Synergetic effects of in situ formed CaH2 and LiBH4 on hydrogen storage properties of the Li-Mg-N-H system

Abstract

Hydrogen storage properties and mechanisms of the Ca(BH(4))(2)-doped Mg(NH(2))(2)-2LiH system are systematically investigated. It is found that a metathesis reaction between Ca(BH(4))(2) and LiH readily occurs to yield CaH(2) and LiBH(4) during ball milling. The Mg(NH(2))(2) -2LiH-0.1Ca(BH(4))(2) composite exhibits optimal hydrogen storage properties as it can reversibly store more than 4.5 wt% of H(2) with an onset temperature of about 90 °C for dehydrogenation and 60 °C for rehydrogenation. Isothermal measurements show that approximately 4.0 wt% of H(2) is rapidly desorbed from the Mg(NH(2))(2) -2LiH-0.1Ca(BH(4))(2) composite within 100 minutes at 140 °C, and rehydrogenation can be completed within 140 minutes at 105 °C and 100 bar H(2). In comparison with the pristine sample, the apparent activation energy and the reaction enthalpy change for dehydrogenation of the Mg(NH(2))(2)-2LiH-0.1Ca(BH(4))(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(BH(4))(2)-doped sample are superior to the individually CaH(2)- or LiBH(4)-doped samples suggests that the in situ formed CaH(2) and LiBH(4) provide a synergetic effect on improving the hydrogen storage properties of the Mg(NH(2))(2)-2LiH system.