Define fuel cell: it is a device that converts the chemical energy of a fuel into electricity by a chemical reaction with oxygen or a other oxidizing agent.IntroductionHydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols such as methanol are often used. Fuel cells differ from batteries in that they require a continuous source of fuel and oxygen/air to maintain the chemical reaction whereas in a cell the battery chemicals react with each other and generate an electromotive force (emf). Fuel cells can produce electricity continuously as long as these inputs are provided. There are many types of fuel cells, but all consist of an anode, a cathode, and an electrolyte that help move charges between the two sides of the fuel cell. Electrons are attracted to the anode along the cathode through an external circuit, producing electricity continuously. As the main difference between fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use followed by the difference in startup time which is 1 second over 10 minutes for PEMFC SOFC. Fuel cells come in a variety of sizes. Individual fuel cells produce a relatively low electrical potential, about 0.7 volts, so cells are "stacked" or placed in series to increase the voltage to meet the needs of an application. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other pollutants. Fuel cell energy efficiency is typically between 40 and 60%, or up to 85% efficient CHP if waste heat is captured for use. Hisalongry:...... middle of paper ...... it does not degrade over time and can theoretically provide electricity continuously.* Power plants must be large to improve their efficiency , but fuel cells can achieve higher performance at any scale, making them perfect for small mobile, residential, and transportation uses.* Because fuel cells are efficient and clean no matter what Their size means they can be installed almost anywhere, including dense urban areas where air quality and transportation congestion may be concerned. Fuel cells could offer an alternative to building new power lines, while reducing the United States' dependence on foreign oil. Fuel cells can provide more reliable power where electricity is needed, making the entire power grid more robust and reliable. Distributed application of small fuel cells will also improve the security of key infrastructure, such as communications and our water systems. .