Investigation On The Potentiation Of Rare Earth And Transition Metal Compounds To Hydroganation Properties Of Li-Mg-N-H System

Li-Mg-N-H system has been extensively investigated due to its high hydrogencapacity and excellent reversibility at moderate conditions. The Li-Mg-N-H systems canabsorb/desorb hydrogen at473K completely, and its theoretical hydrogen storage reaches10.6wt.%. However, the poor hydriding/dehydriding kinetics hinders it from practicalapplications. To improve the hydrogen storage performances of Li-Mg-N-H system, therare earth and transition metal compounds are ball milled with2LiNH2+MgH2compsoite,respectively. Microstructures analysis and hydrogen storage characteristics of thecomposites with additives are investigated. The hydriding/dehydriding reactions of thecomposites as well as the catalytic mechanisms of the additives in the process ofhydrogen absorption/desorption are discussed in detail.The MgH2+x wt.%LaH3(x=0,10,20,30) composites are prepared. The catalyticmechanism of LaH3in composites were investigated. The composite added with0.04LaH3presented a reversible hydrogen storage capacity of5.1wt.%at548K with anenhanced hydrogenation kinetic properties. The improvements were ascribed to the phasetransformation between LaH3and LaH2.3, which leaded to the shrinkage of unit cellvolumes and promotes H to release from MgH2. Compared with2LiNH2+MgH2system,the onset decomposition temperature of2LiNH2+MgH2+0.05REH3(RE=La, Ce, Pr)were dramatically decreased. The2LiNH2+MgH2+0.05PrH3composite showed a395K onset temperature accompanying with better hydrogen desorption properties.According to microstructure analysis, REH3transformed into rare earth nitrides duringthe hydrogen desorption process of the composite. The Rare earth nitrides significantlyimproved the hydrogen storage properties of the composites.Addition of cerium compounds into2LiNH2+MgH2system improved itshydrogenation kinetic properties and reduced its operating temperature.Compared with2LiNH2+MgH2system, the onset temperature of2LiNH2+MgH2+0.05CeF3compositedecreased from413K to390K, meanwhile the hydrogen release of the compositeincreased from2.27wt.%to3.37wt.%at433K in100min. The2LiNH2+MgH2+0.05NbH composite presented a lower onset temperature of21K than that of2LiNH2+ MgH2system. Within100min, the hydrogen desorption of the composite was increasedby0.29wt.%at433K. Meanwhile, The onset temperature of2LiNH2+MgH2+0.05NbF5composite decreased from413K to387K, with a temperature of26K lowerthan that of the pure2LiNH2+MgH2.According to the different effects on2LiNH2+MgH2system of rare earthcompounds and transition metal compounds, LaH3-TiH2and CeF3-NbH compositeadditives was prepared by ball milling method, respectively. Compared to single additive,the enhanced catalytic effect of the composite additive was attributed to the synergisticeffect of the rare earth and transition metal elements with N in amino compound, whichfurther improved the hydrogenation properties of the2LiNH2+MgH2system. The onsettemperature of2LiNH2+MgH2system doped with LaH3-TiH2and CeF3-NbHcomposite additives was386K and390K, respectively, which was lower than those ofboth pure2LiNH2+MgH2and2LiNH2+MgH2with single additive.