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About metal hydrides ... 
How to store hydrogen?
   There are few major ways of storing hydrogen... Gaseous storage, Liquid hydrogen storage, glass microsphere storage, underground storage and metal hydride storage.  Due to deficiencies of the less volume, weight and losses involved in these storage modes, Metal hydrides can be considered as best way of storing atomic
hydrogen and to solve the present perrennial problems regarding  pollution and depletion caused by fossil fuels.  Storing it as metal hydrides seems to be the most cost efficient way.
  Metal hydride ....,  What are Metal Hydrides? 
The metal-hydrogen system consists of a metallic material, hydrogen gas, and an interface region between    them. 
Simplified Model of Metal-Hydrogen Interaction
           Hydrogen gas adsorbs onto the interface region. At the interface, the molecule is disassociated into individual  hydrogen atoms that are able to absorb or dissolve into the metal phase. The random dissolution of hydrogen     atoms in the metal phase is known as the a-phase. Within the metallic phase, the hydrogen atoms can start    to arrange themselves in a specific configuration with the metal atoms, forming the metal hydride phase, called   the b-phase. Where and how the b-phase is nucleated and grows is a characteristic of the material. 

They say the hydrogen   absorb and desorb. Can you explain to me how this can happen? How do they desorb the hydrogen? I realize how it can be absorbed within the matrix of the metal and forms metal hydrides, but how do you release the hydrogen when you  want to use it?

  The reaction of hydrogen with a metal can be written as a chemical reaction:

                                 M + x/2 H2 <----> MHx + Heat

               The double-headed arrow indicates that the reaction is reversible and exists as an equilibrium state. In other  words, by changing conditions, the reaction can be made to go in either the forward or reverse direction. The  heat on the right-hand side indicates that heat or energy is released when the metal hydride is formed, and           thus, heat must be put in to release hydrogen from the metal hydride phase. The heat is the enthalpy (heat of  formation) of the reaction and is an indication of the strength of the metal-hydrogen bond in the metal hydride   phase.


           Simple illustration of hydrogen fuelcell.

Metal Hydride Applications 

               Hydrogen Storage - The ability of metals to absorb large quantities of hydrogen at relatively low pressures   makes them ideal candidates for hydrogen storage reservoirs. In the metal matrix, the hydrogen atom interacts  with the metal atoms and the "sea" of electrons. It is thus possible to bring the hydrogen atoms very close   together, much more so than even in liquid hydrogen. However the weight of the metal matrix is substantially   greater than the stored hydrogen. Nevertheless, the hydrogen density in metal hydrides is significantly greater   than for gaseous or liquid hydrogen. 

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