One of the limitations surrounding the exploitation of hydrogen as a fuel is the difficulty in storing it economically and conveniently. At present, hydrogen is typically stored as a compressed gas or as a cryogenic liquid. However, neither method is very appealing. Gaseous storage requires a large volume and/or weight of storage vessels, while liquid hydrogen requires a rather expensive liquification process and some energy as well. Another possibility for storage that is currently being studied and utilized is metal hydrides. The hydrogen reacts with the metals and forms metal hydrides. Storing hydrogen as a metal hydride has many advantages. Firstly, hydrogen can be stored in metal hydrides more efficiently (in terms of volume) than as liquid, or even solid, hydrogen. Secondly, the formation reaction is easily reversible. The formation of the hydride is an exothermic and usually spontaneous reaction, but the hydrogen can easily be recovered by heating the hydride. Therefore, the metal hydride can be classified as a safe method of storing hydrogen since the hydrides are generally stable below their dissociation temperatures. Moreover, the self-cooling effect will slow down and eventually stop any loss of hydrogen if a leak is developed in the storage system. Several metal hydrides have been developed, but none of them satisfies all these requirements. Some of the more common drawbacks of the currently available materials are that (a) hydrogen-metal ratios are too low, (b) metals involved are too costly, (c) the absorption or release of hydrogen is difficult, slow or sensitive to poisoning phenomena and (d) significant hysteresis is exhibited. The available materials for hydrogen storage are briefly reviewed and a more detailed overview is given of two classes of materials that are being developed in the author's laboratory, namely zirconium-based Laves phase intermetallic compound and body-centred cubic solid solutions based on niobium.
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