Preparation, Structure And Performances Of New Li-Mg-N-H Complex Hydrogen Storage Materials

Based on the review of the research and development of the new Metal-N-Hsystems for the hydrogen storage materials, the Li-Mg-N-H system was selected asthe study object of this work. By means of XRD, SEM, TPD, FTIR and hydrogenstorage performance test, the reaction process during the hydrogen absorption/desorption of the Li-Mg-N-H system and the effect of the particle size on hydrogenstorage performance were systematically studied for developing the new typeMetal-N-H hydrogen storage materials with high storage capacity and excellentkinetic performance. The study includes the followings: the reaction process of theLiNH₂-MgH₂ mixture, the structure and hydrogen storage properties of Li₂MgN₂H₂prepared by sintering the Mg(NH₂)₂-2LiNH₂ mixture, the effect of ball milling onhydrogen absorption/desorption kinetics of Li₂MgN₂H₂, and the improvement on thehydrogen desorption performance of Li₂MgN₂H₂ with benzidine additive.For the LiNH₂-MgH₂ (1:1) system, the exchange of the NH₂ group between LiNH₂and MgH₂ occurred to produce Mg(NH₂)₂ and LiH during first 12h ball milling, thenMgNH was formed by the reaction between Mg(NH₂)₂ and MgH₂ with 1 H atomreleased in the subsequent 12-36h ball milling. The hydrogen desorption in heatingprocess was a two-step reaction, and the Li₂MgN₂H₂ and Mg₃N₂ were formed,respectively, associated with 1 H atom released for each step. A total of 6.1 wt.% ofhydrogen was released from the mixture by first ball milling and subsequent heating.Thermodynamic analysis showed that the hydrogen desorption was an endothermicreaction and the overall heat of reaction calculated was ca. 45.9 kJ/mol-H2. However,only 1 equiv. of H atom can be recharged into the dehydrogenated sample to form amixture of several compounds including Mg₃N₂, LiH and Mg(NH₂)₂ under a highhydrogen pressure. The hydrogen absorption species was identified to be Li₂MgN₂H₂.The enthalpy change (â–³H) for hydrogen desorption calculated by van't Hoff curvewas 39.1 kJ/mol-H₂. Then Li₂MgN₂H₂ was prepared by sintering the Mg(NH₂)₂+2LiNH₂ mixture.Resuts showed that 5.0 wt.% of hydrogen was absorbed by the pristine sample at210℃and was completely desorbed in subsequent hydrogen desorption test. Finally,an orthorhombic Li₂MgN₂H₂ was formed after one cycle.The structure and hydrogen storage performances of Li₂MgN₂H₂ after hand millingand ball milling for 3h and 36h were investigated. After ball milling, the hydrogenabsorption performances were dramatically improved as the starting temperature forhydrogen absorption decreased from 180℃to 80℃. However, the startingtemperature for hydrogen desorption almost was not affected after full hydrogenationat 210℃. With increasing ball mill time, the hydrogen desorption kinetic properties ofLi₂MgN₂H₂ were distinctly improved. The half reduced time (t_1/2) decreased from 210min for hand milling sample to 50 min for 36 h of ball milling sample, and theactivation energy (Ea) was reduced from 116.5 kJ/mol for hand milling sample to 58.7kJ/mol for 36 h of ball milling sample. Sample milled for 36 h showed smallerparticle size by SEM observation, which is responsible for its better kineticperformance. The rare-determining step for hydrogen desorption was identified to bediffusion of the ions of the reactants by Sharp method, and the isothermal hydrogendesorption kinetic curves were well fitted by the Jander equation.Based on the above results, 5 wt.% of bezidine was introduced into the Li₂MgN₂H₂system to further improve the hydrogen storage properties. The addition of benzidinehad little effect on the starting absorption/desorption temperature, and 4.2 wt.% ofhydrogen was reversibly stored in the present study. On the other hand, the bezidinedisperses in the system during ball milling and retards the particle fromconglomerating and growing, consequently inducing the smaller particle size (<400run) during hydrogenation/dehydrogenation cycling. Smaller particle can reduce thediffusion distance and enhance the hydrogen absorption/desorption kineticperformances. The t_1/2 of the samples with hydrogenation at 155℃was reduced from567 min for the pristine sample to 203 min for the sample with benzidine addition.Moreover, the activation energy (E_a) calculated by Jander equation was also decreasedfor the sample with benzidine addition.