Types of Fuel Cells

There are many different kinds of fuel cells, which are distinguished primarily by the electrolyte used and also by the type of fuel that is consumed. The most common fuel is hydrogen gas; hydrocarbons are frequently reformed to produce hydrogen for consumption in the fuel cell. Fuels other than hydrogen can also be used directly, such as in the methanol fuel cell discussed earlier. The electrolyte is important because, more than anything else, the nature of the electrolyte determines the structure of the electrode, the design of the individual cell, how individual cells are combined to form the cell-stack assembly (CSA), and even the architecture and control of the fuel-cell system. In all instances, the electrolyte should have a high ionic conductivity and a low electronic conductivity. It should also provide a reasonably good barrier to the transport of reactants between electrodes. Referring back to Figure 9.1, without a barrier the methanol and oxygen would mix and combust on the electrocatalyst, releasing heat rather than undergoing separate electrochemical reactions to produce electrical work.

Some of the more common fuel-cell types are identified in Table 9.1. Note that the temperature of operation ranges from room temperature up to 1000 °C. When evaluating a new fuel cell, understanding the physical and chemical properties of the electrolyte is a natural place to begin with. The choice of electrolyte and temperature of operation are closely linked. The properties of the electrolyte influence the temperature of operation, the electrocatalysts selected, the approaches to water and thermal management, and the system design. Ultimately, these electrolyte properties also drive the selection of materials.

Table 9.1 Types of Fuel Cells

Each type of fuel cell has its own advantages and limitations. For example, a low-temperature fuel cell may offer the advantages of rapid start-up and shutdown, as well as the possibility of using a broader set of materials for fuel-cell construction. However, such fuel cells may be challenged to find the electrocatalysts needed to carry out the oxidation and reduction reactions effectively at the lower operating temperature. The appropriate type of fuel cell depends on the application, and the references available at the end of the chapter provide information on fuel cells suitable for a wide variety of applications. For our purposes here, we narrow the focus of the rest of this chapter to fuel cells that use hydrogen as the fuel. Natural gas, higher alcohols, and a variety of hydrocarbons can also serve as fuels and will be treated in the next chapter. While it is possible to use other oxidants, oxygen is used almost invariably, and air is likely the source of the oxygen.

Let’s now consider the operation of hydrogen fuel cells in more detail. Operating with hydrogen as the fuel, Table 9.2 shows the electrode reactions for three electrolytes: alkaline, acid, and an oxygen-conducting ceramic. Note that the half-cell reactions are different in different electrolytes. However, the overall reaction obtained from each pair of half-cell reactions is the same, namely, the reaction of hydrogen and oxygen to form water:

Additionally, the half-cell potentials sum to the same value, 1.229 V, under standard conditions. Nonetheless, the operation and design of the fuel cell depend dramatically on the nature of the electrolyte. For example, while hydrogen always reacts at the anode and oxygen at the cathode, water can be produced at either the anode or the cathode, depending on the electrolyte. The current carrier is different for each of the three electrolytes considered. In the acid case, the ionic current is conducted by protons that move from the anode to the cathode. In the case of the alkaline cell, hydroxyl ions carry the current in solution and move in the opposite direction (cathode to anode), although the direction of the ionic current does not change. Finally, for the oxygen-conducting ceramic, oxygen ions move from the cathode to the anode. In all circumstances, electrons flow through the external circuit from anode to cathode.

In spite of important differences, such as those just mentioned, the electrochemical principles that govern the operation of fuel cells are much the same. The application of those principles is explored in the next section.