Bipolar plate based composite

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. 

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). 

Graphite-based composites and metal/alloy materials both have their own advantages and drawbacks. Current research interests in bipolar plate materials include both graphite composites and coated metals. No doubt progress on these materials will eventually lead to substantial reduction in the volume and cost of the fuel cell stack. 

Depending on operation temperature the material chosen for the bipolar plate is graphite (or graphite composites) or iron-based alloys (stainless steel) for low- and medium-temperature fuel cells (PEFC, PAFC, MCFC) and chromium or ceramic-based materials for the high-temperature systems (SOFC). In SOFC, for sealing reasons, tubular concepts have also been developed (see Section 8.1.4.6). 

Bipolar plates are the components performing a number of tasks such as reactant supply, heat exchange, electron transfer to the external circuit, physical strengthening of the cell, etc. The material requirements for bipolar plates include high mechanical strength, corrosion resistance, electronic conductivity, low density, etc. [154]. CNT/polymer compos-ites-based bipolar plates enhance the fuel cell performance considerably [155,156]. The relevant properties such as electrical conductivity, mechanical strength, contact resistance and chemical inertness of the composites exhibit large improvements in PEM fuel cells [157-159]. 

Dweiri and Sahari [16] investigated the electrical properties of fuel cell bipolar plates of carbon-based PP composites. Combinations of carbon black and graphite lead to composites with a higher electrical conductivity. 

From a materials durability point of view, carbon/polymer composite materials are to be preferred. However, metal-based bipolar plates enable the use of very thin plates, thus leading to an increase in volumetric power density. Major car manufacturers such as Honda and Toyota are using metal-based bipolar plates in... 

In general, there are two different types of bipolar plate technologies one based on metal and the other based on graphite as a conductive component. The latter can be pure (expanded) graphite or a graphite composite material containing a polymer binder. Metal plates offer excellent electrical and thermal conductivities and they can be machined to very small thicknesses. Additionally, they do not tend to form cracks or break even at small thicknesses and their nonporous structure offers good gas... 

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. 

TABLE 6.2 Properties of Some Composite-Based Bipolar Plates (Extended)... 

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. 

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). 

Dana Holding Corporation Produce composite graphite-based and metallic bipolar plates. Also developing advanced coatings that enhance the properties of the bipolar plates. 

Although it is a matter of common knowledge that stainless steel is quite prone to corrosion in fuel cells, bare substrates of different alloys were tested in past material investigations. In 1998, Hornung and Kappelt (1998) selected different iron-based materials for Solid Polymer Fuel Cell bipolar plates by using the pitting resistance equivalent (PRE = %Cr + 3.3%Mo + 30%N) as corrosion resistance criterion. The authors exposed that some iron-based materials with PRE >25 (the material compositions are not given... 

Derieth et al. (2008) immersed injection-molded polypropylene-bonded and PPS-based compression-molded composite bipolar plates. The authors exposed some sets of composite bipolar plates to different liquids (10 per investigation). Deviations in weight, thickness, surface topography, and electrical as well as mechanical properties were determined before and after exposure. Furthermore, to identify leachant ionic and organic species, the liquids were analyzed. 

Bornbaum (2010) exposed epoxy-based and phenohc-bonded composite bipolar plates for 14 days in phosphoric acid (W = 85 wt%) at 160°C and after exposure a rockwell hardness test (HR 10/40) was applied to determine the changes in the physical and/or chemical nature of the different thermosetting matrices. 

Although both polymer types are of thermosetting nature the authors operate the epoxy-based bipolar plates successfully in their low temperatm-e fuel cell stacks, it obviously can be seen that there are significant differences between the epoxy-based and the phenoUc-bonded composite and the epoxy one seems to be not suitable for use in high-temperature fuel cells. 

Bipolar plates in two ways can contribute to the limited lifetime of PEM fuel cell stacks first is the physical decomposition of the materials or parts of the materials resulting in mechanical instability, leakage or decrease of electrical conductivity second is the release of additives and materials to the fuel cell environment forcing degradation processes in further fuel cell parts such as gaskets, MEAs, and peripheral components. These phenomena are to be considered both for composite-based and metal-based bipolar plates. 

Dweiri, R. and Sahari, J. 2007. Electrical properties of carbon-based polypropylene composites for bipolar plates in polymer electrolyte membrane fuel cell (PEMFC). Journal of Power Sources 171 424. 

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