Bipolar plate filler

Thermoset-based graphite composite is one of fhe composite materials often used to fabricate bipolar plates. The major filler or reinforcemenf in fhe composite is graphite in a form of powder, flake, or fiber, with additions of carbon powder/fiber (mainly to reduce the cost). 

Mathur RB, Dhakate SR, Gupta DK, Dhami TL, Aggarwal RK (2008) Effect of different carbon fillers on the properties of graphite composite bipolar plate. J Mater Process Tech 203 184-192... 

The effects of the hybrid carbon fillers on the physical properties of the composites for their application as bipolar plates of fuel cells have been investigated. Different types and different sizes of the conducting fillers show synergistic effects on the flexural strengths and the electrical conductivities. 

Not only the binder polymer, but also the carbon fillers have to be characterized with respect to their thermal stability. In the next TGA example, several PPS-bonded bipolar plate samples with different contents of highly conductive carbon nanotubes (0-4 % CNT) are investigated by TGA. As shown in Fig. 19.5, the peak of thermal decomposition at approximately... 

Owing to the chemical inertness of some selected polymers and the corrosion stability of carbon filler materials, composite-based bipolar plates are the first choice for applications demanding long-term stability. 

To achieve adequate conductivities, the composite bipolar plates generally consist of a polymer, which functions as a binding matrix, and a high content of conductive filler materials. Composite plates offer an economic route for producing bipolar plates. For example, the composite material can be produced in an extruder and subsequently injection molded or compression molded to bipolar plates. 

First, a few studies on metal-filled composite bipolar plates are briefly described. At Los Alamos National Laboratory (LANL) composite bipolar plates filled with porous graphite and stainless steel and bonded with polycarbonate (Hermann, 2005) has been developed. Kuo (2006) investigated in composite bipolar plates based on austenitic chromium-nickel-steel (SS316L) incorporated in a matrix of PA 6. Their results showed that these bipolar plates are chemically stable. Furthermore, Bin et al. (2006) reported a metal-filled bipolar plate using polyvinylidene fluorid (PVDF) as the matrix and titanium silicon carbide (TijSiCj) as the conductive filler and obtained an electrical conductivity of 29 S cm" with 80 wt% filling content. 

Owing to the chemically nearly inert nature, low costs and sustainable availability of the substance class of carbon-based fillers are preferred as filler materials for bipolar plates and carbon composite bipolar plates have been extensively investigated. The following section is structured in the different ways of composite bipolar plate production and the resulting eligible materials. 

Wolf and Wilier (2006) reported an investigation with liquid crystal polymer-based bipolar plates. As the conductive filler, a combination of carbon fibers and carbon blacks at a loading level below 40 wt% was used and a conductivity-level up to 6 S/cm was achieved. Furthermore, they suggested a model of synergetic interaction effects of different fillers used in this study as depicted in Figure 6.3. 

Another approach is slurry molding and this technique was firstly used to manufacture carbon-carbon composites by Besmann et al. (2003). This process mixes phenolic resin with carbon fillers in water to create slurry which is fed out and vacuum molded into a preform. A second process called carbon chemical vapor infiltration (CVl) is then used to seal the plate for gas impermeability and for improvement of electrical conductivity. This process has been further developed by Huang et al. (2005) to reduce the cost caused by the CVl process and by Cunningham et al. (2007) to improve the properties of the bipolar plates. However, the mechanical properties of the bipolar plates were not found to be as high as the solely wet-lay-based plates (see Figures 6.7 and 6.8). 

Nonetheless, it can be seen that a range of fillers and polymeric matrices were studied and different ways were used for the production of composite bipolar plates. Graphite and carbon black as fillers are to be found in nearly every study, and accordingly, some polymers are preferred to be used for composite bipolar plates. The reasons for both the fillers and the matrices are obvious. Graphite and carbon black have outstanding chemical stability against corrosion when compared with metallic fillers they achieve an adequate conductivity and are obtainable at a reasonable price. In case of the matrices the chemical corrosion resistance is also a main criterion, and polyolefin materials, fluoropolymers, polyphenylene sulfide (PPS), and phenolic resins are particularly favored. 

Composite bipolar plates generally contain polymeric matrices as binder and filler materials such as graphite, carbon black, and others. As the degradation mechanism of these differ significantly they will be discussed separately in the following, starting with the polymeric matrices. 

Additionally, it has to be pointed out that for composite bipolar plates a high content of filler materials is necessary that leads to more brittle properties. Therefore, such losses in plastic properties lead to mechanical stress to the plate structure owing to swelling of the material. Furthermore, the intermediate formation of peroxides during exceptional fuel cell operation may also lead to decomposition of the polymeric matrix of bipolar plates. This leads in consequence to accelerated embrittlement of the composite material that finally would affect the stability and gas tightness of the bipolar plate. 

Second main material family in composite bipolar plates are the filler substances that on the one hand can contain impurities affecting the PEM fuel cell performance and on the other hand show degradation... 

Low costs, good processability, and high performance are the general targets for composite bipolar plates and they play a major role for selection criteria of the fillers as well as for the polymeric matrices. 

It is well known that the electrical conductivity of polymers loaded with conductive fillers, such as carbon black (CB), graphite particles (GPs), carbon nanotubes (CNTs), and metallic particles, exhibits a discontinuous increase with the filler loading. The phenomenon is explained in terms of the percolation theory [27]. When the concentration of the conductive filler reaches a critical value, termed the percolation threshold, a conductive path is formed in the composite along with a sudden jump in the electrical conductivity by several orders of magnitude [27]. However, even with this jump the conductivity obtained is still too low for bipolar plate applications. As such, filler concentrations much higher than the percolation threshold are required to raise the conductivity to the level suitable for bipolar plate applications [18-23]. Unfortunately, the conductivity derived in this way is obtained at the expense of processability and thus increases the manufacturing cost of bipolar plates. 

Composite bipolar plates are designed to provide high mechanical, electrical, and thermal properties by adding highly conducting fillers such as metals or carbon in an insulating base such as polymers. While metallic composites... 

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