The Bipolar Plates

Different concepts have been studied and used for the configuration of the channel system, from simple parallel channels to more complex arrangements [40], while several materials have been proposed to meet the requirements of chemical compatibility, corrosion resistance, electric and thermal conductivity, gas impermeability, robustness, lightness, and cost [41]. In particular, gas impermeability is a very important requisite, because it is necessary to avoid the direct oxidation of the fuel that would imply consequent loss of useful electrons and local heating dangerous for the MEA. 

An example of a stainless steel serpentine-based gas flow field is reported in Fig. 3.4 for a simple small size PEM stack (64 cm as active area). 

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 

A large diffusion may be found also for composite materials, carbon, or metal based. In the first case different types of polymeric resins (thermoplastics, such as polypropylene, polyethylene, and PVDF, or thermosettings, such as epoxies and phenolics) are filled with carbonaceous powders (graphite or carbon blacks), to provide a material characterized by very high chemical stability in the fuel cell environment and satisfactory properties of electrical conductivity, but which cannot offer sufficient robustness at thickness lower than 2 mm. The metal composite plates are essentially based on combinations (sandwiches of different layers) of stainless steel, porous graphite, and polycarbonates, with the aim to exploit the characteristics of different materials. Their fabrication can be more complex but this is compensated by the possibility to incorporate other functional components, such as manifolds, seals, and cooling layers. 

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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. 

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... 

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. 

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. 

In conclusion, at an intermediate optimum thickness, a diffusion layer allows for (1) gas diffusion toward the CL, (2) liquid transport from the CL toward the flow field channels, (3) good contact with both the bipolar plate... 

The bipolar plate with multiple functions, also called a flow field plate or separation plate (separator), is one of fhe core components in fuel cells. In reality, like serially linked batteries, fuel cells are a serial connection or stacking of fuel cell unifs, or so-called unif cells fhis is why fuel cells are normally also called sfacks (Figure 5.1) [2]. The complicated large fuel cells or module can consist of a couple of serially connecfed simple fuel cells or cell rows. Excepf for the special unit cells at two ends of a simple stack or cell row, all the other unit cells have the same structure, shape, and functions. 

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).)...

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. 

According to the structure, location, and role of the plate in fuel cells mentioned earlier, it is clear that the full function of the bipolar plate would be very important for the electrochemical reactions, heat and water management, and electrical current and power transfer in a stack. The specific functions of bipolar plates include ... 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

The bipolar plates, which separate both electrodes of neighboring cells (one anode of a cell and one cathode of the other), have a triple role ... 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

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. 

Thus, the heat release is directly related to the amount of product water. The next consideration is the amount of heat needed to raise fuel cell temperature from, for example, -30 to 0°C (AT = 30 K). The thermal mass of the fuel cell components comes in large part from the bipolar plates (BPPs), neglecting the end plates. With graphite bipolar plates of 1 mm thickness each, and assuming an adiabatic system, the required heat is... 

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