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.
The Na–H system: from first-principles calculations to thermodynamic modeling
Abstract The Na–H system thermodynamic properties were assessed using Gibbs free energy model parameters obtained from best fit optimizations to combined experimental and first-principles predicted data. The first-principles finite temperature thermodynamic property predictions, based upon density functional theory ground state minimizations and direct method lattice dynamics, were used to supplement the Na–H dataset wherever experimental information was unavailable or unattainable. The predictions proved to be important for extending the evaluation of the heat capacity of the stable NaH phase to cover the complete 0 – 2000 K temperature range. The predicted thermodynamic properties of the hypothetical NaH3 end-member representing complete interstitial H substitution in solid body-centered cubic Na, provided a physical basis for modeling H dissolution in the Na lattice. The modeling also showed satisfactory agreement with experimental measurements of NaH enthalpies of formation, NaH decomposition pressures, and H solubility in liquid Na.
The enthalpies of fusion and the heat capacities of K2MgCl4, Rb2MgCl4, Cs2MgCl4, KMgCl3, RbMgCl3, and CsMgCl3
As part of an extended investigation of the thermochemistry of liquid and solid compounds in the AX + BX 2 and AX + CX 3 systems, the enthalpies of fusion of the congruently melting 2–1 and 1–1 binary compounds in systems between the alkali chlorides KCl, RbCl, and CsCl, and MgCl 2 have been determined. The following molar enthalpies of fusion have been obtained. K 2 MgCl 4 ; Δ H f = ( 10750 ± 200 ) cal th mol − 1 . Rb 2 MgCl 4 ; Δ H f = ( 12040 ± 250 ) cal th mol − 1 . Cs 2 MgCl 4 ; Δ H f = ( 9400 ± 250 ) cal th mol − 1 . KMgcl 3 ; Δ H f = ( 10300 ± 100 ) cal th mol − 1 . RbMgCl 3 ; Δ H f = ( 12300 ± 150 ) cal th mol − 1 . CsMgCl 3 ; Δ H f = ( 13700 ± 200 ) cal th mol − 1 . The new high-precision adiabatic-shield drop-calorimeter which has been used, has made it possible at the same time to give very precise values for heat capacities of mixtures of molten salts. These values have been used to calculate the enthalpies of mixing of MgCl 2 with KCl, RbCl, and CsCl at other temperatures than those given originally by Kleppa and McCarty.