A hydrogen vehicle is an automobile, truck, aircraft, watercraft or other fuel-powered mode of transportation in which hydrogen serves as a fuel. This has been achieved either by burning hydrogen in an internal combustion engine or converting hydrogen's chemical energy into mechanical energy via a fuel cell. In either case, a hydrogen vehicle produces no carbon dioxide or greenhouse gas emissions. Inasmuch as hydrogen is typically produced from natural gas or other fossil fuels, a hydrogen vehicle is not considered a truly "zero emissions" mode of transport, however. The challenges associated with producing a practical, affordable hydrogen vehicle have thus far proved insurmountable, as has producing and storing hydrogen safely, economically, and in sufficient volume for it to be considered a mass market alternative to transportation fuels derived from oil.
Developing a hydrogen vehicle has attracted interest from the U.S. government, the major auto companies, a variety of other private sector businesses, and academic researchers and amateur enthusiasts. Hydrogen cars, trucks, buses, boats of various sizes, aircraft, and submarines have been developed and tested. All have been experimental or demonstration models, and, when available to the public, in very limited quantities as demonstration vehicles, however.
To date, there are no production-scale hydrogen vehicles on the market. Interest, and investment, in developing a hydrogen vehicle began to wane following the financial crisis and economic recession of 2008, along with a shift of public and private interest and funding in favor of electric and hybrid electric vehicles. The high cost of development prompted Ford Motor and Renault-Nissan to curtail, and General Motors to significantly reduce, their respective development initiatives in 2009.
Test hydrogen vehicles have achieved outstanding results in terms of fuel efficiency; hydrogen itself has a much higher energy per unit mass than than gasoline and other fuels. It is much less dense than other fuels, the net result being that a gallon of gasoline holds more than 3.5 times the energy of a gallon of uncompressed hydrogen, however. It is also difficult, relatively costly, and potentially dangerous to store, particularly on-board vehicles. Moreover, scaling hydrogen fuel and hydrogen vehicle production up to levels where it could be considered a mass market alternative mode of transportation would require the building of a nationwide network of filling stations.
Hydrogen is technically considered a fuel carrier as opposed to a fuel source as it does not occur naturally and producing it requires using up primary fuel sources. Generally speaking, hydrogen has been and continues to be produced from natural gas or other fossil fuels, processes that do produce carbon dioxide and other greenhouse gas emissions. Projects to produce a truly zero emissions hydrogen vehicle by producing hydrogen via electrolysis of water using renewable wind and solar power systems have been and continue to be undertaken, however.