About metal hydrides ...
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.
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.