Storing hydrogen as a solid implies that the hydrogen is absorbed on glass or carbon, reacted with metals or is used in an alternative chemical form such as ammonia. For vehicular hydrogen storage, the selection of a solid hydrogen storage material will be based on the following criteria:
|FSEC’s researchers are developing a synthetic route for regeneration of ammonia borane complex and other amine-borane chemical hydrogen storage compounds.
N. Mohajeri. (Photo: N. Waters)
Hydrogen storage in solids may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and at temperatures close to room temperature. It is also possible to achieve volumetric storage densities greater than liquid hydrogen because the hydrogen molecule is dissociated into atomic hydrogen within the metal hydride lattice structure.
Let us now examine some details of each type of solid storage systems.
Metal hydrides offer high volumetric storage densities and a high crash-worthy structure, virtues that makes them very desirable as a transportation fuel storage system. However, they suffer from excessive weight and high cost. Metal hydrides discharge (evolve) hydrogen when the material is heated and uptake (absorb) hydrogen when the material is cooled. The storage tank contains powdered metals that absorb hydrogen and release heat when hydrogen is filled into the tank under pressure. Hydrogen is released when the pressure is reduced and heat is supplied. The most common hydrides include Fe-Ti, La-Ni, Mg and Ti-Zr-V series alloys. Once charged, hydride storage eliminates concerns for high-pressure storage as they function at low gas pressures. A suitable candidate hydride is expected to have low temperature of hydrogenation and dehydrogenation, high hydrogen storage capacity, fast kinetics, high cyclic stability and low cost. However, none of the present day hydride materials fulfill all of these criteria.
Carbon-based hydrogen storage includes a range of materials such as carbon nanotubes, aerogels, nanofibers (including metal doped hybrids), as well as metal-organic frameworks, conducting polymers and clathrates. If structures can be tailored at the nano-scale, hydrogen storage could be enhanced. Single-walled carbon nanotubes are being studied as hydrogen storage materials because of hydrogen gravimetric storage capacities reported to be in the range of 3-10 weight percent at room temperature.
The term "chemical hydrogen storage" or chemical hydrides is used to describe storage technologies in which hydrogen is generated through pyrolysis of hydrogen containing materials or its reaction with water or other compounds such as alcohols. However, these reactions are not easily reversible on-board a vehicle. Hence, the "spent fuel" and/or byproducts must be removed from the vehicle and regenerated off-board.
For additional information on solid storage systems, see Metal Hydride Storage [pdf, 1.3mb] at “Gaseous Hydrogen Storage at Hydrogen for Power Applications – Task 2.0 Storage of Hydrogen in Solid, Liquid and Gaseous Forms” (pages 8-16).