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The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance.

In this work, a thermodynamic picture of the dehydrogenation mechanism of NaBH 4 in NaBH 4 -MgH 2 mixtures with 2:1 and 1:2 molar ratio is drawn, for the first time in literature, thanks to coupled manometric-calorimetric measurements up to 580 °C. Such a new approach also allows, after the measurement of the borohydride melting enthalpy, the evaluation of the dehydrogenation enthalpy of the complex hydride in the mixtures. The thermodynamics of the 2:1 sample (where the borohydride decomposition takes place mainly in liquid state) is more favorable than that of the 1:2 mixture, where the process evolves fully in solid state. The kinetics of the systems is studied at 450 °C, the minimum temperature at which the borohydride decomposition takes place. In these conditions, the 1:2 system is kinetically favored. Several additives (fluorides; chlorides; hydroxides) have been tested as possible destabilizing/catalyzing agents. These substances react with the component hydrides upon discharging, forming stable binary and ternary compounds that do not change the macroscopic desorption pathway of the composites (separate decomposition of the component hydrides) but lead to variations in the desorption temperature and kinetics. In particular, MgF 2 is found to improve the desorption kinetics of both the component hydrides and to reduce the decomposition temperature and enthalpy of NaBH 4 in the 2:1 system. On the contrary, none of the tested dopants exerts any positive effect on the 1:2 system.