Graphite-based bipolar plates (2)

The use of nongraphitic carbon materials like coke seems to be restricted to compsites with binders which can be carbonized with a high carbon yield. The conductivity of nongraphitic carbon is lower than the of graphite; therefore, carbonization and graphitization of the bipolar plate in an niert atmosphere is necessary to achieve reasonable conductivity. Binders with a high carbon yield are phenolic resins, polymerized furfuryl alcohol, polyamides, furan resins and pitches. If the time and energy-consuming step of graphitization is avoided, at least a major fraction of the conductive component needs to be graphitic, but the addition of small amounts of carbon black is helpful in facilitating percolation at a lower amount of conductive filler. Another disadvantage of the graphitization treatment is the shrinkage that accompanies the conversion of the carbonaceous material into graphite because it can damage the structure of the plate. To obtain a dense, compact plate repeated re-impregnation followed by carbonization is necessary, which makes the process ineffective and costly. Thus, it is more advantageous to subject the raw material to graphitization instead of the molded plate.

To obtain the composite, the conductive component must be dispersed within the polymer binder. In earlier applications mainly liquid dispersants were used, i.e., the polymer binder is dissolved in an organic solvent, and the graphite component is dispersed within this solution to obtain a slurry. Then the solvent is removed by air drying and further drying under vacuum. But the use of organic solvents is not desirable because of health and environmental protection concerns. It turned out to be rather difficult to obtain a homogeneous material by a dry-mixing process. Quite severe conditions, i.i., increased temperature and application of high shear, were found to be helpful in achieving a uniform distribution of the filler. Nevertheless cold mixing is sometimes preferred because of the advantage that no encapsulation of the graphite particles by the molten polymer can occur. The dry mixing can be carried out in a discontinuous way using a kneader, or in a continuous way using an extruder. Compounds of superior homogeneity are obtained with a double-screw extruder. The polymer is continuously fed into the extruder where is is preheated, compacted, and subsequently plasticized. According to the desired weight ratio, an adequate amount of the conductive component is added downstream at a point where the polymer is already plasticized, and the conductive particles get dispersed within the plasticized polymer. From the extruding procedure the material is obtained as rods or pellets. After cooling and solidification, the pellets or rods can be granulated or milled into a particle size suitable for further processing.

To process the composite into a plate, techniques commonly used in the processing of plastics such as compression molding can be employed. The mixture is loaded into a mold and heated under pressure to a temperature where the polymer becomes plasticized. For thermoplastics a temperature in the range of 10-50K above the melting temperature is usually applied. For thermosets a temperature at which the polymer is softened is first applied to shape the article, followed by further thermal treatment at a higher temperature to complete curing. Optionally, compression molding can be followed by a carbonization and graphitization treatment. But the shrinking of the polymer within the process of carbonization may cause structural damage to the plate. Therefore using composites with sufficient conductivity is now preferred so that carbonization is not necessary.

When the composite is compression-molded into a blank plate, the further step of engraving the flow field structure by CNC-driven milling machines is necessary, as it was common practice with bulk graphite plates. This process is time-consuming and expensive due to tool wear, the latter also resulting in reduced dimensional accuracy of the flow field structure. So this technique is only useful for prototypes and small series. To obtain structured plates within one process step, molding dies with integrated flow fields are employed. Since die costs are relatively high, this technology becomes more attractive the larger the lot sizes are.

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