Fuel cells convert chemical energy to electrical energy by combining hydrogen from fuel with oxygen from the air. Hydrogen fuel can be supplied in two ways—either directly as pure hydrogen gas or through a "fuel reformer" that converts hydrocarbon fuels such as methanol, natural gas, or gasoline into hydrogen-rich gas. A fuel cell's only emission is water.

Fuel cells have been used in the space program since the early 1960s and are currently used in approximately six hundred office buildings, industrial facilities, and hospitals in the United States. Most automobile makers are experimenting with fuel cell–powered vehicles. DaimlerChrysler and United Parcel Service are testing fuel cell–powered delivery vans and fuel cell–powered city buses are being tested in Washington, DC. In his 2003 State of the Union, President George W. Bush proposed spending $1.2 billion to fund fuel cell research.

All fuel cells contain two...