Abstract
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.
Highlights
Hydrogen is regarded as an efficient energy storage medium and a green and environmental friendly fuel able to help improve environmental issues linked to the use of fossil fuels [1,2], thanks to its high gravimetric energy density and its clean combustion reaction
All the alkali alanates discussed above are unsuitable for practical applications because of kinetic and/or thermodynamic limitations
For the LiAlH4 + LiBH4 + CaCl2 system, a calcium alanate-type phase formed, with a different local coordination geometry of the AlH4− anions with respect to the structure observed in pure calcium alanate
Summary
Hydrogen is regarded as an efficient energy storage medium and a green and environmental friendly fuel able to help improve environmental issues linked to the use of fossil fuels [1,2], thanks to its high gravimetric energy density (more than twice the energy content of the most common fossil fuels) and its clean combustion reaction (leading to the formation of water only). The most promising materials for hydrogen storage concerning the gravimetric and volumetric capacity belong to this last group, suffering anyway from unfavourable working conditions (i.e., high temperature and hydrogen pressure) and scarce cycling ability [1]. Metals 2018, 8, 567 practical applications, thanks to its reversibility in exchanging hydrogen at moderate temperature and pressure conditions, was based on an alanate, that is, titanium-based doped sodium alanate, studied by Bogdanovic and Schwickardi in 1996 [3]. The preparative methods; the crystal structure; the physico-chemical characteristics; the hydrogen absorption and desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Sr, and Eu; and the hydrogen absorption and desorption properties of the most appealing multi-cations alanates and of selected systems based on these compounds will be summarized and discussed. The most promising alanate-based RHC systems developed in the last few years will be described and commented
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