Functions of MgH2 in hydrogen storage reactions of the 6LiBH4–CaH2 reactive hydride composite

Abstract

A significant improvement of hydrogen storage properties was achieved by introducing MgH₂ into the 6LiBH₄–CaH₂ system. It was found that ∼8.0 wt% of hydrogen could be reversibly stored in a 6LiBH₄–CaH₂–3MgH₂ composite below 400 °C and 100 bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine 6LiBH₄–CaH₂ and LiBH₄ samples. Upon dehydriding, MgH₂ first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of ∼290 °C. Subsequently, LiBH₄ reacted with CaH₂ to form CaB₆ and LiH in addition to further hydrogen release. Hydrogen desorption from the 6LiBH₄–CaH₂–3MgH₂ composite finished at ∼430 °C in non-isothermal model, a 160 °C reduction relative to the 6LiBH₄–CaH₂ sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage.