Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. Interest in using hydrogen for on-board storage of energy in zero-emissions vehicles is motivating the development of new methods of storage, more adapted to this new application. The overarching challenge is the very low boiling point of H2: it boils around 20.268 K (−252.882 °C or −423.188 °F). Achieving such low temperatures requires expending significant energy. Established technologies Compressed hydrogen Compressed hydrogen is a storage form whereby hydrogen gas is kept under pressures to increase the storage density. Compressed hydrogen in hydrogen tanks at 350 bar (5,000 psi) and 700 bar (10,000 psi) are used for hydrogen tank systems in vehicles, based on type IV carbon-composite technology. Car manufacturers have been developing this solution, such as Honda or Nissan. Liquefied hydrogen Liquid hydrogen tanks for cars, producing for example the BMW Hydrogen 7. Japan has a liquid hydrogen (LH2) storage site in Kobe port. Hydrogen is liquefied by reducing its temperature to −253 °C, similar to liquefied natural gas (LNG) which is stored at −162 °C. A potential efficiency loss of only 12.79% can be achieved, or 4.26 kW⋅h/kg out of 33.3 kW⋅h/kg. Chemical storage Chemical storage could offer high storage performance due to the high storage densities. For example, supercritical hydrogen at 30 °C and 500 bar only has a density of 15.0 mol/L while methanol has a density of 49.5 mol H2/L methanol and saturated dimethyl ether at 30 °C and 7 bar has a density of 42.1 mol H2/L dimethyl ether. Regeneration of storage material is problematic. A large number of chemical storage systems have been investigated. H2 release can be induced by hydrolysis reactions or catalyzed dehydrogenation reactions. Illustrative storage compounds are hydrocarbons, boron hydrides, ammonia, and alane etc. A most promising chemical approach is electrochemical hydrogen storage, as the release of hydrogen can be controlled by the applied electricity. Most of