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.
Effect of the nanometric LiFePO4 on the hydrogen storage properties of MgH2
Abstract A nanometric phosphoric complex salt additive was synthesized by hydrothermal method. Its catalytic effect on the hydrogen storage properties of MgH 2 was investigated. MgH 2 +20 wt.% LiFePO 4 obtained by ball-milling process had an ability of absorbing 2.03 wt.% hydrogen at 423 K in 21 min, and only 0.98 wt.% could be absorbed by MgH 2 for the identical condition. 3.61 wt.% hydrogen could be released by the composite at 623 K in 21 min, whereas only 2.23 wt.% was desorbed by as-received MgH 2 . Doping by nanometric LiFePO 4 expressively enhanced the hydrogenation/dehydrogenation properties of MgH 2 , and changed the rate-controlling steps from three-dimensional interfacial reaction to one-dimensional diffusion process. Meanwhile, the onset dehydrogenation temperature for MgH 2 +20 wt.% LiFePO 4 was 653 K, which was found to be 60 K lower than the as-received MgH 2 . In addition, the addition of LiFePO 4 also lowered the activation energy ( E a) 31 kJ mol −1 . Based on XRD and XPS analysis, the in situ formed phases LiMgPO 4 and Fe, which are produced by the reaction between MgH 2 and LiFePO 4 , may be responsible for the remarkable improvement in hydrogen storage properties of MgH 2 .
Solubility of hydrogen in metals under high hydrogen pressures: Thermodynamical calculations
The solubility of hydrogen was calculated up to high hydrogen pressures for ten metals having small solubilities under normal pressures. For this purpose, the equation of state of hydrogen was calculated by the procedure of Hemmes et al., and the values obtained for the molar volume, the enthalpy, the Gibbs free energy and the entropy are tabulated in the range 0.1 MPa ⩽ p ⩽ 100 GPa and 400 ⩽ T ⩽ 2000 K. For solubility calculations, the chemical potential of hydrogen in solid solution was estimated from heats of solution observed at low hydrogen pressures and concentrations, by including a pvH term (vH ≊ 2.5 × 10−3nm3/H atom), and concentration-dependent terms arising from interactions between hydrogen atoms. The solubilities obtained for f.c.c. metals Al, γ-Fe, Ni, Cu, Ag, Pt, Au and b.c.c. metals Cr, α-Fe, Mo, W are shown in the form of Arrhenius plots with pressures as a parameter. Strong enhancements of the solubility over Sievert's law are found at high hydrogen pressures (p ⪆1 GPa).