Bipolar fuel cells, Chapter

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. 

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],... 

Because a fuel cell functions at a low voltage (/.c., well below 1 V), it is customary to build up the voltage to the desired level by electrically connecting cells in series to form a stack . This is achieved by means of a bipolar plate-and-frame arrangement similar to that employed for electrolysers see Section 4.2, Chapter 4. There are a number of different designs of fuel cell, but in each case the unit cell has certain components in common. These are as follows. 

The final component of a fuel cell to be discussed in this chapter is the current collector plates (CCP) or more commonly called the bipolar plates (BP) or flow field plate (FFP). As observed in Fig. 7.1 the fuel cell stack is composed by a number of BP, each of which will separate a pair of MEA and with two end plates completing the stack. At either side of the BP, an arrange of channels provide the flow paths for the fuel and oxygen. One of those faces is in contact with the anode of one MEA and the other face with the cathode of the other MEA, hence, the name bipolar plate. The end plates have channels only on one face, and the stack is typically completed by two metal plates with a series of bolts that holds the stack... 

Engineering issues of fuel cell stack and systems design will be dealt with in the following chapters. Hence, issues of, e.g., coflow or counterflow within one cell, stoichiometry and utilization of fuel and oxidant, temperature and current distribution in a fuel cell of technical scale, and, certainly, issues of stacking cells into a bipolar arrangement will not be discussed here. 

It follows from what was said above that to raise the lifetime of PEMFCs a number of problems must be solved, the most important being (1) the development of catalysts less prone to recrystallize and of supports for the catalysts that are more corrosion resistant (Chapter 12) (2) the development of more highly stable membranes (Chapter 13) (3) optimized conditions of water management, to avert temporary flooding of the oxygen electrode and (4) finding new materials for the bipolar plates, the seals, and other structural elements of the fuel cell. 

Basically, the construction of PAFCs differs little from what was said in Section 1.4 about fuel cells with liquid acidic electrolyte. In the development of PAFCs and two decades later in the development of PEMFCs (described in Chapter 3), many similar steps can be distinguished, such as the change from pure platinum catalysts to catalysts consisting of highly disperse platinum deposited on a carbon support with a gradual reduction of platinum content in the catalyst from 4 to 0.4 and then to 0.25 mg/cm, and the change from pure platinum to Pt-Ru catalysts. The bipolar graphite plates that have special channels for reactant snpply and distribution over the entire electrode surface now used widely in PEMFC stacks were first used in PAFCs. 

The basic arrangement for external manifolding is as shown in Chapter 1, Figures 1.12 and 1.13. The electrodes are about the same area as the bipolar plates, and the reactant gases are fed in and removed from the appropriate faces of the fuel cell stack. One advantage of external manifolding is its simplicity, enabling a low-pressure drop in the manifold... 

The fuel cell stacks (5) are arranged as two units connected in parallel. Each half consists of 10 stacks, each of power 13kW. The maximum electrical power is thus 260 kW. The cells are operated at 90°C, and both the fliel and the air are humidified, as explained in Sections 4.4.5 and 4.4.6 of Chapter 4. The operating pressure of the stacks is increased when higher power is needed, up to a maximum of 207 kPa above ambient pressure. Each half unit probably consists of about 750 cells in series. They are constructed in the mainstream fashion for PEMFC, that is, with graphite, or graphite/polymer mixture, bipolar plates. They are water-cooled, as we would expect from Section 4.5.3. 

Abstract This chapter describes the behavior and stability of metallic bipolar plates in polymer electrolyte fuel ceU application. Fundamental aspects of metallic bipolar plate materials in relation to suitability, performance and cell degradation in polymer electrolyte fuel cells are presented. Comparing their intrinsic functional properties with those of carbon composite bipolar plates, we discuss different degradation modes and causes. Furthermore, the influence and possible improvement of the materials used in bipolar plate manufacturing are described. 

In this chapter, the basic components, requirements, and functions of a generic fuel cell were also discussed. The reader should be familiar with the functions of the current collectors (also known as a bipolar plate or cell interconnect), the flow Adds, the anodic and cathode electrodes with the concept of the triple-phase boundary, and the electrolyte. The reader should also understand the flow of current (ionic and electronic) through these components. 

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