Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

• High purity of Mg(AlH4)2 is added to MgH2 as destabilization additive.
• Kinetics and thermodynamics properties of MgH2 are improved by Mg(AlH4)2.
• A possible desorption mechanism of the MgH2–Mg(AlH4)2 is proposed.
• Reversibility of MgH2–Mg(AlH4)2 is favorable than that of pure MgH2.

Both kinetics and thermodynamics properties of MgH2 are significantly improved by the addition of Mg(AlH4)2. The as-prepared MgH2–Mg(AlH4)2 composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH2-relevant desorption is 32.3 kJ mol−1 H2 in the MgH2–Mg(AlH4)2 composite, obviously decreased than that of pure MgH2. The dehydriding mechanism of MgH2–Mg(AlH4)2 composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH4)2 decomposes and produces active Al∗. Subsequently, the in-situ formed Al∗ reacts with MgH2 and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH2 and Al∗, under 3 MPa H2 pressure at 300 °C for 5 h.