Bipolar plate processing

Interconnects are formed into the desired shape using ceramic processing techniques. For example, bipolar plates with gas channels can be formed by tape casting a mixture of the ceramic powder with a solvent, such as trichloroethylene (TCE)-ethanol [90], Coating techniques, such as plasma spray [91] or laser ablation [92] can also be used to apply interconnect materials to the other fuel cell components. 

The major function of a bipolar plate, or simply called "plate," is to connect each cell electrically and to regulate the reactant gas (typically, hydrogen and air in a hydrogen fuel cell) or reactant liquid (typically, methanol in a DMFC) and liquid or gas coolant supply as well as reaction product removal in desired patterns. This plate must be at least electrically conductive and gas and/or liquid tightened. Considering these important functions and the larger fraction of volume, weight, and cost of the plate in a fuel cell, it is worthwhile to construct this chapter with emphasis on the current status and future trend in bipolar plate research and development, mainly for the plate materials and fabrication process. 

The plate at the two ends of a cell row or stack is called the end plate and has a slightly different structure from that of normal bipolar plates in the stack. The end plate actually is a "single-polar" plate with only the fluid field on the inside surface contacting the anode or the cathode of the unit cell at either end of the stack. The outside surface of the end plate is flat with fluid ports as shown in Figure 5.2. The end plate normally contacts the other cell row or system as electrical and fluid input/output connections. Because the end plate is normally made of the same material through similar processing to that of the bipolar plate in a stack, the bipolar plate and end plate will be called a plate hereafter in this chapter unless their differences are addressed. 

Yuan, X. Z., H. J. Wang, J. J. Zhang, et al. 2005. Bipolar plates for PEM fuel cells— From materials to processing. Journal of New Materials for Electrochemical Systems 8 257-267. 

For the HTE process, the electrochemical cell consists of a tri-layer ceramic, well known for its brittleness, which limits applied loads. In addition, the relatively low ionic conduction properties of the electrolyte materials (3% yttrium-stabilised zirconia) requires an operating temperature above 700°C to reduce ohmic losses. This creates difficulties for the involved metallic materials, including bipolar plates and seals. 

Hawkes A, Leach M, (2005). Sohd oxide fuel cell systems for residential micro-combined heat and power in the UK Key economic drivers. Journal of Power Sources, 149 72-83 Hellmana H, van den Hoed R, (2007). Characterising fuel cell technology Challenges of the commercialisation process. International Journal of Hydrogen Energy 32 305 - 315 Hermann A, Chaudhuri T, Spagnol P, (2005). Bipolar plates for PEM fuel cells A review. 

From a cross-flow point of view it may be of interest to mention the phosphoric acid fuel cell with the so-called DiGas system (Fig. 9), which is an air-cooled cross-flow configuration for use in utility-power stations [39]. The process air stream is diverted into two types of channels into individual cells with relatively small cross-sectional area, and into cooling plates (approximately one for every five cells) with a lai ge cross-section. Bipolar plates were molded from a mixture of graphite and phenolic resin, with a Pt-on-carbon cathode and a Pt anode combined with colloidal PTFE on a graphite-paper backing. 

B Bipolar plate with process air L fuel channels CA Anode DIGAS cooling plate CC - Cathode DIGAS cooling plate... 

Bulk phase formation by current or voltage pulses results in different nucleation and crystal growth conditions compared to dc deposition and depends on the electrolyte and the pulse regime (unipolar, bipolar, pulse reverse, etc.) itself [6.98]. Several effects, which are of significance for the pulse plating process, can be distinguished. 

Phase II focuses upon process development to result in a pilot production line capable of producing 300 bipolar plates per hour. Our goal is a complete functional pilot line, including all relevant quality assurance, failure mode and effects analysis, and statistical manufacturing characterization processes. This will be completed by transferring the most promising mass-production technique to laiger-scale and continuous equipment operation in a dedicated production line. 

Preliminary work completed in this project includes laboratory and equipment setup and installation, and preliminary rounds of material optimization and process development. Full size bipolar plate prototypes have been produced with full double-sided flow patterns, demonstrating the potential of the manufacturing process. Process and material development has resulted in the characterization of material properties under a variety of composition levels. Material properties meeting or exceeding DOE targets have been measured, and bipolar plates, both machined and pattern-embossed, have been submitted to UTC Fuel Cells for in and out of cell testing. Phase I work will... 

Thermoplastic carbon composite materials are a favonrable material combination for bipolar plates becanse they can be mannfactnred by the mass production process of injection monlding [102]. Electrical condnctivity of a carbon composite requires a high content of carbon, nsnally a mixtnre of graphite and active coal. The percolation limit of the graphite in the polymer binder has to be exceeded, leading to direct contact between graphite particles. Additionally, a basic condnctivity of the polymer matrix by the smaller active carbon particles is achieved. The injection... 

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