| Hydrogen is a kind of clean energy with high energy storage, high calorific value, pollution-free etc, which is regarded as a potential energy to effectively solve the future energy crisis problem. However, an efficient, safe and economical hydrogen storage technology is still one of the major technical obstacles to widespread use of hydrogen technology. Hydrogen storage materials are one of the key factors to the actual use of hydrogen energy in the future. Among a wide ranges of hydrogen storage materials in many systems, hydrogen storage composites Li-Mg-N-H system have attracting significant attention owing to their high hydrogen storage capacity and a more appropriate thermodynamic properties of hydrogen absorption and desorption.The Li2MgN2H2and the LiMgN are usually prepared by ball milling of2LiNH2-MgH2or2LiH-Mg(NH2)2, and Li3N-Mg3N2or LiNH2-MgH2, respectively. The hydrogen storage properties of the products depend on different preparation methods. Up to now, the preparation of LiMgN by ball milling of Li3N:MgH2(1:1) has been not reported yet. In this paper, the preparation and hydrogen storage properties of the LiNH2/Li3N-MgH2system are studied to optimize the ball milling processing and to obtain the LiMgN with high hydrogen storage capacity as well as excellent hydrogen absorption and desorption behaviors. The mechanism of hydrogen absoption and desorption in the LiMgN system is revealed.The composition and phase structure of the ball-milled, the dehydrogenated and the hydrogenated samples were analyzed by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR). The microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hydrogen storage properties and absorption and desorption rate were tested by Pressure-Composition-Temperature (PCT) measurement.Firstly, the influence of ball milling processing on the hydrogen storage performance in the2LiNH2-MgH2system is studied. The ideal milling processing is obtained:2bar argon protected,500rpm revolution speed,12h ball milling time,40:1ball-to-sample weight ratio,10mm ball diameter,120g ball weight. In the absence of any additives, the as-prepared2LiNH2-MgH2system has a stable hydrogen storage of4.0wt%under5MPa at220℃for several cycles.In the present paper, the Li3N-MgH2system is mainly studied. LiMgN was directly synthesized by ball-milling the Li3N-MgH2as a reaction of Li3N+MgH2â†'LiMgN+2LiH. The phase transformation in the microstructure and different reaction mechanisms of hydrogen absorption and desorption are studied under different hydrogen pressure of5MPa and10MPa, and the hydrogen storage capacities reach～2.2wt%and3.2wt%, respectively.Different additives (5mol%Ti,5mol%TiCl3,5mol%LiNH2) were added to improve the hydrogen storage properties in the Li3N-MgH2system. The hydrogen storage capacity and hydrogenation kinetics of Ti-and TiCl3-added samples are both decreased. In the LiNH2-added sample, the microstructures are uniform and fine, which results in the improvement of hydrogenation kinetics. With an addition of5mol%LiNH2, the hydrogen absorption capacity is improved from3.2wt%to3.7wt%and the onset absorption temperature is decreased from130℃to90℃The improvement of hydrogenation properties is mainly due to a further hydrogenation/dehydrogenation promoted by the intermediate LiNH2as well as well-distributed nano-particles of the hydrogenated/dehydrogenated products.Effect of LiNH2with different amount (0,0.13,1,2mol) on hydrogen storage properties in the Li3N-MgH2system is studied. The addition of LiNH2results in the microstructural change of dispersion of the particles and the grains refinement, which shortens the diffusion distance of hydrogen and the hydrogen storage capacity is significantly improved. The sample with the addition of2mol LiNH2shows the maximum reversible hydrogen storage capacity of approximately5.1wt%. The hydrogenation and dehydrogenation reaction mechanisms are related to LiH, LiNH2and Mg(NH2)2. |
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