| One of the key requirements for widespread utilization of hydrogen energy is the availability of efficient, safe and economic hydrogen storage technologies. In the past few years, considerable attention has been paid to the Li-Mg-N-H hydrogen storage system due to their relatively high gravimetric hydrogen capacity and good reversibility. However, the poor thermodynamics and sluggish kinetics of hydrogenation/dehydrogenation prevent it from practical applications. In this paper, to further improve the hydrogen storage properties, the compositional adjustment, catalytic modification and hydrogen storage mechanisms of the Li-Mg-N-H system were systematically investigated, mainly including the correlation between composition and hydrogen storage behaviors of the Li2NH-MgNH system, the hydrogenation reaction mechanism of the Li3-N-Mg3N2system, the catalytic effect and mechanism of Ca(BH4)2and lithium halides on the hydrogen storage performances of the Mg(NH2)2-2LiH and LiNH2-MgH2systems.The Li3N-xMg3N2composites with x=0,0.25,0.5and1.0were prepared by ball milling, and the hydrogenation/dehydrogenation properties of the as-prepared samples were investigated systematically. It was found that hydrogen storage performances depended strongly on the content of Mg3N2, and the optimal composition was Li3N-0.25Mg3N2. Approximately8.4wt%of hydrogen was stored reversibly in the Li3N-0.25Mg3N2composite with an onset temperature of125℃for dehydrogenation, a40℃reduction in comparison with the pristine Li3N. Investigations on structural and compositional changes at different hydrogenation stages revealed that hydrogen uptake was a stepwise reaction for the Li3N-0.25Mg3N2composite. First, Li3N absorbed hydrogen to convert to Li2NH and LiH and then was further hydrogenated to generate LiNH2. Afterward, the newly developed LiNH2reacted with Mg3N2under hydrogen pressure to produce Li2Mg2(NH)3and MgNH. Finally, Li2Mg(NH)3and MgNH along with LiNH2reacted with hydrogen to form the resultant products of Mg(NH2)2and LiH. In addition, the higher content of Mg3N2facilitated the conversion of LiNH2to Mg(NH2)2by reacting them with hydrogen due to the decreased particle size and the improved contact.The hydrogen storage properties of the Li2NH-xMgNH (x=0,0.5,1and2) system were evaluated by hydrogenation/dehydrogenation experiments, and their reaction mechanisms were elucidated with XRD and FTIR examinations. It was found that the operating temperatures were dramatically decreased for the samples containing MgNH. For the hydrogenated Li2NH-0.5MgNH sample, the onset and ending temperatures of hydrogen desorption decrease by～40℃and220℃. respectively, with respect to the pristine Li2NH sample. Moreover,～5.6wt%of hydrogen was experimentally released from the hydrogenated Li2NH-0.5MgNH sample in the temperature range of140-280℃with a two-step reaction, exhibiting a slightly higher hydrogen capacity than the Mg(NH2)2-2LiH system. Structural examinations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio of MgNH and Li2NH. Thermodynamic analyses reveal that, by changing the reaction routes, a tunable thermodynamic route was obtained for hydrogen storage in the Li2NH-xMgNH system. Furthermore, the hydrogen absorption/desorption kinetics of the Li2NH-0.5MgNH combination was markedly enhanced by adding KH.The Mg(NH2)2-2LiH-xCa(BH4)2composites (x=0,0.1,0.2,0.3) were prepared by ball milling the corresponding chemicals under50bar of H2, and the dehydrogenation/hydrogenation properties of the as-prepared samples were investigated systematically. It was found that a metathesis reaction between LiH and Ca(BH4)2took place during ball milling, leading to the in-situ formation of CaH2and LiBH4. The Mg(NH2)2-2LiH-0.1Ca(BH4)2composite exhibited the optimum hydrogen storage performances as the onset dehydrogenation temperature is decreased to about90℃, a40℃reduction in comparison with the pristine Mg(NH2)2-2LiH system. Approximately4.0wt%of H2was rapidly discharged within100min at140℃, showing a more than4-fold enhancement in the rate constant with respect to the pristine sample. The rehydrogenation of the Ca(BH4)2-doped sample started at about60℃, and～4.0wt%of H2was absorbed quickly within140min at105℃and100bar H2, which is～50times faster than the pristine sample. Further evaluations on kinetics and thermodynamics revealed that the apparent activation energy and the overall heat effect for dehydrogenation of the Mg(NH2)2-2LiH-0.1Ca(BH4)2composite were100.6kJ mol-1and30.2kJ mol-1of H2, respectively, which are lowered by～16.5%and～28.1%, respectively, in comparison with the pristine sample. Moreover, the Ca(BH4)2-doped sample showed better hydrogen storage properties than the individually CaH2or LiBH4-added samples, indicating that the in situ formed CaH2and LiBH4together contribute a synergetic effect on improving the hydrogen storage performances.The hydrogen storage properties of the2LiNH2-MgH2-xCa(BH4)2(x=0,0.1,0.2,0.3) composites were systematically studied. It was found that a metathesis reaction between Ca(BH4)2and LiNH2readily occurred to convert to Ca(NH2)2and LiBH4, and then, the newly formed LiBH4reacted with LiNH2to produce Li4(BH4)(NH2)3during ball milling and/or initial heating process. The Ca(BH4)2-added samples exhibited two distinct dehydrogenation stages upon heating. The in situ formed Ca(NH2)2and Li4(BH4)(NH2)3worked together to not only reduce the onset dehydrogenation temperature but also decrease the first-stage dehydrogenation amount. The2LiNH2-MgH2-0.1Ca(BH4)2sample showed the optimal dehydrogenation properties as it could release ca.5.0wt%with a starting temperature of100℃. Further hydrogenation measurements revealed the low temperature hydrogenation properties of the2LiNH2-MgH2-0.1Ca(BH4)2system are significantly superior to the pristine sample. The dehydrogenated2LiNH2-MgH2-0.1Ca(BH4)2sample could be completely hydrogenated at160℃and100atm while only15%of hydrogen capacity was achieved for the dehydrogenated pristine sample.The hydrogen storage behaviors of LiNH2-MgH2-LiX(X=F, Cl, Br, I) were systematically investigated. It was found that the LiNH2-MgH2-0.05LiBr sample exhibits the optimal hydrogen storage performances. The onset-temperature for dehydrogenation of the LiNH2-MgH2-0.05LiBr sample is120℃, a55℃reduction with comparison to the pristine LiNH2-MgH2sample. Moreover, the LiNH2-MgH2-0.05LiBr sample shows a very sharp dehydrogenation peak at220℃, indicating a fast dehydrogenation kinetics. The dehydrogenation kinetics of the LiNH2-MgH2-0.05LiBr sample is4times faster than the pristine sample, which is due to the reduction of apparent activation energy. The addition of LiBr can also dramatically suppress the ammonia emission during dehydrogenation process. Structural analyses indicate that LiBr can exothermically react with LiNH2to form6Br during dehydrogenation, which can not only weaken the N-H bond of LiNH2, but also promote the mobility of Li+, consequently resulting in the enhanced dehydrogenation performances. |
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