Study On Hydrogen Storage Property Of Y₅Mg_95-xNi_x（x=5,10,15） Alloys

Solid Mg-based hydrogen storage alloys have the advantages of high energy storage density,high safety and abundant raw materials.They are one of the most promising hydrogen storage materials.Because of the complex electronic structure of rare earth elements,alloying with Mg can play a very good catalytic role in hydrogen storage properties of alloys.The addition of yttrium element can greatly improve the properties of alloys.Elemental substitution method is also an effective way to improve the properties of alloys.The properties of alloys are changed by replacing Mg with Ni and changing the content of Ni.Catalysts are also an effective way to change the hydrogen absorption and desorption properties of alloys.The effects of adding TiF₃ catalysts with different content and Cr₂O₃,MoS₂ and NbF₅ catalysts with the same content on the energy absorption of alloys were investigated.By means of vacuum induction melting and planetary ball milling,as-cast,ball-milled and catalyst-added alloy samples were prepared.The phase composition and microstructure of the alloy were observed and analyzed by X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The hydrogen storage capacity,kinetics and thermodynamics of hydrogen absorption and desorption were tested by semi-automatic MH-PCT,and the mechanism of hydrogen absorption and desorption was analyzed and studied.As-cast alloy Y₅Mg_95-xNi_x（x=5,10,15）consists of main phase Mg and auxiliary phase Mg₂Ni and Ni₃Y.The reaction pathway of hydrogen absorption and desorption is Mg+Mg₂Ni+Ni₃Y+H₂→MgH₂+Mg₂NiH₄+YH₃?Mg+Mg₂Ni+YH₂+H₂.YH₂ is decomposed and hydrogen pump effect is exerted in the process of hydrogen absorption and desorption,which can improve the dynamic properties of the alloy.The increase of Ni content will increase the content of beneficial Mg₂Ni and Ni₃Y phases,and the rate of hydrogen absorption and desorption will be increased.The activation energies of hydrogen evolution are 92.64 kJ/mol,77.72 kJ/mol and 82.02 kJ/mol,respectively.The enthalpy of hydrogen evolution is 83.894 kJ/mol,80.426 kJ/mol and 83.155 kJ/mol,and the hydrogen absorption is 6.18 wt.%,5.15 wt.%and 4.93 wt.%respectively.The hydrogen storage kinetics of the alloy was improved by adding Y.The activation energy of hydrogen evolution decreases with the increase of Ni content,and the thermodynamic properties of hydrogen storage alloy fluctuate slightly,among which the enthalpy of hydrogen evolution of Y₅Mg8₅Ni₁₀ is the smallest.When the milling time is set to 5 h,10 h,20 h and 30 h,it can be found that the grain size of Y₅Mg₈₅Ni₁₀ alloy decreases obviously,resulting in nanocrystalline structure and amorphous structure.With the prolongation of milling time,the amorphous phase increases gradually,and the amount of hydrogen absorbed and the rate of hydrogen absorbed also decrease.The activation energies of dehydrogenation of ball-milled alloys are 77.17kJ/mol,75.26kJ/mol,76.79kJ/mol and 79.79kJ/mol,respectively.The enthalpy of dehydrogenation is 77.808kJ/mol,76.779kJ/mol,78.866kJ/mol and 78.726kJ/mol,and the hydrogen absorption is 5.10wt.%,5.0wt.%and 4.69 wt%.Y₅Mg₈₅Ni₁₀ alloy milled for 10 h has the best comprehensive propertiesAfter milling 10hY₅Mg₈₅Ni₁₀ alloy with 1wt.%,3wt.%,5wt.%and 7wt.%TiF₃ as catalysts,the properties of the alloy were studied.TiF₃ forms MgF₂ and TiH₂ in situ after hydrogen absorption and remains stable after hydrogen evolution.With the addition of TiF₃,the hydrogen storage capacity of the alloy decreases,and with the increase of TiF₃content,the rate of hydrogen absorption and desorption increases obviously,and the activation energy of hydrogen desorption decreases first and then increases.The activation energies of hydrogen evolution were 71.57 kJ/mol,70.79 kJ/mol,69.67 kJ/mol and 70.64kJ/mol,respectively.The enthalpy of hydrogen evolution was 76.447 kJ/mol,74.541kJ/mol,72.112 kJ/mol and 74.808 kJ/mol,and the hydrogen uptake was 5.08 wt%,5.05wt.%,5.01 wt.%and 4.82 wt.%.Adding 5wt.%TiF₃ has the best effect on improving the properties of the alloy and the best comprehensive properties.The inert catalysts Cr₂O₃ and MoS₂ will not react with hydrogen when 5wt.%Cr₂O₃,MoS₂ and NbF₅ are added to the 10hY₅Mg₈₅Ni₁₀ alloy by ball milling.However,the grain size of the alloy will be reduced effectively,resulting in a large number of crystal defects caused by lattice distortion,and the hydrogen absorption and desorption properties of the alloy can be improved.In the process of hydrogen absorption and desorption,the active catalyst NbF5 originally formed MgF₂ and NbH₂ phases,which existed stably and did not decompose with the alloy,thus increasing the number of grain boundaries in the alloy.The activation energies of hydrogen evolution are 69.17 kJ/mol,70.37 kJ/mol and 70.53 kJ/mol,respectively.The enthalpy of hydrogen evolution is 80.044 kJ/mol,76.096 kJ/mol and77.150 kJ/mol,and the hydrogen absorption is 5.09 wt.%,5.06 wt.%and 4.94 wt.%respectively.Cr₂O₃-catalyzed alloys have smaller activation energy for hydrogen evolution,MoS2-catalyzed alloys have smaller enthalpy for hydrogen evolution,and NbF₅-catalyzed alloys have faster hydrogen absorption and desorption rate.