An alternative to liquefaction and compression is to store hydrogen in solids, either physically absorbed or chemically bound. Most of the research in this field has concentrated on metals and metallic alloys which have the ability to absorb hydrogen, like a sponge, to form hydrides. In these materials, the metal matrix is expanded and filled with hydrogen. The process, depending on the nature of bonding, can be either reversible or irreversible. Hydrogen stored irreversibly in some materials (including chemical hydrides) can only be released by the chemical reaction of these compounds with another substance, such as water, producing byproducts which must be collected and reprocessed before they can be used again to store hydrogen. Thus, for practical purposes, using irreversibly formed hydrides is not an attractive means of hydrogen storage.
Reversible hydrides are generally solids, which can release the hydrogen contained in the metal in the form of molecular hydrogen, H2. The uptake and release of hydrogen is typically controlled by temperature and pressure, and is different for every hydride. Some metals absorb hydrogen rapidly but release it slowly, while others require higher temperatures before release is possible. The release of the absorbed hydrogen may also be only partial. The storage and release of hydrogen in any case should take place at temperatures and on a time scale suitable for applications in the transportation field. Volume and temperature increases due to hydride formation should also to be taken into account for storage vessel design. Conventional metal hydrides, as well as their hydrogen-storage capabilities, have been well characterized, with most containing relatively heavy elements: TiFe, ZrMn2, LaNi5, etc. As only a few hydrogen atoms can be bound by each metal atom, this explains why typically only 1-3% by weight of these metal hydrides is actually usable hydrogen. In other words, in order to store 5 kg of hydrogen a tank will need to weigh 200 kg or more. So, unless compensated for by the use of lightweight materials in other parts of the vehicle, the additional weight of the tank would reduce fuel efficiency, which is one of the main goals of developing hydrogen-powered vehicles. Metal hydrides however, do have the advantage of being compact, requiring less space than compressed hydrogen for an equal amount of stored energy. Being only under moderate pressure, hydride tanks can also be shaped more freely, and this facilitates their integration into the vehicle body. Today, research into metal hydrides is focused mainly on lighter compounds such as NaAlH4, Na3AlH6, LiAlH4, NaBH4, LiBH4, and MgH2, which offer higher hydrogen contents per unit of mass.
Besides metal hydrides, other solid absorbing materials for potential hydrogen storage are under investigation. Recently, fullerenes and, more significantly, carbon nanotubes have attracted much attention, but they are still a long way from finding their way into fuel tanks, partially because of their still exorbitant price and unproven potential.
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