Thermodynamic Modeling of Hydrogen Storage Capacity in Mg-Na Alloys
Thermodynamic modeling of the H-Mg-Na system is performed for the first time in this work in order to understand the phase relationships in this system. A new thermodynamic description of the stable NaMgH3 hydride is performed and the thermodynamic models for the H-Mg, Mg-Na, and H-Na systems are reassessed using the modified quasichemical model for the liquid phase. The thermodynamic properties of the ternary system are estimated from the models of the binary systems and the ternary compound using CALPHAD technique. The constructed database is successfully used to reproduce the pressure-composition isotherms for MgH2 + 10 wt.% NaH mixtures. Also, the pressure-temperature equilibrium diagram and reaction paths for the same composition are predicted at different temperatures and pressures. Even though it is proved that H-Mg-Na does not meet the DOE hydrogen storage requirements for onboard applications, the best working temperatures and pressures to benefit from its full catalytic role are given. Also, the present database can be used for thermodynamic assessments of higher order systems.
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.
Thermodynamic and structural investigations of chlorides in the systems KCl/MgCl2 and KCl/MnCl2
Abstract By means of a galvanic cell, emf values were measured for the solid-state reactionsnKCl+ MCl2 = KnMCln+2 for all existing compounds in the pseudobinary systems withM = Mg and Mn. ThusΔGr values could be calculated and, from their linear temperature dependence in the range 550–730 K, reaction entropies could be determined. EnthalpiesΔHr were calculated using the Gibbs-Helmholtz relation; they are compared with values found by solution calorimetry at room temperature. The magnitude of the entropy term for the free enthalpy of the formation reactions is discussed for the different compounds. For the modifications ofKMCl3 the lattice parameters for the cubic, tetragonal, and one of the orthorhombic phases were determined by X-ray photographs at varying temperatures. By DSC measurements the transition enthalpy for the tetragonal to cubic transition of KMnCl3 at 659 K was found to be 0.20–0.4 kJ · mole−1, compared to 4.6 kJ · mole for the transition of the stable room-temperature modification with the NH4CdCl3 structure to the metastable GdFeO3 structure.