Examples of Complex Transition Metal Hydrides

It can be summarized that of all the complex aluminum hydrides presented in this chapter only NaAlH4 up till now partially fulfils the requirements for a storage material for mobile fuel cell applications. Over the last decade, the kinetics as well as the cycle stability of doped sodium alanates has been improved. Nevertheless, for application in a low-temperature fuel cell the storage capacity seems to be too low because only the first decomposition step (3.6 wt.% hydrogen) can be used. 

Due to their thermodynamic properties some complex aluminum hydrides are not reversible under acceptable technical conditions. Others have too low hydrogen storage capacities which exclude these materials from broad industrial applications. 

Boron hydrides do have high hydrogen contents. However, the thermodynamics is in most cases unfavorable, the kinetics or the dehydrogenation and rehydrogenation are slow and the possible evolution of diborane is an additional problem. Therefore boron hydrides do not at present not allow any use as reversible hydrogen storage material. 

The complex transition metal hydrides known so far show too low hydrogen contents and can therefore not be considered for hydrogen storage. 

Fundamental research is necessary to understand all the mechanisms involved in the dehydrogenation and rehydrogenation in complex hydrides. Additionally to the understanding of the kinetics it is exceedingly important to develop new systems with much more advantageous thermodynamic properties. 

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Table 5.4 Selected examples of complex transition metal hydrides.

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