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.