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

The synthesized LiBH 4 −MgH 2 −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 −MgH 2 and 2LiBH 4 −Al systems. The major dehydrogenation pathway for the 4LiBH 4 −MgH 2 −Al complex system can be identified as 4LiBH 4 + MgH 2 + Al → 4LiH + MgAlB 4 + 7H 2 by means of X-ray diffraction (XRD) measurements on the as-dehydrogenated 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 −MgH 2 −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 LiBH 4 (Δ H de ° = 68 kJ/mol-H 2 ) by coadditives of MgH 2 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 + MgAlB 4 + 6H 2 → Mg + 4LiBH 4 + Al and Mg + H 2 → MgH 2 . Because of the alloying of Mg with Al, MgH 2 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−MgAlB 4 system compared with LiH−MgB 2 and LiH−AlB 2 systems.