Hydrogen Storage Properties And Mechanisms Of Li-Mg-N-H-based Hydrogen Storage Materials With High Capacity

A safe, efficient and economical hydrogen storage is the enabled technology to the large-scale applications of hydrogen energy. Much attention has been paid to the new Li-Mg-N-H-based hydrogen storage materials owing to their high hydrogen capacity and good reversibility. However, the high operating temperatures and poor kinetics for dehydrogenation/hydrogenation hinder it from practical applications. Based on the review of the progress in the Li-Mg-N-H hydrogen storage systems, the following in-vestigations were conducted by experiments and first-principles calculations. includ-ing the relationship between ball milling time and hydrogen storage properties of the LiNH2-MgH2 mixture. the preparation and hydrogenation/dehydrogenation properties of cubic and orthorhombic Li2MgN2H2, and the effects of NaBH4, NaOH and KOH additives on reversible dehydrogenation/hydrogenation properties of the Mg(NH2)2-2LiH mixture. The correlation between the compositions, phase structures and hydro-gen storage properties of the Li-Mg-N-H material is revealed. The study established a theoretical foundation for further improvements of hydrogen storage properties of the metal-N-H systems.The dehydrogenation/hydrogenation properties of the LiN2-MgH2 mixture milled for different time were investigated. It is found that LiNH2 react with MgH2 to release hydrogen in the ball milling and subsequent heating process. The maximum dehydro-genation amount is 6.3 wt% for the LiNH2-MgH2 mixtures milled for different time. Mechanistic investigation shows that dehydrogenation/hydrogenation reaction path-ways for the LiNH2-MgH2 mixture are strongly dependent on the ball milling time. The LiNH2-MgH2 mixture was first converted to the 1/2Mg(NH2)2-LiH-1/2MgH2 mixture in the initial ball milling process. In the subsequent ball milling and heating process. Mg(NH2)2 reacted with MgH2 and LiH to release hydrogen, respectively. The dehydrogenation reaction between Mg(NH2)2 and MgH2 is thermodynamically favor-able yet kinetically poor, which induces that it is easy to occur during ball milling. However, the dehydrogenation reaction between Mg(NH2)2 and LiH (molar ratio 1:2) prefer occurring in the heating process due to its mildly endothermic nature. Thus, the reaction pathways for dehydrogenation/hydrogenation of the LiNH2-MgH2 mixture depend strongly on the milling duration due to the presence of competing reactions in the ball milling and subsequent heating stages. Li2MgN2H2 is regarded as one of the most promising high-capacity materials for hydrogen storage owing to its reversible capacity of 5.5 wt% and relatively low oper-ating temperatures. The relationship between hydrogen storage properties and crystal structures of Li2MgN2H2 is systematically investigated. Interestingly, the reversible hydrogen storage properties of Li2MgN2H2 are closely related to its crystal structures which determined by the desorption temperature and gas pressure of the Mg(NH2)2-2LiH mixture. The cubic Li2MgN2H2 is produced when the Mg(NH:)2-2LiH mixture dehydrogenates at 250℃and vacuum condition while the dehydrogenated product at 280℃under 9.0 bar hydrogen/argon pressure is the orthorhombic Li2MgN2H2. The reversible phase transformation occurs between the cubic and orthorhombic Li2MgN2H2 under given conditions. The operating temperatures for reversible hydro-gen storage in the cubic Li2MgN2H2 are much lower than those of the orthorhombic Li2MgN2H2. The hydrogenation temperature of the cubic Li2MgN2H2 is lower than that of the orthorhombic phase by～30℃, and it is～10℃lower for dehydrogena-tion of the hydrogenated sample. Further