In addition to the major fuel cell types described above, there are other fuel cells that are mentioned in scientific journals from time to time, and also cells that are described as 'fuel cells', but are not really so.
A fuel cell is usually defined as an electrochemical device that converts a supplied fuel to electrical energy (and heat) continuously, so long as reactants are supplied to its electrodes. The implication is that neither the electrodes nor the electrolyte are consumed by the operation of the cell. Of course, in all fuel cells the electrodes and electrolytes are degraded and subject to 'wear and tear' in use, but they are not entirely consumed in the way that happens with two of the three types of cells briefly described below, both of which are sometimes described as 'fuel cells'.
One type of genuine fuel cell that does hold promise in the very long term is the biological fuel cell. These would normally use an organic fuel, such as methanol or ethanol. However, the distinctive 'biological' aspect is that enzymes, rather than conventional 'chemical' catalysts such as platinum, promote the electrode reactions. Such cells replicate nature in the way that energy is derived from organic fuels. However, this type of cell is not yet anywhere near commercial application, and is not yet suitable for detailed consideration in an application-oriented book such as this.
The biological fuel cell should be distinguished from biological methods for generating hydrogen, which is then used in an ordinary fuel cell. This is discussed in Chapter 8.
1.6.2 Metal/air cells
The most common type of cell in this category is the zinc air battery, though aluminium/air and magnesium/air cells have been commercially produced. In all cases the basis of operation is the same. Such cells are sometimes called zinc fuel cells.
At the negative electrode, the metal reacts with an alkaline electrolyte to form the metal oxide or hydroxide. For example, in the case of zinc the reaction is
The electrons thus released pass around the external electric circuit to the air cathode where they are available for the reaction between water and oxygen to form more hydroxyl ions. The cathode reaction is exactly the same as for the alkaline fuel cell shown in Figure 1.4. The metal oxide or hydroxide should remain dissolved in the electrolyte. Cells that use a salt-water electrolyte work reasonably well when they use aluminium or magnesium as the 'fuel'.
Such cells have a very good energy density. Zinc/air batteries are very widely used in applications that require long running times at low currents, such as hearing aids. Several companies are also developing higher power units for applications such as electric vehicles. This is because they can also be 'refuelled' by adding more metal to the anode - which is why they are sometimes called fuel cells. The fact that the cathode reaction is exactly the same as for a fuel cell, and that the same electrodes can be used, is another reason. However, the electrolyte also has to be renewed to remove the metal oxide. Thus, they consume both the anode and the electrolyte, and cannot really be described as fuel cells. They are mechanically rechargeable primary batteries.
Another type of cell sometimes called a fuel cell is the redox flow cell (Vincent and Scrosati, 1997). In this type of cell, the reactants are removed from the electrodes during charging and are stored in tanks. The capacity of such cells can thus be very large. They are discharged by resupplying the reactants to the electrodes. Because the operation of the cell involves supply of chemicals to the electrodes, these devices are sometimes called fuel cells. However, this is a misnomer, as will become clear.
This type of cell is used to make very large capacity rechargeable batteries and may be used by electricity utilities to balance peaks in supply and demand. There are a number of different chemistries that can be used. Cells based on vanadium have been made (Shibata and Sato, 1999), as have zinc/bromine systems (Lex and Jonshagen, 1999). This type of system is perhaps best exemplified in the so-called Regenesys™ fuel cell (Zito, 1997 or Price et al., 1999).
The operating principles of the Regenesys™ system is shown in Figure 1.17. Two fluids ('fuels') are involved. When fully charged, a solution of sodium sulphide (Na2S2) in water is fed to the negative electrode, and a sodium tribromide (NaBr3) solution is fed to the positive electrode. The reaction at the negative electrode is
The electrons flow around the external circuit, and the sodium ions pass through the membrane to the positive electrolyte. Here the reaction is
So, as the system discharges, the sodium sulphide solution gradually changes to sodium polysulphide, and the sodium tribromide solution changes to sodium bromide. Figure 1.18 shows such a system under construction in Cambridgeshire, England. The two tanks to hold the solutions can be seen, together with a building that will hold the cells. This particular system has a storage capacity of 100 MWh, which is equivalent to the energy held in about 240,000 typical lead acid car batteries, and is believed to be the largest electrochemical electrical energy storage system in the world.
The rationale for calling this system a fuel cell is presumably that the electrodes are simply a surface where reactions take place and are not consumed. Furthermore, the electrodes are fed an energy-containing liquid. However, the electrolyte most certainly changes during operation, and the system cannot work indefinitely. Also, the electrolyte solutions are not fuels in any conventional sense. Indeed, this is a rather unusual and very high capacity rechargeable battery. Exactly the same arguments apply to the other cells of this type.
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