Thermodynamic analysis of dehydrogenation path of Mg–Al–Li–Na alloys

Abstract A thermodynamic study on the Mg–Al–Li–Na–H system is conducted in this work in order to investigate its hydrogen storage properties. For that purpose, a thermodynamic database is constructed using the CALPHAD technique. In this work, Li is added to the previously studied Mg–Al–Na–H system and the related binaries (Li–Mg, Li–Na) and ternaries (Al–Li–Mg, Al–Li–H, Li–Mg–H) are reassessed using the Compound Energy Formalism (CEF) for the terminal solid solutions and the Modified Quasichemical Model (MQM) for the liquid phase. The quaternary hydride Na 2 LiAlH 6 is assessed for the first time. The constructed database is used to calculate de/re-hydrogenation reaction pathways of MgH 2 –LiAlH 4 /Li 3 AlH 6 composites in a wide temperature and pressure ranges. The results are analyzed and compared with the experimental data to better understand and predict the reaction mechanisms. It is shown that Al resulted from the decomposition of LiAlH 4 /Li 3 AlH 6 hydrides destabilizes MgH 2 by the formation of Mg–Al phases: β and γ, and that LiH does not contribute to these reactions. Only negligible amounts of hydrogen are released from LiH because of the limited solubility of Li (from LiH) in Mg at high temperatures. It is concluded that MgH 2 –Li 3 AlH 6 system has better reversibility than MgH 2 –LiAlH 4 system and promising compositions (e.g. 37.09 mol% and 53.65 mol% MgH 2 contents) are predicted. Also, a new destabilization reaction is predicted in the system. The MgH 2 and Na 2 LiAlH 6 hydrides destabilize mutually at 308.4 K (35.25 °C) and 1 bar because of the formation of MgNaH 3 . However, the hydrogen capacity is reduced. It is concluded that Na considerably reduces the hydrogen storage potential of the system.