Energy

Chemical storage

Other technologies exploit the ability of hydrogen to bind to chemical compounds or metals in order to facilitate its storage and transport. Hydrogen can chemically bind to different metals and metal alloys by forming hydrides, i.e. compounds that are able to trap the hydrogen at relatively low pressures (the gas penetrates inside the crystal lattice of the metal) and release it at high temperatures. This technology allows to reach energy densities that are potentially higher than compressed hydrogen and can be compared with liquid hydrogen. Storage volume could be reduced by 3-4 times, making its use in cars possible, while specific energy depends on the specific gravity of the basic metal. The percentage of hydrogen weight as compared to the total metal weight varies from 1% to 12.7% (lithium hydride), while for ordinary bottles that percentage is slightly higher than 1%. Despite all these positive characteristics, there are still many problems to be solved in order to have a competitive storage. For example, it is necessary to work better to improve the structural and thermal stability of the material, to make pressure and temperature compatible with the expected applications, etc. Anyhow, at present, the available materials provoke a very heavy storage: with the same weight, the vehicle has three time less autonomy than what would be obtained with liquid or compressed hydrogen and advanced tanks. Instead, there are clear advantages in terms of convenience, compact shape, and safety. As an alternative, it is possible to transport molecules rich in hydrogen from which the gas can be extracted only where and when it is needed by means of a device called reformer (but in this way a certain quantity of waste will be obtained, too). There are different molecules that could be used in order to reach this objective, like methane (CH4) and methanol (CH3OH). These chemical procedures are advantageous as they allow to use already-existing transport and storage facilities. Although they are very promising, these technologies have many negative characteristics: methanol, for example, is toxic. At the moment a chemical compound, sodium boronhydride, is being tested, as it is able to bind with plenty of hydrogen. A aqueous solution, composed of half of sodium boronhydride and half of water, supplies hydrogen with an energy ratio that is similar to petrol, in terms of volume. Sodium boronhydride without hydrogen converts into borax, a substance that is present in ordinary detergents and that can be used again.