Composite Bipolar Plate

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

Schematic indication of bipolar plates in simplified PEM fuel cells. The bipolar plates and end plates (ElectroPhen ) were designed by Bac 2 Conductive Composites Inc. (http //www.bac2. co.nk/fuel-cell-appHcations/ (accessed Dec. 2008).)...

Section 5.2.2 include composites and metals. From a cost reduction point of view, it is estimated, according to the cost model, that the cost percentage of the plate in a stack can be reduced from -60 to 15-29% if the graphite plate were replaced by the composite plate or metal plate [15]. However, many uncertain factors are involved in the estimation. The progress and major challenges in development of bipolar plates fabricated by these candidate materials will be introduced in the following parf of this section. 

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). 

Besmann, T. M., J. J. Henry, Jr., E. Lara-Curzio, et al. 2003. Optimization of a carbon composite bipolar plate for PEM fuel cells. Materials Research Society Proceedings 756 F7.1.1-F7.1.7. 

Cho, E. A., U. S. Jeon, H. Y. Ha, et al. 2004. Gharacteristics of composite bipolar plate for polymer electrolyte membrane fuel cells. Journal of Power Sources 125 178-182. 

The bipolar plate design is illustrated in Fig. 47. It consists of a cross-flow arrangement where the gas-tight separation is achieved by dense ceramic or metallic plates with grooves for air and fuel supply to the appropriate electrodes. A porous cathode, a dense and thin electrolyte and a porous anode form a composite flat layer placed at the top of the interconnected grooves. The deposition of the porous electrodes can be achieved by mass production methods. Moreover, the bipolar plate configuration technology makes it possible to check for defaults, independently and prior to assembly of the interconnection plate and the anode-electrolyte-cathode structure. 

FIGURE 12.11 Comparison of liquid water transport for two 50-cm single-cell PEM fuel cells using commercial graphite composite bipolar plates (a) surface modified and (b) as received (0.1 A/cm, 1.5/2.0 Hj-air stoichiometry, 100% RH). 

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. 

Overall, the component reliability is a challenge to fnel cell mannfactnrers as well as their component snppliers. The stack is only one of several snbsystems in a PEM fuel cell system with hundreds of parts and components. Component compatibility, which includes both chemical and mechanical properties, plays an important role in system reliability and overall performance. To select the best materials/design for a system component, one mnst first stndy its properties (physical, chemical, mechanical, and electrochanical) nnder relevant conditions snch as temperature, pressure, and composition. Eor example, the reactant side of a PEM fuel cell bipolar plate (all sealing materials and plate components) mnst be able to tolerate high humidity, temperature... 

Blunk, R., Zhong, R, and Owens, J., Automotive composite fuel cell bipolar plates hydrogen permeation concerns, J. Power Sources, 159, 533, 2006. 

Blunk, R. et al.. Polymeric composite bipolar plates for vehicle applications, J. Power Sources, 156, 151, 2006. 

Besmann, T.M. et al., Carbon/carbon composite bipolar plate for proton exchange membrane fuel cells, J. Electrochem. Soc., 147, 4083, 2000. 

Wolf, H. and Willert-Porada, M., Electrically conductive LCP-carbon composite with low carbon content for bipolar plate application in polymer electrolyte membrane fuel cell, J. Power Sources, 153, 41, 2006. 

Huang, J., Baird, D.G., and McGrath, J.E., Development of fuel cell bipolar plates from graphite filled wet-lay thermoplastic composite materials, J. Power Sources, 150, 110, 2005. 

The MCFC is a promising power generating source because of its unique characteristics such as high fuel efficiency and ability to use various carbonaceous fuels. Although Ni-10wt% Cr is used in the state-of-the-art MCFC as anode, it needs to be improved in terms of better creep and sintering resistance. In spite of the development in the alternate cathode material research, lithiated nickel oxide has been the choice of cathode material in the kilowatt-level MCFC stacks developed by many companies. Continuous research in the development of stable electrolyte retention matrix, identification of suitable molten carbonate electrolyte composition, and additives to the electrolyte will be a significant milestone. Also, research in the area of current collector/bipolar plate to overcome... 

PVDF/graphite composites can lend themselves as the best materials for both molded and coated plates. A key component of a fuel cell stack is the bipolar plate. Also referred to as a water transport plate or a separator... 

The most suitable materials result to be non-porous graphite, metals (aluminum, stainless steel, titanium, and nickel), and composite sohds. Graphite made nonporous by impregnation with impermeable substance was early used for bipolar plates, but its applicability is limited by difficulties in machining and consequent costs. The metal plates present the obvious advantages of high robustness and low... 

Develop high conductivity composite bipolar plates to allow high power density designs to be... 

For the fabrication of the 50-kW PEM fuel cell stack, compression-molded composite bipolar plates... 

Figure 5. Performance of 63-cell PEM Fuel Cell Stack with Compression Molded Composite Bipolar Plates...

Figure 1. Infrared Images of Carbon Composite Bipolar Plate Material Samples...

During this period, the mechanical properties of the carbon composite bipolar plate material were assessed, including measurements of uniformity and defect distribution by infrared imaging. The results indicate good material uniformity and substantial resistance to torsional stresses. Significant improvement in the wetting of the material was demonstrated after a simple oxidation treatment. 

---