Synthesis of Zn2TiO4 via solid-state method as a promising additive for dehydrogenation properties of NaAlH4
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Adding a small amount of Co3O4 significantly reduces the operating temperatures of dehydrogenation and improves the hydrogen storage reversibility of the LiBH4–2LiNH2 system. The LiBH4–2LiNH2–0.05/3Co3O4 composite desorbs ∼9.9 wt% hydrogen by a four-step reaction with a 96 °C reduction in the midpoint temperature with respect to the pristine sample. The first and third steps of the dehydrogenation of the 0.05/3Co3O4-added sample are endothermic in nature, which is different from the pristine sample. Upon thermal dehydrogenation, the Co3O4 additive undergoes a series of chemical transformations and finally converts to the metallic Co, which is responsible for the improved thermodynamics and kinetics of the Co3O4-added sample. More importantly, 1.7 wt% of hydrogen is recharged into the 0.05/3Co3O4-added system under 110 bar hydrogen at 220 °C, which is superior to the pristine system.
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KSiH3 exhibits 4.1 wt% experimental hydrogen storage capacity and shows reversibility under moderate conditions, which provides fresh impetus to the search for other complex hydrides in the K-Si-H system. Here, we reproduce the stable Fm3̄m phase of K2SiH6 and uncover two denser phases, space groups P3̄m1 and P63mc at ambient pressure, by means of first-principles structure searches. We note that P3̄m1-K2SiH6 has a high hydrogen content of 5.4 wt% and a volumetric density of 88.3 g L-1. Further calculations suggest a favorable dehydrogenation temperature Tdes of -20.1/55.8 °C with decomposition into KSi + K + H2. The higher hydrogen density and appropriate dehydrogenation temperature indicate that K2SiH6 is a promising hydrogen storage material, and our results provide helpful and clear guidance for further experimental studies. We found three further potential hydrogen storage materials stable at high pressure: K2SiH8, KSiH7 and KSiH8. These results suggest the need for further investigations into hydrogen storage materials among such ternary hydrides at high pressure.
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The Mg(NH2)2-2LiH system with KOH additive is a promising high-capacity hydrogen storage material in terms of low dehydrogenation temperatures, good reversibility, and excellent cycling stability. Various mechanisms have been reported to elucidate the reasons for the K-containing additive improving the hydrogen storage performance. Herein, the dehydrogenation performance of Mg(NH2)2-2LiH-0.07KOH is found to be strongly associated with hydrogen pressures. The Li2K(NH2)3 and KH produced from the reaction between KOH, LiH, and Mg(NH2)2 in the ball milling process are converted into Li3K(NH2)4, MgNH, and LiNH2 in the heating dehydrogenation process under Ar carrier gas or very low hydrogen pressure, exhibiting a two-peak dehydrogenation process. For the sample under high hydrogen pressure, Li2K(NH2)3 can react with LiH to convert into Li3K(NH2)4 and further to form KH and LiNH2 in the heating process, showing a one-peak dehydrogenation process under 5 bar hydrogen. The hydrogen pressure-dependent reactions of K-containing additives in the Mg(NH2)2-2LiH system lead to a different hydrogen storage performance under different dehydrogenation conditions.
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The design of benign and safe hydrogen storage systems is the priority in the development of new energy carriers. The storage of hydrogen in a liquid or compressed state, as well as in metal hydrides and adsorbents, depends on pressure and temperature and under normal conditions does not meet the criteria of the target hydrogen storage capacity, energy consumption for hydrogen storage or safety. The storage of hydrogen in chemical compounds in which it is naturally included in the composition is the only alternative. Aromatic hydrocarbons capable of reversible hydrogenation–dehydrogenation reactions are of the greatest interest among regenerable hydrogen-containing compounds and can be used for hydrogen storage. The role of the metal in the catalytic reactions of the hydrogenation–dehydrogenation of cyclic hydrocarbons for hydrogen storage is discussed in the present review in close relation to the structure and composition of the cyclic substrates.
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Hydrogen storage properties and mechanisms of the Li3N–xMg3N2 (x = 0, 0.25, 0.5, 1.0) composites were investigated in this paper. It was found that the Li3N–0.25Mg3N2 composite exhibited optimal hydrogen storage performances as it can store reversibly ∼8.4 wt % hydrogen with an onset temperature of 125 °C for dehydrogenation. Upon absorbing hydrogen, Li3N converted to Li2NH and LiH first and was further hydrogenated to generate LiNH2. The newly developed LiNH2 then reacted with Mg3N2 under hydrogen pressure to produce Li2Mg2N3H3 and MgNH. Finally, Li2Mg2N3H3 and MgNH along with LiNH2 further reacted with hydrogen to form the resultant products of Mg(NH2)2 and LiH. More Mg3N2 in the Li3N–xMg3N2 composites retarded Li3N to react with H2 at the beginning of hydrogenation due to the baffle effect but facilitated the hydrogenation of Mg3N2 at the second-stage hydrogenation because of the decreased particle size and the frequent contact of the constituent species.
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The hydrogen storage properties of LiBH4 ball milled with various ratios of carbon nanotubes (Cnano) were investigated. The LiBH4∕Cnano mixtures showed superior dehydrogenation, hydrogen desorption starting at 250°C, and the majority of hydrogen being released below 600°C. The rehydrogenation results revealed that the Li2C2, formed during the dehydrogenation, could be reversed to LiH, in which the hydrogen capacity corresponds to 1∕4 of the original hydrogen content of LiBH4, and C at 10MPa hydrogen pressure and 400°C.
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