H - Hydrogen
Hydrogen - Hydrogen is a flammable gas which occurs naturally in water and organic matter. It is also one of the main alternative fuel sources being developed by many car manufacturers for use in fuel cell vehicles. (See Fuel cell.)
In a fuel-cell car, the hydrogen is stored in a tank or extracted from methanol. When combined with oxygen, it creates electricity and the only by-product is water, expelled as steam.
Hydrogen only provides about half the power of diesel for the same amount burnt, and has its own emissions problems in the form of corrosives. Some car manufacturers are working on pure hydrogen-powered cars, but the main problem here is that it has to be stored at extremely low temperatures.