Hydrogen fuel production. Laboratory apparatus for the production of hydrogen gas from water using a semiconductor. Hydrogen gas is a clean, renewable and sustainable fuel. The development of efficient and renewable hydrogen production is needed for it to replace fossil fuels. Here, a semiconductor is immersed in the water and splits water molecules using the energy in sunlight. The water molecules split into hydrogen and oxygen gas. Burning the hydrogen in oxygen releases the stored energy and reforms water, completing the cycle. Photographed at the National Renewable Energy Laboratory site in Golden, Colorado, USA, in 1998.