Hydrogen fuel pump at a roadside service station. In hydrogen-powered vehicles, hydrogen (from the fuel tank) and oxygen (from the air) react to form water and electrical power. The only by-products are heat and water, making this technology a clean source of power compared to the pollutant gases and carbon dioxide produced by the burning of fossil fuels. Photographed in June 2018, at Cobham services on the M25, Greater London, Surrey, UK.