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

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.