Potentials of individual cells are about 1 V. Most applications require at least a few volts, but often hundreds of volts are desired. Single cells can be fabricated and then connected in series to increase the voltage, just as is done in batteries. The first approach, which is used commonly in batteries, is referred to as a monopolar design as shown Figure 10.3a. The monopolar design consists of distinct positive and negative electrodes. Multiple electrodes are often connected in parallel to form a cell with increased area as shown in the figure. The cells are then connected in series to increase the voltage as required for the application. Series connections are made external to the individual cells as the current is collected from one cell and then directed into the next cell via an external electrical connection.
Figure 10.3 Monopolar and bipolar designs. The diagram on the left (a) shows the monopolar design; below this the connections required to build voltage are illustrated. The bipolar design is shown on the right (b).
In contrast to batteries, the most common approach to building voltage in fuel cells is to use what is known as a bipolar design or bipolar stack as shown in Figure 10.3b. In the bipolar configuration, one positive and one negative electrode are mated together. The reactants are still physically separated, but the negative electrode of one cell is electrically connected to the positive electrode of the next cell in the stack. Thus, there is a single plate that serves as the current collector for two cells—hence the name bipolar plate and bipolar configuration. Note that in the bipolar design the current does not have to be directed in and out of the cells, rather current can pass in a straight line along the axis of the stack. Because external wires are not needed to connect cells in series in order to build voltage, the bipolar configuration is more compact than the monopolar design.
Current collection for the monopolar design has been discussed previously in Chapter 8 for batteries. Similarly, at each end of a bipolar fuel-cell stack, current is collected and directed into wires or bus bars. Here, the design challenges are similar to those found in the monopolar design.
The bipolar plates between cells in the fuel-cell stack are critical to our ability to utilize effectively the advantages of the bipolar configuration. Of primary importance is that this bipolar plate serves as a barrier so that reactants from the anode do not mix with those of the cathode from the next cell. The prevention of mixing is essential from both safety and efficiency perspectives. A second important function of the bipolar plate is to collect current from the cathode of one cell and pass it to the adjoining anode of the next cell. A highly conductive bipolar plate will tend to even out any local nonuniformities in the current from an individual cell.
Assuming that the bipolar design is preferred, are we always going to put all of the cells in series in a single stack? The answer is no. As the number of cells increase, it become progressively more difficult to assemble the cell stack. Further, there are times when one long stack just doesn’t suite the available space allocated for the fuel cell. In these instances, two, three, or more cell stacks may be fabricated and then the stacks connected together in series with external wires. In some instances, we may face the reverse problem, although this is rare. Namely, the area of the individual cell calculated as in Illustration 10.2 cannot be used because of manufacturing limitations or space constraints. In such cases, it is possible to split the fuel cell in two cells with smaller areas, and then connect them in parallel with external wires.
We have determined the area of individual cells in the stack, but not their shape. For bipolar configurations, planform is used to describe the shape of the cell when viewed from above. A rectangular planform is the norm, but how might we decide on the aspect ratio? Often, space limitations dictate the aspect ratio. Assuming that we aren’t constrained by the available space, then ease of manufacturing and pressure drops associated with reactant flows are critical factors. As shown in Figure 10.4, another function of bipolar plates is to direct the flow of reactants over the planform area of the cell. Details of these flow fields will be addressed in Section 10.6. Of course, the bipolar stack is not the only approach used to build voltage in fuel cells. A notable example is the so-called tubular design used in SOFCs. Some of the challenges and advantages of this design were explored in Problem 9.9 of the previous chapter.
Figure 10.4 Basic components of a bipolar fuel-cell design.
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