Hydrogen De/Resorption Properties of the LiBH4-MgH2-Al System

The synthesized LiBH4-MgH2-Al (4:1:1 mole ratio) composite exhibits reversible de/rehydrogenation properties. Thermogravimetry and differential scanning calorimetry indicate that the dehydrogenation onset temperature is reduced by 100 K from that of 2LiBH(4)-MgH2 and 2LiBH(4)-Al systems. The major dehydrogenation pathway for the 4LiBH(4)-MgH2-Al complex system can be identified as 4LiBH(4) + MgH2 + Al -> 4LiH + MgAlB4 + 7H(2) by means of X-ray diffraction (XRD) measurements on the as-de hydrogenated samples. The isothermal dehydrogenation measurements exhibit that the maximum dehydrogenation amount (9.4 wt % H-2, 673 K) approaches the theoretical value (9.9 wt % H-2) of the reaction. Through pressure-composition isotherms (P-C-T) and the van't Hoff equation, the dehydrogenation enthalpy and entropy of the 4LiBH(4)-MgH2-Al system can be determined as 57 kJ/mol-H-2 and 75 J/K.mol-H-2, respectively. The system is slightly destabilized from pristine LiBH4 (Delta H-de degrees = 68 kJ/mol-H-2) by coadditives of MgH2 and Al. The XRD measurements on the rehydrogenated samples suggest that the above reaction is partially reversible and the backward reaction takes place in two steps as 4LiH + MgAlB4 + 6H(2) -> Mg + 4LiBH(4) + Al and Mg + H-2 -> MgH2. Because of the alloying of Mg with Al, MgH2 in the complex system cannot be fully recovered below the temperature of 673 K. The isothermal rehydrogenation measurements exhibit significantly enhanced kinetics for the LiH-MgAlB4 system compared with LiH-MgB2 and LiH-AlB2 systems.