Impact of Stoichiometry on the Hydrogen Storage Properties of LiNH₂−LiBH₄−MgH₂ Ternary Composites

Abstract

We recently reported (Yang, J.; et al. Angew. Chem., Int. Ed. 2008, 47, 882) a novel hydrogen storage composite involving a 2:1:1 LiNH2:LiBH4:MgH2 ratio. On the basis of in-depth experimental and computational analysis, this composite was found to release hydrogen via a complex multistep reaction cascade, which seeded the products of a subsequent reversible hydrogen storage reaction. This so-called autocatalytic reaction sequence was found to result in favorable kinetics, ammonia attenuation, and partial low-temperature reversibility. Here, we extend our original study by examining the effects of reactant stoichiometry on the ensuing hydrogen storage desorption pathway and properties. In particular, we examine four (LiNH2) X −(LiBH4) Y −(MgH2) Z composites, where X:Y:Z = 2:1:2, 1:1:1, 2:0.5:1, and 2:1:1 (original stoichiometry). For each sample, we characterize the postmilled mixtures using powder X-ray diffraction (PXRD) and infrared spectroscopy (IR) analyses and observe differences in the relative extent of two spontaneous milling-induced reactions. Variable-temperature hydrogen desorption data subsequently reveal that all composites exhibit a hydrogen release event at rather low temperature, liberating between 2.3 (1:1:1) and 3.6 (2:0.5:1) wt % by 200 °C. At higher temperatures (200−370 °C), the hydrogen release profiles differ considerably between composites and release a total of 5.7 (1:1:1) to 8.6 (2:0.5:1) wt %. Utilizing variable-temperature IR and PXRD data coupled with first-principles calculations, we propose a reaction pathway that is consistent with the observed phase progression and hydrogen desorption properties. From these data, we conclude that premilled reactant stoichiometry has a profound impact on reaction kinetics and high-temperature reaction evolution because of reactant availability. From this enhanced understanding of the desorption process, we recommend and test a stoichiometrically optimal ratio (3:1:1.5) which releases a total of 9.1 wt % hydrogen. Finally, we assess the reversibility (at 180 °C) of the four primary composites over two desorption cycles and find that only the 2:1:1 and 2:0.5:1 are reversible (3.5 wt % for 2:0.5:1).

Applications of high throughput (combinatorial) methodologies to electronic, magnetic, optical, and energy-related materials

“…%), and reversibility were observed at ratios of 2:1:1 and 2:0.5:1 in this system. 340 An interesting aspect of work on this ternary system was the comprehensive use of both experiment and theory to first identify promising systems, followed by larger scale powder experiments.…”

H2sorption performance of NaBH4–MgH2composites prepared by mechanical activation

“…In the last years, many efforts have been made to improve both the thermodynamic and the kinetic characteristics of LiBH 4 desorption by addition of suitable catalyzing/destabilizing agents. A promising solution seems the preparation of the so called "reactive hydrides composites" (RHC) systems (Barkhordarian et al [11], Vajo et al [12]), made of the borohydride, the light hydride MgH 2 (Dornehim et al [13], Vajo et al [14,15]), and eventually other complex hydrides such as LiAlH 4 and/or LiNH 2 (Pinkerton et al [16], Sudik et al [17], Yang et al [18,19]). The borohydride reacts with the destabilizing hydride or with its dehydrogenation product, leading to the formation of a stable discharged compound (the general reaction scheme being AH + MBH → AB + MH).…”

Recent Progress and New Perspectives on Metal Amide and Imide Systems for Solid-State Hydrogen Storage

“…Inspired by the richness of the chemistry observed in the binary systems 2LiBH 4 [75]. In their following works [114,115], an optimum composition of this ternary system was found to be 6LiNH 2 + 3MgH 2 + LiBH 4 by combinatorial synthesis and screening techniques. A self-catalyzed reaction mechanism for this ternary composition was proposed: (1) during milling LiNH 2 reacts with LiBH 4 to form Li 4 BN 3 H 10 (Equation (9)); (2) Li 4 BN 3 H 10 interacts with MgH 2 producing Li 2 Mg(NH) 2 (Equation (10)); (3) the formed Li 2 Mg(NH) 2 functions as seeds for the reaction reported in Equation (5) leading to a general improvement of the hydrogen storage properties [116].…”