experimental and first-principles investiga-tions exhibit a 5.2 kJ/mol reduce in the enthalpy for the orthorhombic Li2MgN2H2 with respect to the cubic phase while the activation energy for hydrogenation from the orthorhombic Li2MgN2H2 is 48.5 kJ/mol higher than that of cubic Li2MgN2H2. The different kinetic barriers are responsible for the change in the operating temperatures for hydrogenation/dehydrogenation of the cubic and orthorhombic Li2MgN2H2.In order to improve the dehydrogenation/hydrogenation kinetics of the Li-Mg-N-H materials, the effects of NaBH4 addition on the dehydrogenation/hydrogenation prop-erties of the Mg(NH2)2-2LiH mixture were investigated. It is found that the Mg(NH2)2-2LiH-0.1NaBH4 mixture shows the optimal hydrogen storage properties, as the onset desorption temperature is decreased from～130℃for the Mg(NH2)2-2LiH mixture to～117℃and the desorption rate at 150℃is also 3 times that of the Mg(NH2)2-2LiH mixture. Structural analyses indicate that the NaBH4 species seems almost unchanged during the dehydrogenation/hydrogenation process. Further studies reveal that the presence of NaBH4 in the Mg(NH2)2-2LiH mixture facilitates the formation of Mg vacancies in Mg(NH2)2-The appearance of Mg vacancies not only weakens the N-H bonds but also promotes the diffusion and migration of small atoms/ions, conse-quently resulting in the improvement of the dehydrogenation/hydrogenation kinetics of the NaBH4-added Mg(NH2)2-2LiH mixture.Dehydrogenation/hydrogenation behaviors of the NaOH-added Mg(NH2)2-2LiH mixture were investigated. With increasing the amount of NaOH. the operating tem-peratures for dehydrogenation/hydrogenation of the Mg(NH2)2-2LiH mixture are de-creased. For the Mg(NH2)2-2LiH-0.5NaOH mixture, the onset dehydrogenation tem-perature is lowered to～90℃, a 36℃reduction relative to the Mg(NH:)2-2LiH mix-ture. Mechanistic investigations reveal that NaOH reacts with Mg(NH2)2 and LiH to convert to NaH, LiNH2 and MgO during ball milling. The synergetic effect of NaH, LiNH2 and MgO remarkably improves the reversible dehydrogenation/hydrogenation properties of the Mg(NH2)2-2LiH mixture. Therefore, the synergetic effect of compos-ite catalysts is an important approach to develop the high-efficiency catalysts for the Li-Mg-N-H hydrogen storage materials.Furthermore, the reversible dehydrogenation/hydrogenation properties of the KOH-added Mg(NH2)2-2LiH mixture were investigated. The operating temperatures for dehydrogenation/hydrogenation of the Mg(NH2)2-2LiH mixture are significantly de-creased by the addition of KOH associated with the reduction in the byproduct of NH3 during hydrogen desorption. The onset dehydrogenation temperature of the Mg(NH2)2-2LiH-0.07KOH mixture is lowered to～75℃. Moreover, the dehydrogenation rate of the Mg(NH2)2-2LiH-0.07KOH mixture at 140℃is about 22 times that of the Mg(NH2)2-2LiH sample and it is a 25-fold increase in the re-hydrogenation rate for the dehydrogenated Mg(NH2)2-2LiH-0.07KOH mixture at 100℃. Mechanistic inves-tigations reveal that KOH can react with Mg(NH2)2 and LiH during ball milling, which results in the change of reaction pathways for dehydrogenation/hydrogenation of the Mg(NH2)2-2LiH mixture. In the heating process, partial Mg(NH2)2 reacts first with LiH to convert to LiNH2 and MgNH due to the presence of the potas-sium-contained compounds, and then the newly developed MgNH and LiNH2 react with the potassium-contained compounds and the rest Mg(NH2)2 and LiH to further liberate hydrogen and produce the cubic Li2MgN2H2. The changes in dehydrogena-tion/hydrogenation pathways are the primary reason for the improvement on thermo-dynamics and kinetics of hydrogen storage of the KOH-added Mg(NH2)2-2LiH mix-ture.