricate graphite-based composite for fuel cell applications. Injection molding of thermoplastics and compression molding of thermosets are the most common techniques. The most essential advantage of compression molding is that in this process, the viscosity of the final formulation is not needed to be very low. However, in the injection molding, lower viscosity is required to ensure the appropriate flowing of the material [7,34].
In recent years, Li et al. [24] fabricated composite bipolar plates contain water-soluble phenolic resin and reinforced with expanded graphite using resin vacuum impregnation and hot press method. They concluded that the resin concentration has considerable effects on mechanical properties, resistivity, gas permeability, and corrosion current density of the fabricated plates. Chaiwan and Pumchusak [35] compared wet and dry methods for the fabrication of multiwalled carbon nanotubes - reinforced graphite/phenolic resin. They observed that the dry nanoparticle method is an appropriate option for such composites and can lead to better electrical and mechanical properties. Pandey et al. [21] prepared high-density polyethylene composites filled with different percentages of flake graphite by a combination of melt mixing followed by injection molding. They asserted that higher content of graphite flake could increase the thermo-mechanical properties of the composites. Lee et al. [36] implemented different sizing of graphite particles and used the extrusion and compression molding process to produce fluorinated ethylene-propylene/graphite composite bipolar plates. They found that controlling the orientation and dispersion of graphite particles inside the composite can enhance the mechanical and electrical properties. Boyaci San and Okur [37] applied the response surface methodology to survey the effects of process parameters during the compression molding process on the physical and electrical properties of the synthetic graphite/carbon/phenolic resin composites. They found that the molding temperature has considerable effects on surface appearance and electrical conductivity. In a recent publication, Liao et al. [38] proposed a new low-carbon composition consists of thermoplastics filled with graphene/graphite/carbon fiber ternary carbon materials. They used a hot press to mold the samples and applied carbon coating treatment on both surfaces of the composites. Using this method, they reached high flexural strength and high in-plane electrical conductivity.
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 thermoset-based multi-filler 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. Our goal is to manufacture bipolar plates with 1.8 mm of thickness containing a gas flow field pattern with 0.65 mm of channels depth and 0.5 mm of channels width in a single press cycle. We have used Novolac phenolic resin as polymer binder and carbon fiber, flake graphite powder, expandable, and expanded graphite as reinforcements. We have also employed a ready-to-mold BMC material (Bulk Moulding Compound Inc., USA) for comparison purposes and also to discuss the processing parameters. Then, we will characterize the molded composites in terms of their physical, mechanical, and electrical properties, which are fundamental in PEMFC applications. The BMC material datasheet, and also requirements of the US department of energy (DOE) are employed to assess our measurements.
Furthermore, remedies for molding defects during the manufacturing process of the composite bipolar plate are less discussed in the literature. Therefore, we try to address some molding defects, which can result in blemished parts and defects such as inside/surface porosities and delamination. These defects exacerbate the surface appearance of the parts and deteriorate the physical, mechanical, or electrical properties of the composites.
Section snippets
As-received materials
Different composite plates are manufactured using different types and amounts of fillers. Novolac phenolic resin powder with a purity of 97–99% and a size of less than #150 mesh with 2.06 g/cm3 of density was purchased from Moheb Group (Tehran, Iran). The melting point of this phenolic resin is 95–105 °C and contains 8 wt% of Hexamethylenetetramine (HMTA) curing agent. The flake graphite powder with 99% purity was received locally. It is proven that flake-shaped graphite has superior flexural
Material and composite preparation
The GICs are graphite flakes which underwent an acid treatment process. During this process, the dried graphite mixed and saturated with a mixture of nitric and sulfuric acids. Then, the GICs can be expanded up to hundred times by exposing them to microwave radiation or high-temperature ovens [40,41,44]. In this work, the as-received GICs exposed to a heat treatment in a muffle oven with a temperature of 800 °C for over 15–20s, then cooled slowly at room temperature. This process produced puffy
Molding defects and troubleshooting
Several blemishes can occur during compression molding that spoils the appearance of the plate [51,52]. During the curing process of thermoset resins, different volatiles such as water vapor or ammonia will be generated as a result of cross-linking reactions between Novolac resin and the HMTA. We designed air vents on the molds to allow gases to escape, but it is observed that venting is not enough, especially in short curing cycles. Trapping of gases and volatiles inside the resin matrix
Conclusion
The primary purpose of the current investigation was to fabricate thermoset-based composite bipolar plates using a hot compression molding process. We observed that inappropriate process parameters during the process could dramatically spoil the surface appearance and also cause severe internal defects for the produced BPs. These defects can intervene in the final desired properties of the compositions, and we tried to mitigate the defects during the compression molding process. We concluded
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14 - compression molding
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2023, International Journal of Hydrogen EnergyCitation Excerpt :By adding thermoplastics to epoxy resin, fracture toughness can be improved without reducing other required mechanical properties. Thermoplastics have the characteristics of high temperature melting and cooling curing, thus they can form a stable homogeneous phase after blending with epoxy resin and curing [22]. In addition, thermoplastics have a wide temperature resistance range, so the cryogenic environment of has little impact on their performance.