Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion-conducting membrane. In the basic fuel cell, oxygen passes over one electrode and hydrogen over the other; in doing so, it generates electricity, water, and heat. Fuel cells chemically combine the molecules of a fuel and oxidizer without burning or having to dispense with the inefficiencies and pollution of traditional combustion.
Some other important characteristics of fuel cells are as follows:
• Charge carrier. The charge carrier is the ion that passes through the electrolyte. The charge carrier differs among different types of fuel cells. For most types of fuel cells, however, the charge carrier is a hydrogen ion, H+, which has a single proton.
• Contamination. Fuel cells can be contaminated by different types of molecules. Such a contamination can lead to severe degradation in their performance. Because of the difference in electrolyte, catalyst, operating temperature, and other factors, different molecules can behave differently in various fuel cells. The major contamination agent for all types of fuel cells is sulfur-containing compounds, such as hydrogen sulfide (H2S) and carbonyl sulfide (COS).
• Fuels. Hydrogen is currently the most popular fuel for fuel cells. Some gases, such as CO and CH4, have different effects on fuel cells, depending on the type of fuel cell. For example, CO is a contaminant to fuel cells operating at relatively low temperatures, such as the proton exchange membrane fuel cell (PEMFC). However, CO can be used directly as a fuel for the high-temperature fuel cells, such as the solid oxide fuel cell (SOFC).
• Performance factors. The performance of a fuel cell depends on numerous factors, such as the electrolyte composition, the geometry of the fuel cell, the operating temperature, and gas pressure. The geometry of the fuel cell is affected mainly by the surface area of the anode and cathode.
A valuable source that covers introductory to highly technical information on different types of fuel cells is the Fuel Cell Handbook published by the U.S. Department of Energy. It is freely available on the Internet (U.S. Department of Energy, 2000) or from the fuel cell test and evaluation center of the U.S. Ministry of Defense (FCTec, 2008).
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