Plates bipolar

Design Principles An individual fuel cell will generate an electrical potential of about 1 V or less, as discussed above, and a current that is proportional to the external load demand. For practical apph-cations, the voltage of an individual fuel cell is obviously too small, and cells are therefore stacked up as shown in Fig. 27-61. Anode/ electrolyte/cathode assemblies are electrically connected in series by inserting a bipolar plate between the cathode of one cell and the anode of the next. The bipolar plate must be impervious to the fuel... 

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. 

The bipolar plate material of the PAFC is graphite. A portion of it has a carefully controlled porosity that sei ves as a resei voir for phosphoric acid and provides ffow channels for distribution of the fuel and oxidant. The plates are elec tronically conductive but impervious to gas crossover. 

The bipolar plates are made from either Type 310 or Type 316 stainless steel, which is coated on the fuel side with nickel and aluminized in the seal area around the edge of the plates. Both internally and externally manifolded stacks have been developed. 

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. 

Since a typical voltage output from one cell is around 0.4-0.8 V, many cells must be connected together in series to build up a practical voltage (e.g., 200 V). A bipolar plate performs this cell-connecting function and also helps to distribute reactant and product gases to maximize power output. 

Molded bipolar plates for high-voltage battery and fuel power sources.b l... 

Bipolar plates for connecting fuel cells in a series. (Reproduced with permission from Wiley Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 12, Wiley, New York, 2002.)... 

Decreased cost, thickness, and weight of bipolar plates... 

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. 

Tai L-W and Lessing PA. Tape casting and sintering of strontium-doped lanthanum chromite for a planar solid oxide fuel cell bipolar plate. J. Am. Ceram. Soc. 1991 74 155—160. 

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Fuel cell technology continues to advance with materials research. The catalyst material has been one of the major expenses in fuel cell design. An anode with about 40% less catalyst has been developed at Forsc-hungszentrum Julich GmbH in Julich, Germany. It has a bipolar plate with areas of different catalytic activity levels. The anode substrate has one phase that does not act as catalyst to methane-vapor reforming reactions, and another phase where it acts as a catalyst. 

The costs of a PEMFC stack are composed of the costs of the membrane, electrode, bipolar plates, platinum catalysts, peripheral materials and the costs of assembly. For the fuel-cell vehicle, the costs of the electric drive (converter, electric motor, inverter, hydrogen and air pressurisation, control electronics, cooling systems, etc.) and the hydrogen storage system have to be added. Current costs of PEM fuel-cell stacks are around 2000/kW, largely dominated by the costs of the bipolar plates and... 

Cost targets exist for all parts of the fuel cell for bipolar plates, from 10/kW (2004) to 3/kW in 2015 for electrocatalysts, from 40/kW (2005) to 3/kW in 2015 and for membrane electrode assemblies (MEA), from 50/kW (2005) to 5/kW in 2015 (Freedom Car, 2005 these cost targets are somewhat different from those mentioned by the IEA (2005)). Since 2004, the number of fuel-cell cars has been growing and at the time of writing they numbered approximately 1000 worldwide there are also around 100 fuel-cell buses in use worldwide in several demonstration projects. But these cars are produced as individual (hand-built) models and are extremely expensive, with production costs per vehicle currently estimated at around one million large-scale production is not expected before 2015, see Section 13.1. 

An analysis of the individual PEM components offers evidence of almost unbroken R D see Fig. 13.10 (Jochem et al., 2007). The overall importance of the membrane is striking. Furthermore, the numbers of annual applications for bipolar plates (BPP) and the gas-diffusion layer (GDL) decrease after 2002, while the increase in membrane applications flattens out. This correlates with the equally lower number of fuel cell patents in the field of mobile applications. 

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]. 

There has been an accelerated interest in polymer electrolyte fuel cells within the last few years, which has led to improvements in both cost and performance. Development has reached the point where motive power applications appear achievable at an acceptable cost for commercial markets. Noticeable accomplishments in the technology, which have been published, have been made at Ballard Power Systems. PEFC operation at ambient pressure has been validated for over 25,000 hours with a six-cell stack without forced air flow, humidification, or active cooling (17). Complete fuel cell systems have been demonstrated for a number of transportation applications including public transit buses and passenger automobiles. Recent development has focused on cost reduction and high volume manufacture for the catalyst, membranes, and bipolar plates. 

Several designs for the bipolar plate and ancillary stack components are used by fuel cell developers, and these are described in detail (9, 10, 11, and 12). A typical PAFC stack contains... 

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. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

It helps to distribute the reactant gases or liquids evenly from fhe FF channels of the bipolar plates to the CL so that most of fhe active zones (and catalyst particles) are used effectively. Thus, the DL has to be porous enough for all the gases or liquids (e.g., liquid fuel cells) to flow without major problems. 

If provides mechanical support to the CL and the membrane in order for these two components to be unaffected by the pressure that the landings or ribs of the bipolar plate put on them. Therefore, fhe DL... 

It helps to conduct electron flow from the bipolar plates to the CL and vice versa with low resistance between them, hr order for the DL to be able to do this successfully, it has to be made of a material that is a good electronic conductor. 

It helps to transfer the heat produced from the CL to the bipolar plates in order to keep the cell at the desired temperature of operation. Thus, the DL should be made out of a material that has a high level of thermal conductivity so that removal of heat is as efficient as possible. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

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