Alternative Sources of Energy— An Introduction to Fuel Cells By E.A. Merewether U.S. Geological Survey Bulletin 2179 U.S. Department of the Interior U.S. Geological Survey Alternative Sources of Energy— An Introduction to Fuel Cells By E.A. Merewether U.S. Geological Survey Bulletin 2179 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2003 Version 1.0, 2003 This publication is available only online at: http://pubs.usgs.gov/bul/b2179 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. iii Contents Abstract ............................................................................................................................................................ Introduction .................................................................................................................................................... History ........................................................................................................................................................... 3 Components .................................................................................................................................................. 3 New Developments ...................................................................................................................................... 5 Evaluations..................................................................................................................................................... 5 Conclusion ..................................................................................................................................................... 6 References..................................................................................................................................................... 6 Glossary ......................................................................................................................................................... 9 Figure 1. A schema for a typical fuel cell wherein the electrolyte is a proton exchange membrane (PEM) and the catalysts are platinum.................................................................. 2 Table 1. Types of fuel cells—their components and characteristics ................................................ 4 Alternative Sources of Energy— An Introduction to Fuel Cells By E.A. Merewether Abstract Introduction Fuel cells are important future sources of electrical power This report describes and compares fuel cells and is one and could contribute to a reduction in the amount of petro of a series of reports from the Energy Program of the U.S. leum imported by the United States. They are electrochemi Geological Survey prepared in response to requests from the cal devices similar to a battery and consist of a container, an public for information regarding contemporary sources of anode, a cathode, catalysts, an intervening electrolyte, and an power and unconventional sources of electricity. The Energy attached electrical circuit. In most fuel cell systems, hydro Program is concerned mainly with resources of natural, gen is supplied to the anode and oxygen to the cathode which energy-rich materials, particularly with accumulations of results in the production of electricity, water, and heat. Fuel coal, crude oil, natural gas, and uranium minerals. However, cells are comparatively efficient and reliable, have no moving the Program also includes investigations of less developed parts, operate without combustion, and are modular and scale- sources of energy, including deposits of tar sand, oil shale, able. Their size and shape are flexible and adaptable. In opera and biomass. Additionally, there are tentative plans to review tion, they are nearly silent, are relatively safe, and generally do the energy potential of hydropower and gas-to-liquids tech not pollute the environment. nology as well as areas that might supply geothermal, solar, During recent years, scientists and engineers have or wind power. developed and refined technologies relevant to a variety of A fuel cell is an electrochemical device, similar to a bat tery, that generally combines hydrogen from any of several fuel cells. Types of fuel cells are commonly identified by sources and oxygen (which can come from air) to produce the composition of their electrolyte, which could be either electricity, heat, and water (Baird and Hayhoe, 1993). Basi phosphoric acid, an alkaline solution, a molten carbonate, a cally, a fuel cell is composed of an anode (a negative elec solid metal oxide, or a solid polymer membrane. The elec trode) and a cathode (a positive electrode), which are sepa trolyte in stationary power plants could be phosphoric acid, rated by a liquid or solid electrolyte (fig. 1). Generally, the molten carbonates, or solid metal oxides. For vehicles and electrodes are permeable or contain channels that distribute smaller devices, the electrolyte could be an alkaline solution hydrogen or other substances and oxygen. The electrodes are or a solid polymer membrane. For most fuel cell systems, the frequently accompanied by catalysts, commonly platinum or fuel is hydrogen, which can be extracted by several procedures palladium (Geyer, 2000). In most fuel cells, hydrogen atoms from many hydrogen-bearing substances, including alcohols, enter the cell at the anode where their electrons are removed, natural gas (mainly methane), gasoline, and water. producing direct current electricity and positively charged There are important and perhaps unresolved technical hydrogen ions (cations). Direct current can be converted to problems associated with using fuel cells to power vehicles. alternating current by an inverter. The electrons flow through The catalysts required in several systems are expensive metals an external circuit that extends from the anode to the cath of the platinum group. Moreover, fuel cells can freeze and not ode. The external circuit can include electric motors, lighting work in cold weather and can be damaged by impacts. Storage systems, or other electrical devices. The hydrogen ions travel tanks for the fuels, particularly hydrogen, must be safe, inex through the electrolyte to the cathode where they recombine pensive, of a reasonable size, and contain a supply sufficient with the electrons and oxygen to produce water and heat for a trip of several hundred miles. Additional major problems (Smithsonian Institution, 2001). will be the extensive and costly changes in the national infra Fuel cells are almost endlessly rechargeable and pro structure to obtain, store, and distribute large amounts of the ductive, operate without combustion, have no moving parts, fuels, and in related manufacturing. are nearly silent, and have an excellent safety record. Those 2 Alternative Sources of Energy—An Introduction to Fuel Cells electrons electrons H+ oxygen H+ 2H2 O2 H+ + 4H+ 4H+ + hydrogen + 4e­ ions 4e- hydrogen H+ 2H2O water H+ and heat H+ anode (-) catalyst electrolyte catalyst cathode (+) Figure 1. A schema for a typical fuel cell wherein the electrolyte is a proton exchange membrane (PEM) and the catalysts are platinum. Hydrogen molecules when added to the anode and a catalyst lose their electrons, which travel through an external circuit from the anode to the cathode, activating electrical devices enroute. Hydrogen ions formed at the anode and a catalyst move through the electrolyte to the cathode and another catalyst where they combine with the electrons and oxygen to produce water and heat. recently developed for powering automobiles have an esti extracts hydrogen from accompanying supplies of methanol, mated life span of decades (Baum, 2002). Fuel cells can be gasoline, or other substances (Geyer, 2000). Pure hydrogen for modular and scaleable; many joined together are called a fuel- the storage systems can be obtained from alcohols, naphtha, cell stack. These characteristics allow the gradual addition of benzene, methane, propane, gasoline, and diesel fuel. Hydro electrical capacity in response to increases in demand, as well gen is released when hydrocarbon-bearing materials in the as flexibility in the selection of sizes and locations for new presence of catalysts are subjected to pressurized steam (gas stationary power plants. If stationary power plants are built at ification) (U.S. Department of Energy, 1999, 2002). It can also sites of electrical need, less of the electricity generated is lost be obtained from water by electrolysis, where the electricity during transmission and distribution (Geyer, 2000). Presently, could be supplied by hydroelectric generators, wind turbines, the main planned uses of fuel cells are for the production of solar cells, or other producers of power. Hydrogen can also be electricity at stationary power plants and to supply electricity generated by photoelectrochemical and photobiological proce for motors that move buses, trucks, and cars. Other con dures (U.S. Department of Energy, 2002). In a few commercial templated applications include power for dwellings, trains, fuel cells, gaseous mixtures of hydrogen and carbon dioxide motorcycles, snowmobiles, watercraft, aircraft, and assorted are extracted from fossil fuels or biomass