Theory
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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.
