A process for synthesizing the Li-Mg-N-H hydrogen storage system from Mg and LiNH2

We successfully established a method to synthesize the Li-Mg-N-H hydrogen storage system composed of Mg(NH2)2 and LiH from Mg and LiNH2 as starting materials. First of all, Mg and Li2NH were homogeneously mixed by ball-milling for 2 h under 1 MPa argon atmosphere. After that, the mixture was heat-treated at 250 °C for 16 h under vacuum condition to be transformed into Mg3N2 and Li2NH. Target materials, Mg(NH2)2 and LiH, were synthesized by a further heat-treatment for Mg3N2 and Li2NH at 200 °C under 10 MPa hydrogen atmosphere for 12 h. The phases in the sample at each processing stage were examined by X-ray diffraction and Fourier transformation infra-red absorption measurements and confirmed to be the expected ones. From experimental results of a thermal desorption mass spectroscopy, the final hydrogenated sample indicated that ~7 mass% hydrogen was desorbed starting from about 150 °C and peaked around 230 °C under helium flow condition at 5 °C/min heating rate without ammonia emission in our experimental accuracy

Hydrogen absorption properties of Li-Mg-N-H system

Isothermal hydrogen absorption properties of the ball milled mixture of 3Mg(NH2)2 and 8LiH after dehydrogenation at 200 °C under high vacuum were investigated at two different temperatures of 150 and 200 °C. The pressure –composition isotherm (PCT) curve at 200 °C revealed a two-plateaus-like behavior, while the PCT curve at 150 °C showed a single-plateau-like behavior. The hydrogenated phases were composed of LiH and Mg(NH2)2 under 9 MPa at 200 °C, while those were observed as mixed phases of LiH and LiNH2 at 150 °C without any trace of Mg(NH2)2 in XRD measurements. These results indicate that there are 2 step hydrogenation processes corresponding to high and low pressures at 200 °C, but the kinetics at 150 °C is too slow to proceed with the second hydrogenating step at high pressure region

Hydrogen desorption/absorption properties of Li-Ca-N-H system

Hydrogen storage properties of two ball-milled composites of Ca(NH2)2+2LiH and CaH2+2LiNH2 were investigated as a series of searching studies of high performance hydrogen storage materials. About 4.5 mass% hydrogen is desorbed from about 100 °C and the thermal desorption profiles show a peak around 200 and 220 °C for the composites of CaH2+2LiNH2 and Ca(NH2)2+2LiH, respectively, under a helium flow at 5 °C/min heating rate without any NH3 emission within our experimental accuracy. The powder X-ray diffraction and infrared absorption spectroscopy indicated that the dehydrogenated states of both composites form "an unknown imide phase" after heating up to 400 °C, while the dehydrogenated states after heating up to 200 °C in vacuum are the mixed phases of Li2NH and CaNH. The rehydrogenated state for the ball-milled composite of CaH2+2LiNH2 is transformed into the composite of Ca(NH2)2 and 2LiH by repeating the dehydrogenation and rehydrogenation cycles at 180~200 °C

Evaluation of enthalpy change due to hydrogen desorption for lithium amide/imide system by differential scanning calorimetry

Enthalpy change (ΔH) due to hydrogen desorption (H-desorption) for the lithium amide/imide system was evaluated by differential scanning calorimetry (DSC) measurement. In order to obtain the accurate and precise value of ΔH, we have paid special attention to following two points for correcting raw experimental data. One is to determine a cell constant of DSC equipment, which was evaluated by using the TiO2-doped MgH2 compound as a reference because of its quite similar hydrogen desorption properties to that of the lithium amide/imide system. The other is to estimate the sample amount corresponding to the H-desorption reaction from weight loss in the thermogravimetric (TG) analysis. By performing both the corrections, the ΔH value due to the H-desorption reaction from LiNH2 + LiH to Li2NH + H2 was evaluated to be 67 kJ/mol H2

Evaluation of enthalpy change due to hydrogen desorption for lithium amide/imide system by differential scanning calorimetry

Enthalpy change (ΔH) due to hydrogen desorption (H-desorption) for the lithium amide/imide system was evaluated by differential scanning calorimetry (DSC) measurement. In order to obtain the accurate and precise value of ΔH, we have paid special attention to following two points for correcting raw experimental data. One is to determine a cell constant of DSC equipment, which was evaluated by using the TiO2-doped MgH2 compound as a reference because of its quite similar hydrogen desorption properties to that of the lithium amide/imide system. The other is to estimate the sample amount corresponding to the H-desorption reaction from weight loss in the thermogravimetric (TG) analysis. By performing both the corrections, the ΔH value due to the H-desorption reaction from LiNH2 + LiH to Li2NH + H2 was evaluated to be 67 kJ/mol H2

Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System

Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH2)2) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH and Mg(NH2)2 lead to the fast hydrogen desorption. To preserve the potential hydrogen desorption properties, thermochemical and mechanochemical rehydrogenation processes are investigated. Although the only thermochemical process results in slowing the reaction rate due to the crystallization, the ball-milling can recover the original hydrogen desorption properties. Furthermore, the mechanochemical process at 150 °C is useful as the rehydrogenation technique to preserve the suitable crystalline size and mixing state of the reactants. As a result, it is demonstrated that the 8LiH and 3Mg(NH2)2 system is recognized as the potential hydrogen storage material to desorb more than 5.5 mass% of H2 at 150 °C