Carbon bipolar plates

The carbon/carbon bipolar plate fabricated by U.S. Oak Ridge National Laboratory. (Besmann, T. M. et al. In U.S. National Laboratory R D Meeting, http //www.pnl.gov/microcats/ott-review/ottmeeting/14-Besman.pdf (accessed Dec. 2008).)... 

Bac2 Ltd Formed in 2002 to develop commercial-scale electrically conductive polymer composites. Manufactures composite carbon bipolar plates using their patented Electrophen polymer. 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The elec trodes contain veiy finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite or metal. 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

The area of contact between the outer edge of the bipolar plate and the electrolyte structure prevents gas from leaking out of the anode and cathode compartments. The gas seal is formed by compressing the contact area between the electrolyte stmcture and the bipolar plate so that the hquid film of molten carbonate at operating temperature does not allow gas to permeate through. 

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. 

In this chapter, after recalling the working principles and the different kinds of fuel cells, the discussion will be focused on low-temperature fuel cells (AFC, PEMFC, and DAFC), in which several kinds of carbon materials are used (catalyst support, gas-diffusion layer [GDL], bipolar plates [BP], etc.). Then some possible applications in different areas will be presented. Finally the materials used in fuel cells, particularly carbon materials, will be discussed according to the aimed applications. To read more details on the use of carbon in fuel cell technology, see the review paper on The role of carbon in fuel cell technology recently published by Dicks [6],... 

Carbon meets many of these requirements and has been used by fuel cell makers over many years. Nevertheless, the development of low-cost, nonporous, carbon materials continues to be a challenge and the bipolar plate remains one of the most costly components in a PEMFC system. 

Ni-state-of-the-art anodes contain Cr to eliminate the problem of sintering. However, Ni-Cr anodes are susceptible to creep, while Cr can be lithiated by the electrolyte and consumes carbonate, leading to efforts to decrease Cr. State-of-the-art cathodes are made of lithiated-NiO. Dissolution of the cathode is probably the primary life-limiting constraint of MCFCs, particularly under pressurised operation. The present bipolar plate consists of the separator, the current collectors, and the seal. The bipolar plates are usually fabricated from thin sheets of a stainless steel alloy coated on one side by a Ni layer, which is stable in the reducing environment of the anode. On the cathode side, contact electrical resistance increases as an oxide layer builds up (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

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. 

Wu, M. and Shaw, L.L., A novel concept of carbon-filled polymer blends for applications in PEM fuel cell bipolar plates, Int. J. Hydrogen Energy, 30, 373, 2005. 

The individual components used in an AFC are not necessarily expensive compared to those of other fuel cell t3q>es under development. Use of Ft catalysts can be avoided, while the bipolar plates collecting the electron flows typically have to be made of fairly expensive black carbon to avoid corrosion. The peripherals needed for water management and electrolyte draining add to the cost, but do not necessarily lead to drawbacks such as long start-up... 

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. 

Fig. 1. Schematic presentation of a PEFC cross-section. The cell (left) consists of a membrane catalyzed on both sides (referred to as a membrane/electrode (M E) assembly ), gas-diffusion backing layers and current collectors with flow fields for gas distribution. The latter become bipolar plates in a fuel cell stack. The M E assembly described schematically here (right) shows catalyst layers made of Pt/C catalyst intermixed with ionomer and bonded to the membrane (large circles in the scheme correspond to 10 nm dia. carbon particles and small circles to 2 nm dia. platinum particles).

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