Effects Of Composition And Catalyst On Hydrogen Storage Properties Of Li-Mg-N-H System

Energy is the material basis of human activities. In our era, the development ofthe energy is the issues of common concern in the world. At present, the developmentand utilization of the clean energy become hot research spot in the world, and thehydrogen energy is an ideal green energy. In the application of hydrogen, energyhydrogen storage is the most critical problem, and the solid hydrogen storage waswhispered concerned by researchers. In the solid hydrogen storage materials, M-N-Hhydrogen storage materials is considered to be one of the most promising hydrogenstorage materials based on its high hydrogen storage capacity, good reversibility,relatively mild hydrogen absorption and desorption condition, and relatively stablematerials. In this paper, composition and catalyst of hydrogen storage properties ofLi-Mg-N-H system were studied and the results showed that:（1） Hydrogen storage performance of tow different compositions（2LiNH₂-1.1MgH₂and Mg（NH₂）₂-2.2LiH） Li-Mg-N-H system was checked by theequipment of PCT （pressure-composition-Temperature）, and there are obviousdesorption plateau among150²40℃for both two systems. The capacity of thereversible hydrogen storage can reach over4.5wt.%at above200℃.When thetemperature of the tow systems increased,the dehydrogenation equilibrium pressureis also increasing.The relationship between system compositions and equilibriumpressures satisfied the Van’t Hoff equation at different temperature, entropy andenthalpy of the dehydrogenation reaction can be calculated according to the Van’tHoff equation. Dehydrogenation enthalpy of LiNH₂-MgH₂system and Mg（NH₂）₂-LiHsystem are ΔH=40.4kJ/molH₂and ΔH=42.8kJ/molH₂respectively, furthermore, theirentropy are ΔS=123.6JK^-1/molH₂and ΔS=149.2JK^-1/molH₂. Gibbs free energy of thedehydrogenation reaction are approximately same to the two systems.（2） Hydrogen absorption and desorption kinetics showed that LiNH₂-MgH₂hydrogen storage system has faster kinetics, that is the absorption speed and thedesorption speed are both faster than the Mg（NH₂）₂-LiH hydrogen storage system.XRD result showed that the hydrogenation products of the two different systems areboth Li₂Mg（NH）2, which has a good reversible hydrogen absorption and desorptionproperties. According to the principle and the dehydrogenation data of kinetics, the activation energy of the desorption reaction can be calculated, the value of Ea of theformer is42.5kJ/mol, and that of the latter is51.7kJ/mol. The difference is that theformer system has MgH₂left, and the latter system has LiH left. The absorption anddesorption capacity of the former system is slightly stronger than the catalytic abilityof the latter. However, the ability of inhibition ammonia released for LiH is strongerthan that of the former, because lithium hydride and ammonia reaction is faster thanthat of magnesium hydride.（3） After50hours by high-energy ball milling, the particles of CaCO₃can bereduced into nanometer, and its size can reached about100nm. The equilibriumpressure of Li-Mg-N-H system can not be changed after mixing nanometer CaCO₃catalyst, but the absorption and desorption kinetics of the system is obviouslyincreased. It is showed that after CaCO₃was mixed into the system as a catalyst, theviscosity of the sample was reduced, and the contact area among the particles wereincreased, so the reaction rate became faster.