Synthesis And Hydrogen Storage Properties Of Core-Shell Structured Mg-Based Ultralfine Composite Powders

In recent years, magnesium and its derived alloys have drawn a lot ofattention due to their abundant reserves, low cost, light weight and hightheoretical hydrogen storage capacity （7.6wt%）. Nevertheless, the majorobstacles to practical applications include both sluggish hydriding anddehydriding kineticsand high absorption anddissociation temperatures. Itisoptimal to discover effective solutions to improve both thermodynamic andkinetic properties of magnesium-based hydrogen storage materials.In the present study, the Mg-Nb, Mg-Nb₂O₅and Mg-TM（TM=Ti, Ni,Fe）-La hydrogen storage composite powders were prepared through arcplasma method followed by in-situ passivation, in order to obtain particleswhich can remain stable in dry air. The composition, phase component,morphology, particle size and hydrogen storage properties of these powderswere systematically analyzed by using inductive coupled plasma emissionspectrometer （ICP）, X-ray diffraction （XRD）, transmission electronmicroscopy （TEM）, pressure-composition-temperature （P-C-T） andthermogravimetry （TG）/differential thermal analysis （DTA） or differentialscanning calorimetry （DSC） techniques.ICP analyses revealed a reduction in Nb content of both Mg-Nb andMg-Nb₂O₅powders when compared with the initial composition. XRDresults showed the presence of MgNb₂O_3.67phase in Mg-Nb powder andNbO_2.46phase in Mg-Nb₂O₅powder. TEM observations revealed that theMgNb₂O_3.67particles distribute more homogeneous and the particle size issmaller than the NbO_2.46particles in Mg-Nb₂O₅powder. P-C-T analysesshowed that the hydrogen sorption plateaus of Mg-Nb powders are wider andsmootherwith asmallerabsorption/desorptiongap comparedtothoseofMg-Nb₂O₅powder. Based on van’t Hoff equation, the hydrogenationenthalpy of the Mg-Nb powder is determined to be-73.33kJ/mol H₂, lowerthan the value of-82.45kJ/mol H₂for Mg-Nb₂O₅powder. TG/DTAmeasurements showed that the hydrogenated Mg-Nb powder has a fasterdesorption rate and a sharper endothermic desorption peak than those ofhydrogenated Mg-Nb₂O₅powder. The better hydrogen storagethermodynamicandkineticproperties ofMg-Nbpowdercanbeattributedtothe catalytic effect of MgNb₂O_3.67generated in passivation. In contrast, theNbO_2.46phase has a low content and heterogeneous distribution inMg-Nb₂O₅powder and thus has poor catalytic effect. The above resultsshowed that arc plasma method followed by in-situ passivation is anefficient approach to produce metal-oxide hydrogen storage composite withsuperior catalytic effect of oxide than that of directly evaporated oxide.With regard to Mg-TM-La（TM=Ti, Fe, Ni） ternary composite powders,ICP composition analyses revealed reduction in TM and La contents for allcomposites when compared with their initial compositions. XRD resultsindicate introduction of La₂O₃during passivation and formation ofMg₂NiH₄in hydrogenated Mg-Ni-La-H particles. It is observed in TEMimages that the composites are all mainly composed of ultrafine Mgpowders covered by MgO and La₂O₃nanoparticles, forming a specialcore-shell structured metal-oxide composite. P-C-T measurements revealthat the addition of3d transition metals（Ti, Fe, Ni） change the formationenthalpy of composite hydride and reduce activation energy ofhydrogenation of ternary composite powder. In addition, TG/DTA（DSC）curves show that the addition of3d transition metals （Ti, Fe, Ni） andLa（La₂O₃） also decrease hydrogen desorption temperature of MgH₂, whilethe dissociation temperatures of composite hydrides are measured to be653K for Mg-Ti-La-H,623K for Mg-Fe-La-H and633K for Mg-Ni-La-H,respectively. Meantime, the hydrogen absorption kinetics can besignificantly improved when compared to those in the pure Mg. This is especially true for the Mg-Ni-La composite powder, which can absorb1.47wt%of hydrogen at303K after3.5h. The rapid absorption kinetics atlow temperatures can be attributed to the catalytic effects from bothMg₂NiH₄and oxide nanoparticles, especially La₂O₃, which can act aschannels for hydrogen sorption and result in formation of core-shellstructure. The above results showed that simultaneous addition of3dtransition metals （Ti, Fe, Ni） and4f rare earth metal （La） to Mg can catalyzeboth the thermodynamic and kinetic properties of Mg ultrafine powders,which is effective solution to improve hydrogen storage properties of Mg.