Li4(NH2)3Cl amide-chloride: a new synthesis route, and hydrogen storage kinetic and thermodynamic properties

Abstract

Amide-halide compounds were identified as possible promoters of the dehydrogenation kinetics of the Li–N–H system. However, reversible hydrogen storage capacities and sorption kinetics of Li₄(NH₂)₃Cl and Li₃Mg_0.5(NH₂)₃Cl have not been reported yet. In the present work, Li₄(NH₂)₃Cl was synthesized using a new synthesis route that involves the pre-milling of a LiNH₂–LiCl mixture. Attempts to synthesize Li₃Mg_0.5(NH₂)₃Cl by applying similar synthesis procedures using LiNH₂ and 0.5MgCl₂ were unsuccessful; instead, a mixture of Li₄(NH₂)₃Cl–0.5Mg(NH₂)₂ was obtained. The hydrogen storage properties of the Li₄(NH₂)₃Cl–3LiH and Li₄(NH₂)₃Cl–0.5Mg(NH₂)₂–3LiH composites were evaluated between 200 °C and 300 °C. The onset of hydrogen release was reduced by 20 °C when Li₄(NH₂)₃Cl–3LiH decomposed in the presence of Mg(NH₂)₂ (180 °C with respect to 200 °C) and its hydrogen desorption rate increased by 83%. However, no change in the dehydrogenation activation energy was observed for Li₄(NH₂)₃Cl–3LiH decomposition due to minor amounts of Mg(NH₂)₂. The hydrogen storage capacity under cycling was reduced from about 3.0 wt% to 1.5 wt% at 300 °C, after rehydrogenation at 6.0 MPa. The formation of Li₇(NH)₃Cl was clearly identified in the dehydrogenated material. Unfortunately, the sloped plateau and the thermodynamic stability of Li₄(NH₂)₃Cl–3LiH precludes its hydrogen storage applicability.