For a long time, alternative fuels have been researched, and one of the most prominent is the Hydrogen. Altough its combustion process is reported to be the cleanest one due to the emissions restricted to vaporized water and being the most abundant chemical element in the Universe, Hydrogen is also the most reactive, then it must be extracted from chemical compounds like water or methane, that is currently the most common Hydrogen source besides its usage as a fuel (usually labeled as CNG - compressed natural gas - or NGV - natural gas for vehicles) widely accepted commercially in India, Pakistan, Argentina, Italy and Brazil.
Unlike water, which electrolysis releases only Oxygen, methane processing releases carbon dioxide, the same compound intended to be cut away from tailpipe emissions in hydrogen-powered vehicles. Considering that a biggest chunk of all the methane turned into Hydrogen is coming from fossil reserves, turns its usage as a fuel pointless in an environmental aspect, while other options as the biodiesel and even the ethanol are more effective in this matter. Also, due to all the energy spent to process the methane and subsequently store the Hydrogen in a safe way, it does worth more to use the methane itself as a fuel.
One of the ways intended to make use of Hydrogen is thru the fuel-cells (also often quoted as combustion-cells), promoting a combustion that releases an electric current used to power an electric-drive motor. First experiences in a fuel-cell vehicle were attempted by General Motors in 1966 in a prototype badged as Electrovan and based in the ChevyVan available at the time that was chosen over a regular sedan due to the enormous size and the extra weight of the Hydrogen-powered setup. Currently the systems evolved to a point that it can be bolted into a smaller vehicle such as the Opel Zafira and the Chevrolet Equinox without too many interferences in the passenger space and cargo capacity. The first manufacturer to release a fuel-cell vehicle to the general public was Honda with the FCX Clarity in 2008, but the availability is extremely limited to some areas in California and Japan due to the absence of Hydrogen refuelling stations enough to a worldwide offer.
Another way to use Hydrogen is into internal-combustion engines, with highlights to BMW and Mazda, since it also allows an easier usage of more traditional fuels such as gasoline and ethanol. Hydrogen, however, has a lower energy density and the on-board storage is quite problematic. Usually, a carbon-fibre storage cylinder is certified to 900bar, and while the most usual CNG setups have a 200bar working pressure Hydrogen is kept at 700bar, with a smaller safety margin. Also, beyond the safety valves required, due to the size of a Hydrogen atom it easily escapes from micro-fissures in the tank walls, requiring the vehicle to be stored in places with a good air circulation to avoid an explosive atmosphere to be generated around the parked vehicle, turning it into a less attractive option not just in the economic viability matter but also due to the safety.
So, despite the daydreams in late Twentieth Century years and early 2000s, Hydrogen kept a distant dream to the motor world in general...