The hydrogen storage properties and reaction mechanism of the MgH2-NaAlH4 composite system

In this study, we report the hydrogen absorption/desorption properties and reaction mechanism of the MgH2–NaAlH4 (4:1) composite system. This composite system showed improved dehydrogenation performance compared with that of as-milled NaAlH4 and MgH2 alone. The dehydrogenation process in the MgH2–NaAlH4 composite can be divided into four stages: NaAlH4 is first reacted with MgH2 to form a perovskite-type hydride, NaMgH3 and Al. In the second dehydrogenation stage, the Al phase reacts with MgH2 to form Mg17Al12 phase accompanied with the self-decomposition of the excessive MgH2. NaMgH3 goes on to decompose to NaH during the third dehydrogenation stage, and the last stage is the decomposition of NaH. Kissinger analysis indicated that the apparent activation energy, EA, for the MgH2-relevent decomposition in MgH2–NaAlH4 composite was 148 kJ/mol, which is 20 kJ/mol less than for as-milled MgH2 (168 kJ/mol). X-ray diffraction patterns indicate that the second, third, and fourth stages are fully reversible. It is believed that the formation of Al12Mg17 phase during the dehydrogenation process alters the reaction pathway of the MgH2–NaAlH4 (4:1) composite system and improves its thermodynamic properties