Hydrogen storage properties and mechanisms of Mg(BH4)2⋅2NH3–xMgH2 combination systems

Abstract

• A composite system composed of Mg(BH 4 ) 2 ⋅2NH 3 and MgH 2 was prepared. • Ammonia release from Mg(BH 4 ) 2 ⋅2NH 3 is fully depressed in the presence of MgH 2 . • ∼13.9 wt% H 2 is released from Mg(BH 4 ) 2 ⋅2NH 3 –MgH 2 initiated at 70 °C. • The dehydrogenated sample can take up 3.7 wt% H 2 at 450 °C and 100 bar. The hydrogen storage properties and mechanisms of the Mg(BH 4 ) 2 ⋅2NH 3 – x MgH 2 combination systems were systematically investigated. The release of ammonia from pristine Mg(BH 4 ) 2 ⋅2NH 3 was completely depressed in the presence of MgH 2 . Approximately 13.9 wt% of hydrogen was released from the Mg(BH 4 ) 2 ⋅2NH 3 –MgH 2 combination system at a molar ratio of 1:1 with an onset temperature of 70 °C, which is 75 °C less relative to the onset temperature of pristine Mg(BH 4 ) 2 ⋅2NH 3 . In the initial dehydrogenation stage, the Lewis base H δ− in MgH 2 first attacks H δ+ in the NH 3 group of Mg(BH 4 ) 2 ⋅2NH 3 to release hydrogen and generate a MgBH 4 NH 2 compound. Further elevation of the operating temperature results in the decomposition of MgBH 4 NH 2 , releasing hydrogen to form MgH 2 and BN. Finally, MgH 2 decomposes to release the remaining hydrogen to produce Mg. In addition, the dehydrogenated product absorbed approximately 3.7 wt% hydrogen at 450 °C with an initial hydrogen pressure of 100 bar, which demonstrates the partial reversibility for hydrogen storage.