Functions of LiBH4 in the hydrogen sorption reactions of the 2LiH–Mg(NH2)2 system

Abstract

LiBH₄ is an effective additive in enhancing hydrogen desorption and absorption properties for the 2LiH–Mg(NH₂)₂ hydrogen storage system. The improving factors brought by LiBH₄ were studied experimentally. Although a minimum in activation energy was observed in the middle stage of hydrogen desorption for the samples with and without LiBH₄ doping, a reduction in the activation energy was manifest in the overall conversion range due to the LiBH₄ introduction. In contrast to the pure 2LiH–Mg(NH₂)₂, the system component Mg(NH₂)₂ crystallized in the presence of LiBH₄ at about 150 °C in situ from the post-milled amorphous mixture prior to hydrogen desorption. We believe the formation of the N-atom matrix by the crystalline Mg(NH₂)₂ is relevant to the subsequent dehydrogenation as it shares a similar sublattice structure with the desorption product. The formation and disappearance of Li₄(BH₄)(NH₂)₃ in the dehydrogenation step witnessed the capturing and releasing of LiNH₂ that is generated from the dehydrogenation of 2LiH–Mg(NH₂)₂, which alters not only the kinetics but also thermodynamics in favor of an improved hydrogen sorption performance. The reaction mechanism could be further clarified by the refinement of powder neutron diffraction data of the intermediate compound Li₄(BD₄)(NH₂)₃.