Hydrogen bus. Woman boarding a hydrogen bus, which is powered by a fuel cell that runs on hydrogen gas. Inside the proton exchange membrane (PEM) fuel cell, hydrogen is combined with air, without combustion, to form heat and water. The electricity generated drives the bus. This is far cleaner than conventional fossil fuels. Hydrogen, which is created by the electrolysis of water, is a cheaper and more readily available fuel than oil. Photographed in Reykjavik, Iceland, in 2003.