Clarifying the dehydrogenation pathway of catalysed Li4(NH2)3BH4–LiH composites

Abstract

The effect of different metal oxides (Co₃O₄ and NiO) on the dehydrogenation reaction pathways of the Li₄(NH₂)₃BH₄–LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B–H bonds in the Li–B–N–H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH₂ and Li₃BN₂ formation as the end product with a strong reduction of NH₃ emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li₄(NH₂)₃BH₄–LiH–NiO and Li₄(NH₂)₃BH₄–NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li₃BN₂H₂ at the early stages of dehydrogenation and the end products LiNH₂ and monoclinic Li₃BN₂. We provided evidence for the interaction between NiO and LiNH₂ during heating and proposed that the presence of Li facilitates a NH_x-rich environment and the Ni catalyst mediates the electron transfer to promote NH_x coupling.