Electric vehicle
Hydrogen Fuel Cells – Basic Principles
Download 3.49 Mb. View original pdf
|
| 5.2 Hydrogen Fuel Cells – Basic Principles 5.2.1 Electrode Reactions We have seen that the basic principle of the fuel cell is the release of energy following a chemical reaction between hydrogen and oxygen. The key difference between this and simply burning the gas is that the energy is released as an electric current, rather that heat. How is this electric current produced? To understand this we need to consider the separate reactions taking place at each electrode. These important details vary for different types of fuel cell, but if we start Figure 5.2 A fuel-cell-powered bus in use in Germany. Vehicles like this, used all day in cities, and refuelling atone place, are particularly suited to being fuel cell powered. (Reproduced by kind permission of MAN Nutzfahrzeuge AG.) 90 Electric Vehicle Technology Explained, Second Edition with a cell based around an acid electrolyte, we will start with the simplest and the most common type. At the anode of an acid electrolyte fuel cell the hydrogen gas ionises, releasing electrons and creating H + ions (or protons): 2H 2 → H+ 4e − (5.2) This reaction releases energy. At the cathode, oxygen reacts with electrons taken from the electrode, and H + ions from the electrolyte, to form water: O 2 + e+ H 2H 2 O (5.3) Clearly, for both these reactions to proceed continuously, electrons produced at the anode must pass through an electric circuit to the cathode. Also, H + ions must pass through the electrolyte. An acid is a fluid with free H + ions, and so serves this purpose very well. Certain polymers can also be made to contain mobile H + ions. These materials are called proton exchange membranes (PEMs), as an H + ion is also a proton, and their construction is explained below in Section Comparing Equations (5.2) and (5.3) we can see that two hydrogen molecules will be needed for each oxygen molecule if the system is to be kept in balance. This is shown in Figure 5.3. It should be noted that the electrolyte must only allow H + ions to pass through it and not electrons. Otherwise, the electrons would go through the electrolyte, not round the external circuit, and all would be lost. 5.2.2 Different Electrolytes The reactions given above may seem simple enough, but they do not proceed rapidly in normal circumstances. Also, the fact that hydrogen has to be used as a fuel is a disadvantage. To solve these and other problems many different fuel cell types have been tried. These different types are usually distinguished by the electrolyte that is used, though there are always other important differences as well. Most of these fuel cells have somewhat different electrode reactions than those given above however, such details are given elsewhere (e.g. Larminie and Dicks, 2003). Download 3.49 Mb. Share with your friends:
|