Study On The Modification Of Microstructure And Hydrogen Storage Performances Of Mg-Al-Y Alloys

Magnesium is considered as one of the most potential hydrogen storage materials due to its large hydrogen storage capacity（7.6 wt.%,110 kg H₂/m³）,abundant storage capacity（about 2.35%in the Earth’s crust）and low price.In this work,starting from the Mg-Al binary alloy,the light rare earth element Y is selected to partially replace Mg,so as to optimize the Mg-Al-Y ternary alloy.Subsequently,the effects of mechanical ball milling,the addition of porous carbonaceous catalysts and the preparation of Mg-Al-Y-based composites on the improvement of hydrogen storage kinetics and thermodynamics of the Mg-Al-Y ternary alloy were studied.Mg_95-xAl₅Y_x（x=0-5）was prepared by vacuum induction melting,and the phase composition and microstructure of the alloys were studied.It was found that the Mg₉₅Al₅ alloy was mainly composed of Mg and Mg₁₇Al₁₂ phases.After hydrogen absorption,the phase composition of the alloy changes to Mg H₂ and Al.After hydrogen desorption,the phase composition of the alloy returns to Mg and Mg₁₇Al₁₂.Compared with pure Mg,the hydrogen desorption enthalpy（ΔH）of the Mg₉₅Al₅ alloy has no significant change,but the activation energy of hydrogen desorption（_aE^{d e}）decreases from 172.61 k J/mol（pure Mg）to 155.562 k J/mol(Mg₉₅Al₅).The introduction of Y element leads to the emergence of Mg₂₄Y₅ and Al₂Y phases in the alloy system,in which the Mg₂₄Y₅ phase will undergo disproportionation reaction Mg₂₄Y₅+H₂→Mg+YH₂→Mg+YH₂+YH₃ after hydrogen absorption,while the Al₂Y as a stable phase does not participate in hydrogen absorption and desorption reaction.With the increase of Y substitution,the diffraction peaks of Mg₁₇Al₁₂ phase gradually become small or even disappear,while the diffraction peaks of Mg₂₄Y₅ and Al₂Y phases increase in number and intensity.The introduction of Y element has no significant effect on the thermodynamics of the Mg-Al-based alloy,but the hydrogen absorption and desorption kinetics of the alloy is improved obviously.In addition,the hydrogen absorption and desorption rate of the Mg-Al-based alloy is accelerated,the activation energy decreases from 155.562 k J/mol(Mg₉₅Al₅)to 89.62 k J/mol(Mg₉₀Al₅Y₅),and the initial hydrogen desorption temperature decreases with the increase of Y content.The effects of ball milling time（5,10,15,20 h）on the microstructure and hydrogen storage properties of Mg₉₁Al₅Y₄ alloy were studied.The results show that the phase composition of the alloy is not changed by ball milling,but the particle size and grain size of the alloy are changed.With the prolongation of the ball milling time,the diffraction peaks of the main phase Mg in the alloy is slightly widened,and the partial diffraction peaks of the Mg₂₄Y₅ phase disappear.The average grain size of the alloy particles and the main phase Mg decrease continuously when the ball milling time is prolonged from 5h to 15h.When the ball milling time prolonging to 20 h,the alloy particles agglomerate obviously,and the grains no longer be refined.The as-milled alloy can achieve the saturation hydrogen absorption within 40 min at a high temperature of 573 K.However,only the alloy ball milling for 15 and 20 h can absorb about 3 wt.%H₂ within 100 min at a low temperature of 473 K.Porous and layered carbon supported Fe nanoparticles（Fe@C）catalyst were prepared by chemical blow molding carbonization method.The catalytic mechanism of the Fe@C catalyst（0,2,5,8 wt.%）on the as-milled Mg₉₁Al₅Y₄ alloy was investigated.The results show that the Fe@C catalyst effectively improves the ball milling efficiency,refines the alloy particles and modifies the alloy surfaces.Fe@C catalyst introduces its high-density defects into the alloy to facilitate the diffusion of hydrogen and the nucleation of new phase.In addition,Fe nanoparticles loaded on Fe@C plays an active role in promoting the dissociation and recombination of hydrogen molecules on the alloy surface.Appropriate addition of Fe@C can accelerate the hydrogen absorption and desorption rate of the alloy,but excessive addition inhibits the hydrogen absorption and desorption.The optimal addition of Fe@C is finally determined to be 5 wt.%.The Tm@C（Tm=Fe,Co,Cu）catalysts were prepared and the effect of adding 5wt.%Tm@C（Tm=Fe,Co,Cu）catalysts on as-milled Mg₉₁Al₅Y₄ alloy was investigated.The results show that Co@C catalyst not only has the largest defect density and specific surface area（190.9 m²/g）but also plays the optimal role in improving the ball milling efficiency and refining particles/grains.The Mg₉₁Al₅Y₄-5wt.%Co@C system shows the excellent kinetic properties,which can absorb a small amount of hydrogen at a low temperature of 323 K and the dehydrogenation activation energy of this alloy is reduced to 84.8 k J/mol.The difference of Tm@C catalysts in improving the alloy kinetics is due to the difference in the distribution of defect density,specific surface area and pore size,which leads to the difference in the defects introduced into the alloy and the refinement of alloy particles/grains.In addition,the highly electronegative and polyvalent Fe,Co,Cu transition metal particles supported by Tm@C may play various catalytic roles in the dissociation of hydrogen molecules and the recombination of hydrogen atoms.Mg₉₁Al₅Y₄-5 wt.%Tm F_x（Tm F_x=Ni F₂,Cr F₂,Zr F₄）-5 wt.%Co@C composites were prepared by mechanical milling to explore the possible synergistic catalytic effect between different Tm F_x and Co@C.The effect of Ni and F doping on hydrogen dissociation performance of Mg H₂ was studied by using first-principles pseudopotential plane wave method.According to the change of heat of formation(ΔH_sys)of the super cellular model,whether the F atom replaces the H atom in Mg H₂or occupies a gap position,it will induce the formation of a more stable structure.The replacement of Mg by Ni can destabilize the original system.According to the energy required to dissociate individual hydrogen atoms（ΔE）from the system,the entry of Ni and F atoms into the Mg H₂ supercell is conducive to the improvement of hydrogen dissociation performance.Mg H₂ in the hydrogenated Mg₉₁Al₅Y₄ alloy could react with Ni F₂ during ball milling to form Mg₂Ni H₄ and Mg F₂ phases.Cr F₂ did not change in the ball milling process,but changed into Cr and Mg F₂ after hydrogen desorption.Zr F₄remains stable and its addition is simple doping.The co-addition of Ni F₂ and Co@C in the alloy can produce a better catalytic effect.