Hydrogen can be used as a fuel for a modified internal combustion engine or in a fuel cell. Fuel cells are electrochemical devices that directly convert hydrogen, or hydrogen-rich fuels, into electricity using a chemical rather than a combustion process. Fuel cells do not need recharging or replacing and can produce electricity as long as they are supplied with hydrogen and oxygen. Hydrogen is the fuel for fuel cells, and can be produced by the electrolysis of water.
The environmental acceptability of hydrogen fuel cells depends on how the hydrogen is produced. If a renewable energy source such as solar energy is used to generate the electricity needed for electrolysis, vehicles powered by hydrogen fuel cells would be relatively clean since hydrogen combustion emits water vapor, but it also emits NOx compounds. Nitrogen dioxide (x=2) contributes to photochemical smog and can increase the severity of respiratory illnesses. Shipping and storage of hydrogen are important unresolved issues that hinder the widespread acceptance and implementation of hydrogen fuel cell technology.