44 Connecting Cells in Series the Bipolar Plate

As has been pointed out in the previous section, the voltage of a working fuel cell is quite small, typically about 0.7 V when drawing a useful current. This means that to produce a useful voltage many cells have to be connected in series. Such a collection of fuel cells in series is known as a 'stack'. The most obvious way to do this is by simply connecting the edge of each anode to the cathode of the next cell all along the line, as in Figure 4.11. (For simplicity, this diagram ignores the problem of supplying gas to the electrodes.)

The problem with this method is that the electrons have to flow across the face of the electrode to the current collection point at the edge. The electrodes might be quite good conductors, but if each cell is only operating at about 0.7 V, even a small voltage drop is important. Unless the current flows are very low, and the electrode a particularly good conductor, or very small, this method is not used.

The method of connecting to a single cell, all over the electrode surfaces, while at the same time feeding hydrogen to the anode and oxygen to the cathode, is shown in Figure 4.12. The grooved plates are made of a good conductor such as graphite or stainless

Cathode

Oxygen fed to each cathode

Hydrogen fed to each anode

Electrolyte

Anode

For reactions in this part the electrons have to pass all along the face of the electrode.

Figure 4.11 Simple edge connection of three cells in series

Oxygen fed to each cathode

Hydrogen fed to each anode

Electrolyte

Anode

For reactions in this part the electrons have to pass all along the face of the electrode.

Figure 4.11 Simple edge connection of three cells in series

Figure 4.12 Single cell, with end plates for taking the current from all over the face of the electrodes, and also supplying gas to the whole electrode
Figure 4.13 Two bipolar plates of very simple design. There are horizontal grooves on one side and vertical grooves on the other

steel. This idea is then extended to the 'bipolar plate' shown in Figure 4.13. These make connections all over the surface of one cathode and also the anode of the next cell (hence 'bipolar'). At the same time the bipolar plate serves as a means of feeding oxygen to the cathode and hydrogen to the anode. A good electrical connection must be made between the two electrodes, but the two gas supplies must be strictly separated, otherwise a dangerous hydrogen/oxygen mixture will be produced.

However, this simple type of bipolar plate shown in Figure 4.13 will not do for PEM fuel cells. Because the electrodes must be porous (to allow the gas in) they would allow the gas to leak out of their edges. The result is that the edges of the electrodes must be sealed. This is done by making the electrolyte somewhat larger than the electrodes, and fitting a sealing gasket around each electrode, as shown in Figure 4.14.

Rather than feeding the gas in at the edge, as in Figures 4.12 and 4.13, a system of 'internal manifolding' is used with PEM fuel cells. This arrangement requires a more complex bipolar plate, and is shown in Figure 4.15. The plates are made larger relative to the electrodes, and have extra channels running through the stack which feed the fuel and oxygen to the electrodes. Carefully placed holes feed the reactants into the channels that run over the surface of the electrodes. It results in a fuel cell stack that has the appearance of the solid block, with the reactant gases fed in at the ends, where the positive and negative connections are also made.

Figure 4.16 shows a fairly high power PEM fuel cell system. It consists of four stacks made as described above, each a block of approximately square cross-section.

Figure 4.14 The construction of anode/electrolyte/cathode assemblies with edge seals. These prevent the gases leaking out of the edge of the porous electrodes

Hydrogen removed Air removed

Hydrogen removed Air removed

hydrogen to surface of anode

Figure 4.15 A simple bipolar plate with internal manifolding, as is usually used in PEM fuel cells. The reactant gases are fed to the electrodes through internal tubes hydrogen to surface of anode

Figure 4.15 A simple bipolar plate with internal manifolding, as is usually used in PEM fuel cells. The reactant gases are fed to the electrodes through internal tubes

Figure 4.16 An example of a fairly large PEM fuel cell system. Four separate fuel cell stacks can be seen. Each stack consists of 160 cells in series (Reproduced by kind permission of MAN Nutzfahrzeuge AG. This is the stack from the bus in Figure 4.2, and is made by Siemens.)

A further complication is that the bipolar plates also have to incorporate channels in them for cooling water or air to pass through, as fuel cells are not 100% efficient and generate heat as well as electricity.

It should now be clear that the bipolar plate is quite a complex item. A fuel cell stack, such as those of Figure 4.16, will have up to 80 cells in series, and so a large number will be needed. As well as being a fairly complex item to make, the question of its material is often difficult. Graphite, for example, can be used, but this is difficult to work and is brittle. Stainless steel can also be used, but this will corrode in some types of fuel cell. To form the gas flow paths, and to make the plates quickly and cheaply, plastic would be ideal. However, the bipolar plate must clearly be a very good conductor of electricity, and this is a great difficulty for plastics. The present situation is that no entirely satisfactory way of making these items has yet been developed, but many of the most promising options are discussed elsewhere, such as Ruge and Buchi (2001). It is certainly the case now, and will be for many years, that the bipolar plate makes a major contribution to the cost of a fuel cell, as well as its size and its weight.

Anyone who has made fuel cells knows that leaks are a major problem. If the path of hydrogen through a stack using internal manifolding (as in Figure 4.15) is imagined, the possibilities for the gas to escape are many. The gas must reach the edge of every porous electrode, so the entire edge of every electrode is a possible escape route, both under and over the edge gasket. Other likely trouble spots are the joins between each and every bipolar plate. In addition, if there is the smallest hole in any of the electrolyte, a serious leak is certain.

The result is that a fuel cell is quite a difficult system to manufacture, requiring parts that are complex to form rapidly and cheaply. Very careful assembly is required, and each fuel cell stack consists of a large number of components. The system has a very low level of fault tolerance.

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