FUEL CELL

any of a class of devices that convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell is much more efficient than most other types of energy converters. A fuel cell resembles a primary celli.e., a voltaic battery. In both types of devices, chemical reactions occur so that electrons are released on one electrode and caused to flow through an external circuit to a second electrode. There is, however, one major difference between batteries and fuel cells. In the former, the active ingredients are incorporated within the electrodes and are chemically altered during the reaction, becoming depleted with use. In fuel cells, a gas or liquid fuel is supplied continuously to one electrode and oxygen or air to the other from an external source. Because of this feature, fuel cells are able to produce electrical energy for a substantially longer period of time than can batteries (see also battery). During the late 1830s, William R. Grove of Great Britain built what is often considered the first fuel cell. His device produced electric current from hydrogen and oxygen reacting on platinum electrodes. Throughout the balance of the century there was a continuing effort to produce electricity from direct electrochemical oxidation of a conventional fuel such as coal or from the carbon monoxide and hydrogen derivable from coal. Molten salts as well as aqueous solutions were tried as electrolytes. Although all the basic principles of operation were recognized and considerable ingenuity applied to development, none of this early work led to devices able to compete with electric generators driven by steam or water power. Reactions were inefficient, rates were too slow, and cell life too short. After World War II there was a significant increase in interest in the development of practical fuel cells and batteries capable of large-scale electric-power generation. The promise of improvements resulting from new technology, the demanding requirements of military and space-vehicle programs, and the concentrated effort to reduce atmospheric pollution from power plants and internal-combustion engines accelerated fuel-cell research and development by the early 1960s. Many kinds of fuel cells were shown to be operable, and a number of them were refined into practical systems. Hydrogen, methyl alcohol, hydrazine, and some of the simpler hydrocarbons have been used directly as fuels. Air and oxygen have been employed as oxidants. A variety of electrolytes have been utilized: concentrated alkaline or acid solutions usually at temperatures below 150 C (300 F), and molten carbonates and other salts at temperatures of several hundred degrees Celsius. Certain cells have used ionically conducting modified zirconium oxide as a solid electrolyte at temperatures near 1,000 C (1,830 F). In most modern fuel cells, the electrodes consist of porous metal or carbon and at lower temperatures include catalysts to increase reaction rates to reasonable levels. Some fuel-cell types have been built into systems incorporating a means for storage and controlled supply of fuel and oxidant and for the removal of heat and reaction products. Two such systems, using liquid hydrogen and oxygen, have served as a primary electric-power source for U.S. manned spacecraft. For possible future space flights of long duration (e.g., missions to other planets), solar or nuclear generators are considered more suitable, though regenerative fuel cells (i.e., those whose operation can be reversed to regenerate hydrogen and oxygen) may be used to supplement these devices. Hydrogen-oxygen fuel cells also have been used to power forklift trucks and small automotive vehicles on an experimental basis. Various exploratory attempts have been made to introduce fuel cells into commercial use, but so far none have been particularly successful. Fuel cells that operate on methanol, however, have been employed on a limited scale to power television repeater stations and navigation beacons. any of a class of devices that convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell is much more efficient than most other types of energy converters. Additional reading Stanley W. Angrist, Direct Energy Conversion, 4th ed. (1987), provides a historical introduction and overview. Other general references include Manfred Breiter, Electrochemical Processes in Fuel Cells (1969); and Robert Noyes (ed.), Fuel Cells for Public Utility and Industrial Power (1977). David Linden (ed.), Handbook of Batteries and Fuel Cells (1984), provides comprehensive information on types and applications. Brooke Schumm, Jr.