Understanding the hydrogen storage behavior of promising Al–Mg–Na compositions using thermodynamic modeling

Abstract

Thermodynamic modeling of the Al–Mg–Na–H system is performed in this work to understand the phase relationships and reaction mechanisms in this system. The Al–Na system is reassessed using the modified quasichemical model for the liquid phase. All the terminal solid solutions were remodeled using the compound energy formalism. The thermodynamic properties of the ternary systems are estimated from the models of the binary systems and the ternary compound using the CALPHAD method. The reaction pathways for the systems MgH2/AlH3, MgH2/NaAlH4, and MgH2/Na3AlH6 are calculated and compared to the experimental data from the literature. Details about the reaction mechanisms and temperatures, the amount of the products, and their composition are revealed and discussed in this work. The calculations show that in the composites MgH2/NaAlH4 and MgH2/Na3AlH6, the components spontaneously destabilize mutually in specific relative amounts by forming NaMgH3, which may play only a catalytic role on the decomposition of (MgH2 + Al) mixture, NaAlH4, or Na3AlH6. Also, Al destabilizes MgH2 and NaMgH3 by forming β phase and reducing the decomposition temperatures of these hydrides by more than 50 °C. The constructed database is successfully used to reproduce the pressure–composition isotherms (PCIs) for Mg-10 at% Al and Mg-4 at% Al alloys at 350 °C. The results provide a better understanding of the reaction mechanisms in the PCIs found in the literature concerning the number of plateau pressures and their sloping. It is shown that the first plateau pressure observed during the PCIs of Al–Mg alloys depends on Al content and is higher than that of pure Mg. This difference is due to Al solubility in hcp-Mg.