In the last years the international interest towards the development of hydrogen based technologies, particularly the area of fuel cell electric vehicles, is greatly increased. The more and more alarming news about climate changes caused by fossil fuels and, moreover, the technological advances of fuel cells, particularly the so called PEMFC (proton exchange membrane fuel cells), have contributed to this interest. In a fuel cell hydrogen reacts electrochemically whit oxygen producing electricity (and heat). The efficiency of process can be 50-60%, more than twice that of a thermal cycle, and the exhaust produced is only pure water. Hydrogen can be employed as a fuel even in current internal combustion engines, where it burns quickly with atmospheric oxygen. However, the efficiency of the conversion from chemical to mechanical energy is limited by the typical efficiency of a thermal engine which hardly exceeds 25%. There are many problems to be solved before seeing a large scale marketing of technologies related to hydrogen as an energy vector.

Hydrogen storage in a high pressure cylinder (up to 700 atm) [Quantum]		Cryogenic tank for liquid state hydrogen storage [Linde]

Besides the lack of infrastructures for the development of hydrogen-based technologies (production, distribution, refuelling, etc.), very important is the problem of hydrogen storage, specially for vehicular applications. Hydrogen can be stored as compressed gas in high pressure cylinders, as liquid hydrogen in suitable cryogenic tanks, or in solid state as in metallic hydrides or in materials as light hydrides suitable to retain it in considerable quantities. Even if the use of high pressure cylinders is simple and inexpensive, it presents serious problems of size and weight. A tank of liquid hydrogen concentrates a larger quantity of hydrogen in the volume unit, but involves problems related to refrigeration costs, safety, manipulation and losses by evaporation.

Prototype tank based on hydrides for hydrogen storage in solid state [Sandia Laboratories]

 

The use of solid absorbers such as metal hydrides allows volumetric density comparable to or greater than liquid hydrogen and presents no particular hazards. However, the weight of the absorber and that of the tank may be too high compared to the weight of hydrogen stored. It is therefore necessary to develops new materials that are easily loaded with hydrogen and release it in a controlled manner in conditions of temperature and pressure acceptable in practical applications.