Bipolar plate fabrication

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. 

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).)... 

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. 

Many alternative materials have been investigated to replace graphite in the fabrication of bipolar plates. The major candidate materials with potential to overcome the technical barriers and reach the targets mentioned in... 

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). 

Injection molded thermoplastic bipolar plates made of Vectra LCP (liquid crystal polymer) and Fortron PPS fabricated by Ticona Engineering Polymers, (http //www.ticona.com/redesign/ index/markets/innovation/fuel cell.htm Ticona Engineering Polymers. 2008 accessed March 2008.)... 

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. 

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). 

Alternative materials for bipolar plates include graphite, stainless steels, titanium and aluminium, all with a developed fabrication technique, and coating technique if needed. Major competitors UTC Fuel Cells has an active fuel cell bus programme, but give sparse details of its flow plate and other technology. (See UTC web site.)... 

Figures 3-5 describe how, by fabricating the framed MEA and stacking multiple MEA/bipolar plate building blocks containing flow fields for air and fuel streams (Fig. 2), the PEFC stack fabrication is completed. The stack is indeed the heart of any PEFC power system and the quality of a PEFC power system depends to large degree on the quality of the stack,...

It should be noted that the products of this decomposition are water, carbon dioxide, and HF. While PFSA membrane FCs have been demonstrated for many thousands of hours, the flux of HF is significant enough so that uncoated metallic bipolar plates are precluded. Hard to machine graphite bipolar plates must be used or an electrically conducting corrosively resistive coating must be developed for easily fabricated metal bipolar plates. Lifetime studies of PEM... 

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

Fabricate fibrous component preforms for the bipolar plate by slurry molding techniques using carbon fibers of appropriate lengths. 

The present market situation is characterized by a small number of units and hand fabrication. Figure 8.18 compares the present situation of the stack production by a research institution with the prediction of ADL by plotting the cost distribution of PE fuel cell stacks. The present cost for the manufacture of a single unit is about a factor of 10-50 more expensive than the values of the ADL analysis. As can be seen, the importance of bipolar plates is presently about equal to that of the MEA and labor-intensive tasks like assembling and quality control contribute significantly to the cost of the stack system. These are the cost drivers that can be... 

Lu and Wang [48] have reported that by using stainless steel plates as bipolar plates (electrodes) with the flow field machined with photochemical etching technology a micro-DMFC produce maximum power density of 62.5 mW cm" at 40°C and 100 mW cm at 60°C at atmospheric pressure. The active electrode area is only 1.625 cm. The power generated by the stainless steel DMFC is double to that of the Si-based DMFC which the authors had previously fabricated. 

Bipolar plate materials have historically been metals coated with corrosion-resistant layers or graphite with a seal treatment (to lower the gas permeability). In recent years, major efforts have been made for developing RP bipolar plates. The new plates usually have molded-in gas flow channels so that they can be fabricated rapidly and cost-effectively. However, the cost of bipolar plates ( 10/plate with 400 cm ) today is still too high to be applied to automotive and other civil power applications, and the conductivity is marginal. 

Carbon constitutes the most abundant element of the different FC components. Setting aside the membrane, which is a polymer with a carbon backbone, all the other components, i.e. the CL, GDL and current collector plates (bipolar plates) are made almost entirely of graphitic carbon. The electrocatalyst support of the CL is commonly carbon black in the form of fine powder. GDLs are thin porous layers formed by carbon fibers interconnected as a web or fabric, while current collector plates are carbon monoliths with low bulk porosity. As explained above each of these components has a particular function within the fuel cell and in particular in the triple phase boundary. The structure and properties of the carbon in each of the different FC components will determine the whole performance of the cell. 

Similarly, research conducted on materials and fabrication methods for GDL and bipolar plates aim to tune their properties in order to improve the fuel cell performance. It is clear that the current trend is the integration of the MEA components in order to improve the architecture of the triple phase boundary region and, consequently, the mass and charge transport. 

The anode is usually a thin layer of Ni-YSZ cermet. At temperatures over 1173K, the kinetics of steam reforming reactions are faster than the electrochemical oxidation of hydrogen or carbon monoxide. The other principal component is the bipolar plate, which is commonly fabricated from LaCrOa (Mg-doped) which is predominantly an electronic conductor and is compatible with other cell components at 1273 K. It is however relatively brittle and expensive, and there is a continuous effort to replace it with other materials preferably with a metallic system. 

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