Reaction Paths between LiNH₂ and LiH with Effects of Nitrides

Abstract

The solid-state reaction between LiNH2 and LiH potentially offers an effective route for hydrogen storage if it can be tailored to meet all the requirements for practical applications. To date, there still exists large uncertainty on the mechanism of the reaction--whether it is mediated by a transient NH3 or directly between LiNH2 and LiH. In an effort to clarify this issue and improve the reactivity, the effects of selected nitrides were investigated here by temperature-programmed desorption, X-ray diffraction, in-situ infrared analysis, and hydrogen titration. The results show that the reaction of LiNH2 with LiH below 300 degrees C is a heterogeneous solid-state reaction controlled by Li+ diffusion from LiH to LiNH2 across the interface. At the LiNH2/LiH interface, an ammonium ion Li2NH2+ and a penta-coordinated nitrogen Li2NH3 could be the intermediate states leading to the production of hydrogen and the formation of lithium imide. In addition, it is identified that BN is an efficient "catalyst" that improves Li+ diffusion and hence the kinetics of the reaction between LiNH2 and LiH. Hydrogen is fully released within 7 h at 200 degrees C with BN addition, rather than several days without the modification.

Unique Hydrogen Desorption Properties of LiAlH4/h-BN Composites

“…Aguey-Zinsou et al reported that h-BN would enhance the Li ion mobility across the interface of LiNH 2 and LiH [20]. Thus, the enhancement of Li ion mobility could be one of the reasons for the unique hydrogen desorption properties of LiAlH 4 /h-BN composites.…”

“…It was assumed that the excellent rehydrogenation property of LiBH 4 would be related with the enhanced hydrogen and lithium diffusion capability by the nanoscale h-BN, which was synthesized by ball-milling of h-BN at 490 rpm for 20 h [19]. The enhancement of Li + and/or H − diffusion by adding h-BN was firstly reported in the LiNH 2 /LiH system [20]. Hydrogen was fully desorbed from the LiNH 2 /LiH/h-BN composite in less than 7 h, whereas the LiNH 2 /LiH composite desorbed hydrogen in several days.…”

“…Hydrogen was fully desorbed from the LiNH 2 /LiH/h-BN composite in less than 7 h, whereas the LiNH 2 /LiH composite desorbed hydrogen in several days. They proposed that h-BN is an efficient catalyst that improves Li + diffusion and hence the kinetics of the reaction between LiNH 2 and LiH [20]. The mobility of Li + ions between LiH and LiNH 2 was also enhanced by adding LiTi 2 O 4 catalyst [21].…”

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

“…The most important step in this mechanism would be the movement of a lithium ion to an interstitial site, forming a lithium Frenkel defect pair. 16 In addition to the polar mechanism and the ammoniamediated mechanism, Aguey-Zinsou et al 17 have recently suggested that the reaction between LiNH 2 and LiH below 300 • C is a heterogeneous solid-state reaction, controlled by the diffusion of Li + from LiH to LiNH 2 across the interface. In this mechanism, the reaction is direct rather than ammonia-mediated.…”

“…In this mechanism, the reaction is direct rather than ammonia-mediated. 17 Theoretical studies of LiNH 2 and Li 2 NH to date have focused mainly on structural, electronic, and thermodynamic properties of the bulk compounds. [18][19][20][21][22][23][24] Experimental data, 16 on the other hand, suggest that the ratelimiting process in the Li amide/imide reaction involves mass transport mediated by point defects.…”