Synergetic Effects of In Situ Formed CaH2 and LiBH4 on Hydrogen Storage Properties of the Li-Mg-N-H System

Hydrogen storage properties and mechanisms of the Ca(BH4)2-doped Mg(NH2)22?LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH4)2 and LiH readily occurs to yield CaH2 and LiBH4 during ball milling. The Mg(NH2)22?LiH0.1?Ca(BH4)2 composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt?% of H2 with an onset temperature of about 90?degrees C for dehydrogenation and 60?degrees C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt?% of H2 is rapidly desorbed from the Mg(NH2)22?LiH0.1?Ca(BH4)2 composite within 100 minutes at 140?degrees C, and rehydrogenation can be completed within 140 minutes at 105?degrees C and 100 bar H2. In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH2)22?LiH0.1?Ca(BH4)2 composite are decreased by about 16.5?% and 28.1?%, respectively, and thus are responsible for the lower operating temperature and the faster dehydrogenation/hydrogenation kinetics. The fact that the hydrogen storage performances of the Ca(BH4)2-doped sample are superior to the individually CaH2- or LiBH4-doped samples suggests that the in situ formed CaH2 and LiBH4 provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH2)22?LiH system.