The effect of Al on the hydrogen sorption mechanism of LiBH4

Abstract

We demonstrate the synthesis of LiBH₄ from LiH and AlB₂ without the use of additional additives or catalysts at 450 °C under hydrogen pressure of 13 bar to the following equation: 2LiH + AlB₂ + 3H₂↔ 2LiBH₄ + Al. By applying AlB₂ the kinetics of the formation of LiBH₄ is strongly enhanced compared to the formation from elemental boron. The formation of LiBH₄ during absorption requires the dissociation of AlB₂, i.e. a coupled reaction. The observed low absorption-pressure of 13 bar, measured during hydrogen cycling, is explained by a low stability of AlB₂, in good agreement with theoretical values. Thus starting from AlB₂ instead of B has a rather low impact on the thermodynamics, and the effect of AlB₂ on the formation of LiBH₄ is of kinetic nature facilitating the absorption by overcoming the chemical inertness of B. For desorption, the decomposition of LiBH₄ is not indispensably coupled to the immediate formation of AlB₂. LiBH₄ may decompose first into LiH and elemental B and during a slower second step AlB₂ is formed. In this case, no destabilization will be observed for desorption. However, due to similar stabilities of LiBH₄ and LiBH₄/Al a definite answer on the desorption mechanism cannot be given and neither a coupled nor decoupled desorption can be excluded. At low hydrogen pressures the reaction of LiH and Al gives LiAl under release of hydrogen. The formation of LiAl increases the total hydrogen storage capacity, since it also contributes to the LiBH₄ formation in the absorption process.