Study On Hydrogen Storage Properties Of Mg-Ag Alloys

In this paper, the development for the Mg-based hydrogen storage materials has been briefly described. Based on the review of the research progress and existing problems of Mg-based hydrogen storage materials, the constructed Mg?Ag alloys are prepared through alloy melting and mechanical milling in present work. De-/hydrogenation properties of Mg-Ag alloys are characterized by the Sievert’s method. The de-/hydrogenation reaction mechanisms and the kinetics as well as thermodynamic characteristics of the Mg–Ag alloys are analyzed by means of X–ray diffraction(XRD), pressure–composition–isotherm(PCI) measurement, scanning electron microscope/energy spectrum(SEM/EDS) and diffferential scanning calorimetry(DSC).The hydrogen storage properties of Mg4 Ag and Mg6 Ag alloy are studied firstly. It is found that the processes of de-/hydrogenation reaction for Mg4 Ag and Mg6 Ag alloys are completely reversible after activation. The kinetics of Mg4 Ag alloy could be improved by ball-milling and adding of catalyst. The kinetic properties of Mg6 Ag alloy are superior to that of Mg4 Ag alloy.The researches on the hydrogen storage properties and the de-/hydrogenation reaction mechanisms of ternary Mg?In?Ag alloy show that the processes of de-/hydrogenation reaction for Mg5.7In0.3Ag alloy are completely reversible after activation. The as-cast Mg5.7In0.3Ag alloy is composed of Mg(In) solid solution and Mg3 Ag phase. Indium atoms have a stronger affinity with Mg?Ag compound in H2 atmosphere. During initial hydrogenation, indium atoms transfer from Mg(In) solid solution to MgAg phase to form(Mg, In)Ag phase. Subsequently, the(Mg, In)3Ag phase and Mg phases are formed after dehydrogenation.Kinetic modeling studies indicate that the hydrogen desorption of Mg?H2, Mg6Ag?H2 and Mg5.7In0.3Ag?H2 systems are rate?limited by a two-dimension phase boundary migration. The activation energy of Mg?H2 and Mg6Ag?H2 systems are calculated as 155 and 89.8 kJ/mol H2, respectively. The hydrogen desorption kinetics of the Mg5.7In0.3Ag?H2 system is remarkably enhanced due to the activation energy of Mg5.7In0.3Ag?H2 system greatly reduce to 78.2 kJ/mol H2.The thermodynamic properties of Mg4 Ag compound are the same to that of Mg phase, but the thermodynamic properties of Mg3 Ag compound are superior to Mg4 Ag compound. The desorption enthalpy changes of Mg?H2 and Mg6Ag?H2 systems are calculated as 75.5 and 71.8 kJ/mol H2, respectively. Significantly, the desorption enthalpy change of(Mg, In)3Ag?H2 system is further reduced to 62.6 kJ/mol H2 due to the synergy of In and Ag. The thermodynamics of(Mg, In)3Ag?H2 system has a dual improvement as compared to that of the Mg?H2 system.