a fuel cell is an electrical device which when fed with fuel at a continuous rate produces an electrical output indefinately as long as it is supplied with fuel (unlike a chemical battery where once chemicals have exhausted there is no more charge produced). Cells convert hydrogen or hydrogen containing fuels directly into electrical energy. With the electrochemical reaction of hydrogen and oxygen, heat is produced. The level of heat is dependent on the fuel cell type employed. The type of electrolyte used dictates their performance characteristics, making each classification of fuel cell appropriate for a particular application. A fuel cell operates in the reverse mode to that of electrolysis, being efficient relative to various forms of heat engines. They have virtually silent operation and if the fuel employed is hydrogen then there is no pollution. Fuel cells are in principle similar to primary batteries, excepting that the fuel and oxidant ( i. E. Hydrogen and oxygen) are stored externally. Individual fuel cells typically generate dc voltage of 0. 7volts-0. 8 volts with a power output of a few tens of watts. Fuel cells are assembled in modules known as "stacks" to produce larger outputs. Fuel cells offer the prospect for high electrical efficiency, quiet operation, modular construction and significantly lower emissions of pollutants relative to conventional energy conversion techniques.