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

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 Na2LiAlH6 is assessed for the first time. The constructed database is used to calculate de/re-hydrogenation reaction pathways of MgH2-LiAlH4/Li3AlH6 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 LiAlH4/Li3AlH6 hydrides destabilizes MgH2 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 MgH2-Li3AlH6 system has better reversibility than MgH2-LiAlH4 system and promising compositions (e.g. 37.09 mol% and 53.65 mol% MgH2 contents) are predicted. Also, a new destabilization reaction is predicted in the system. The MgH2 and Na2LiAlH6 hydrides destabilize mutually at 308.4 K (35.25 °C) and 1 bar because of the formation of MgNaH3. However, the hydrogen capacity is reduced. It is concluded that Na considerably reduces the hydrogen storage potential of the system.