A hydrogen vehicle is a vehicle that uses hydrogen as its on-board fuel for motive power. The term may refer to a personal transportation vehicle, such as an automobile, or any other vehicle that uses hydrogen in a similar fashion, such as an aircraft. The power plants of such vehicles convert the chemical energy of hydrogen to mechanical energy (torque) in one of two methods: combustion, or electrochemical conversion in a fuel-cell:

• In combustion, the hydrogen is burned in engines in fundamentally the same method as traditional gasoline cars.
• In fuel-cell conversion, the hydrogen is reacted with oxygen to produce water and electricity, the latter of which is used to power an electric traction motor.

The molecular hydrogen needed as an on-board fuel for hydrogen vehicles can be obtained through many thermochemical methods utilizing natural gas, coal (by a process known as coal gasification), liquefied petroleum gas, biomass (biomass gasification), by a process called thermolysis, or as a microbial waste product called biohydrogen or Biological hydrogen production. Hydrogen can also be produced from water by electrolysis. If the electricity used for the electrolysis is produced using renewable energy, the production of the hydrogen would (in principle) result in no net carbon dioxide emissions. On-board decomposition to produce hydrogen can occur when a catalyst is used.

Hydrogen is an energy carrier, not an energy source, so the energy the car uses would ultimately need to be provided by a conventional power plant. A suggested benefit of large-scale deployment of hydrogen vehicles is that it could lead to decreased emissions of greenhouse gases and ozone precursors. Schultz, M.G., Thomas Diehl, Guy P. Brasseur, and Werner Zittel. Air Pollution and Climate-Forcing Impacts of a Global Hydrogen Economy. Science 24 October 2003 302: 624-627http://www.sciencemag.org/cgi/content/full/302/5645/624 Further, the conversion of fossil fuels would be moved from the vehicle, as in today's automobiles, to centralized power plants in which the byproducts of combustion or gasification can be better controlled than at the tailpipe. However, there are both technical and economic challenges to implementing wide-scale use of hydrogen vehicles, as well as less expensive alternatives. The timeframe in which challenges may be overcome is likely to be at least several decades, and hydrogen vehicles may never become broadly available. From TechnologyReview.com