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conventional thermal power plants (International Fuel Cells, 2000a, 2000b). Power plants based on fuel cells are more dependable than diesel-powered generators (Beardsley, 1996). They have lower maintenance costs and longer life expec- tancies than the alternatives (Baird and Hayhoe, 1993). The operation of fuel cells provides heat for space heating and is virtually pollution free, although when fueled by natural gas, fuel cells can produce a small amount of carbon dioxide. Compared with combustion-based processes for generating electricity, a one-megawatt fuel cell system would save more than 200,000 pounds of air pollutants and 11 million pounds of carbon dioxide from the atmosphere during each year of operation (International Fuel Cells, 2000a, 2000b). A typical, medium-sized car powered by fuel cells using hydrogen will have a system efficiency of more than 50 percent (Bellona Foundation, 2000). The same car with an internal combustion engine has an average fuel efficiency of 12 percent. History The possibility of generating electricity by reversing the electrolysis of water was discovered by Sir William Grove in about 1839 (Society of Automotive Engineers, 2001). Charles Langer and Ludwig Mond first used the term “fuel cell” in 1889 while attempting to create a practical fuel cell using coal gas (a mixture of hydrogen, methane, carbon monoxide, other hydrocarbons, carbon dioxide, nitrogen, and oxygen) and air. In the first few years of the 20th century, there were efforts to develop a fuel cell that would use carbon or coal to produce electricity. Francis Bacon developed a usable hydro- gen-oxygen cell containing an alkaline electrolyte and nickel electrodes in 1932. However, a practical system was not dem- onstrated by Bacon and his associates until 1959. In the same year, Harry Karl Ihrig presented a tractor of 20 horsepower that was powered by fuel cells (Society of Automotive Engi- neers, 2001). NASA began developing a compact generator of electricity for use on space missions in the late 1950s and fuel cells have been providing electricity and water on spacecraft since the 1960s. More recently, many companies and govern- mental agencies have supported research concerning fuel cell technology for possible use in stationary power plants, homes, vehicles, water craft, and small electronic devices including cell phones. Mainly because of the dependence of the United States and other nations on imported crude oil, research to develop less conventional sources of energy has accelerated during the past decade. Components The electrolyte in a fuel cell can be phosphoric acid, alkaline solutions (generally potassium hydroxide in water), molten carbonate (sodium, potassium, lithium, or magne- sium carbonate), a solid metal oxide (commonly calcium or zirconium oxide), or a solid polymer membrane [the proton- exchange membrane (PEM), a thin permeable sheet] (table 1). Using phosphoric acid, a silicon carbide matrix holds the elec- trolyte, and the electrodes are composed of finely dispersed platinum on carbon paper (U.S. Department of Energy, 2002). This type of fuel cell requires relatively pure hydrogen for the anode, operates at temperatures of 150–200 degrees Celsius (C) [about 300–400 degrees Fahrenheit (F)] (U.S. Depart- ment of Energy, 2002), and has an electrical output of as much as 200 kilowatts. This fuel cell is comparatively heavy and inappropriate for use in vehicles but can be used in stationary installations such as power plants. Fuel cells containing molten carbonate and solid metal oxide electrolytes can be used as highly efficient power-gen- erating stations, although these fuel cells are not portable and operate at much higher temperatures than cells using phos- phoric acid or PEM. The high temperatures enable hydrogen and carbon monoxide to be electrochemically oxidized at the anode (Steele and Heinzel, 2001). Potential fuels include natural gas and coal-derived gas. Existing units using molten carbonate electrolytes have electrical outputs of as much as two megawatts and their nickel electrode catalysts are inex- pensive compared to the platinum or palladium used in other cells. A pilot plant for the city of Santa Clara, California, that uses a molten carbonate electrolyte, operates at a temperature of 650 degrees C (1202 degrees F). Molten carbonate salts are highly corrosive and require carefully designed and main- tained facilities. If the electrolyte is a solid metal oxide, commonly a thin ceramic layer of zirconium oxide, the cathode is usually com- posed of lanthanum manganate and the anode is composed of nickel-zirconia (U.S. Department of Energy, 2002). This fuel cell (a direct fuel cell) is a promising option for high-pow- ered applications, such as industrial uses or central electricity generating stations. The cell operates at temperatures of nearly 1000 degrees C (1,832 degrees F) and has an electrical output of as much as 100 kilowatts. The high operating temperature allows the extraction of hydrogen from fuels without the use of a reformer. However, where the electrolyte consists of solid metal oxides, the fuel cells can suffer from leakage and sealing problems (Beardsley, 1996). In lightweight fuel cells for use mainly in automobiles, buses, and trucks, the electrolyte can be an alkaline solution or a solid polymer membrane [the proton-exchange membrane (PEM)]. These cells require relatively pure hydrogen for the anode (Steele and Heinzel, 2001). In alkaline electrolytes, the operating temperature is 150–200 degrees C (about 300–400 degrees F) and the electrical output ranges from 300 watts to 5 kilowatts (Smithsonian Institution, 2001; Society of Auto- motive Engineers, 2001). Alkaline cells were used in Apollo spacecraft to provide both electricity and drinking water. However, one disadvantage is that containers filled with liquid can leak. The PEM electrolyte is a solid, flexible, fluoride polymer film, a fluorocarbon ion exchange with a polymeric membrane (U.S. Department of Energy, 2002). This fuel cell will not leak Components 3PDF Image | An Introduction to Fuel Cells
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