Reversible Metal Hydride Hydrogen Stores

The reader might well question the inclusion of this method in this section, rather than with the chemical methods that follow. However, although the method is chemical in its operation, that is not in any way apparent to the user. No reformers or reactors are needed to make the systems work. They work exactly like a hydrogen sponge or absorber . For this reason it is included here. 

Certain metals, particularly mixtures (alloys) of titanium, iron, manganese, nickel, chromium, and others, can react with hydrogen to form a metal hydride in a very easily 

One example of snch an alloy is titanium iron hydride, the last entry in Table 8.16 of the following section. In terms of mass this is not a very promising material it is the volumetric measure that is the advantage of these materials. It requires one of the lowest volumes to store 1kg in Table 8.16 certainly it is one of the lowest practical materials. It actually holds more hydrogen per unit volnme than pnre liquid hydrogen.  

To the right, the reaction of 8.15 is mildly exothermic. To release the hydrogen then, small amounts of heat must be supplied. However, metal alloys can be chosen for the hydrides so that the reaction can take place over a wide range of temperatures and pressures. In particular, it is possible to choose alloys suitable for operating at around atmospheric pressure and room temperature. 

The system then works as follows hydrogen is supplied at a little above atmospheric pressure to the metal alloy, inside a container. Reaction 8.15 proceeds to the right, and the metal hydride is formed. This is mildly exothermic, and in large systems some cooling will need to be snpplied, but normal air cooling is often sufficient. This stage will take a few minutes, depending on the size of the system, and if the container is cooled. It wiU take place at approximately constant pressure. 

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The van t Hoff plot shows the reason why NaAlH4 is one of the most promising candidates for reversible hydrogen storage. NaAlH4 as a typical low-temperature metal hydride exhibits an equilibrium pressure of 0.1 MPa at 35 °C and a storage capacity of 3.7 wt.% for the first decomposition step. This is twice the amount stored in interstitial metal hydrides. 

Actinide metals react with hydrogen at about 300 "C to form non-stoichiometric metallic hydrides of composition MH2 to MH.n The foimation of uranium hydride is reversible at higher temperatures and can be used to store tritium. 

An electron is removed from the Ni (OH)2 at the anode, and the reverse reaction occurs to yield NiOOH and H+, which goes to the cathode to react to produce the metal hydride. The merits of this type of battery are that the cathode reaction is simple, and the metalhydride can store large amount of hydrogen. It is possible to replace the liquid electrolyte by the solid polymer electrolyte. 

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