Tailoring the hydrogen storage properties of Li4BN3H10 by confinement into highly ordered nanoporous carbon

Abstract

The effects of nanoconfinement on the H₂ release and uptake, and the emission of toxic ammonia (NH₃) and diborane (B₂H₆) during the decomposition of quaternary phase Li₄BN₃H₁₀ have been comprehensively investigated. Li₄BN₃H₁₀ confined in highly ordered nanoporous carbon (Li₄BN₃H₁₀@NPC) with cylindrical pores and an average pore size of 4.4 nm (NPC-4.4 nm) did not show Bragg peaks or melting of the Li₄BN₃H₁₀ due to nanoconfinement-induced amorphization. Li₄BN₃H₁₀ confined in NPC starts to decompose at 110 °C, lower by more than 160 °C with respect to bulk and the dehydrogenation kinetics was significantly enhanced. More importantly, in comparison with the exothermic decomposition of bulk Li₄BN₃H₁₀, a distinct two-step endothermic decomposition process was observed in the nanoconfined Li₄BN₃H₁₀ system indicating the decomposition pathway was altered by nanoconfinement. A direct line-of-site residual gas analyzer mass-spectrometer (RGA-MS) analysis indicated NH₃ was released earlier than H₂ during the decomposition of nanoconfined Li₄BN₃H₁₀ but it evolved slowly and the overall emissions of NH₃ and B₂H₆ were significantly suppressed by nanoconfinement. More interestingly, some other possible gases, diimine (N₂H₂) and triborane or heavier species, produced during the decomposition of Li₄BN₃H₁₀ or its constituent parts (LiBH₄ and LiNH₂), were identified by RGA-MS for the first time. The observation of diimine in the reaction product may provide insight into the reaction mechanisms for rehydriding of compounds in the LiNH₂ and Li₂NH systems as well.