Study On Hydrogen Absorption And Desorption Properties Of Mg_95-xAl₅Y_x（x=0-5） Alloy

Among all kinds of hydrogen storage alloys,magnesium-based hydrogen storage alloys have become one of the hot hydrogen storage materials that researchers at home and abroad focus on because of their high hydrogen storage capacity.Over the years,researchers have tried many improvements on the basis of pure magnesium,mainly including alloying,adding catalysts,surface treatment,amorphization and other methods.However,the high reaction temperature and poor hydrogen absorption and desorption kinetics of magnesium-based hydrogen storage alloy still restrict its further development.Considering the low cost and mature technological process of catalyst and ball milling process,this paper studies the effects of ball milling process and catalyst addition on the thermodynamic and kinetic properties of hydrogen absorption and desorption of Mg-Al alloy.When Mg is partially replaced by Y,Mg₂₄Y₅ phase is formed in addition to Mg as the main phase.The hydrogen absorption capacity of the alloy increases first and then decreases with the increase of Y content.The hydrogen absorption and desorption capacity of Mg₉₁Al₅Y₄ alloy is the largest,and the hydrogen release rate of the alloy also increases significantly with the addition of Y element.The activation energy of hydrogen release of the alloyE_a^de decreases with the increase of Y content.All samples have only one platform.The platform pressure increases first and then decreases.When Y content x=3,the corresponding equilibrium hydrogen pressure is the largest.The entropy change and enthalpy change are consistent with the platform pressure change:increase first and then decrease.The reason is the cell volume change caused by the interaction between atomic radius differences of Mg,Al and Y alloys.The comprehensive hydrogen storage performance of Mg₉₁Al₅Y₄ is the best.On the basis of the above,the influence of ball milling time on the hydrogen storage performance of the alloy was studied.The results show that the alloy is significantly improved after dynamic ball milling,but the hydrogen absorption of the sample after ball milling is slightly reduced.It is possible that the ball milling further solutes Al,reduces the unit cell volume,and reduces the octahedral gap for hydrogen storage.The hydrogen storage capacity of the alloy is 6.233wt.%when the sample is milled for 50h and the reaction temperature is 633K..The activation energy of hydrogen release first decreases and then increases with ball milling time,with a minimum value of 82.0734 kJ/mol at 50h.Different ball milling times all cause sub-elevation of the platform,with the maximum corresponding value of 50h.Subsequently,carbon-supported copper was prepared as a catalyst,and the effect of the catalyst on the thermodynamic and kinetic properties of hydrogen absorption and desorption of ball-milled Mg₉₁Al₅Y₄ alloy was studied.The results show that,for the kinetic properties of ball-milled alloy,appropriate addition of graphene can significantly improve the hydrogen absorption and desorption kinetic properties of the alloy.Carbon materials can reduce particle agglomeration and refine alloy grains.However,the addition of catalyst slightly reduced the hydrogen absorption of the alloy and the activation energy of the reaction was reduced to a certain extent.The changes of thermodynamic performance parameters show that P-C-T platform pressure slightly increases,reducing the absolute value of reaction enthalpy,which is conducive to the hydrogen evolution reaction of alloy.The change of reaction entropy further illustrates the influence of catalyst on the system performance.