Hydrogen Storage Behavior Of Amorphous TiMgVNi₃ And The Effect On Hydrogen Storage Performance Of MgH₂

On the basis of summarizing the classification,hydrogen absorption and desorption principle and research status of hydrogen storage alloys,Mg based hydrogen storage composites were prepared by adding catalysts,so as to improve the hydrogen absorption and desorption kinetics and cycle stability of MgH₂.Firstly,the amorphous powder of TiMgVNi₃was prepared by melting and high-energy ball milling to explore its own hydrogen storage performance.On this basis,MgH₂-10 wt%TiMgVNi₃ and MgH₂-10 wt%TiMgVNi₃-5 wt%Pr H₂ composites were prepared by ball milling.The structure,microstructure and hydrogen storage properties of the composites were studied by means of X-ray diffraction（XRD）,pressure composition temperature（P-C-T）and transmission electron microscope（TEM）.The main research contents and conclusions are as follows:（1）The amorphous powder of TiMgVNi₃ was prepared by melting and high-energy ball milling,and its hydrogen storage performance and corresponding microstructure were studied.The results show that the amorphous powder can begin to absorb hydrogen at 100℃.With the increase of temperature,the hydrogen absorption rate and amount gradually increase,and the hydrogen absorption amount reaches 0.9 wt%at 400℃.XRD of the sample after hydrogen absorption showed that amorphous began to crystallize with the increase of temperature,but no obvious hydride was found during hydrogen absorption at low temperature.MgH₂ and Mg₂Ni H₄ were observed after hydrogen absorption at 400℃.At the same time,it is found that amorphous powder can form（Ti,Mg,V,Ni）H_x hydride phase after ball milling for 10 h in hydrogen atmosphere.The hydrogen storage reversibility of the hydride is poor because the hydride has good high temperature stability,but it is a potential effective catalyst to improve the performance of MgH₂.（2）MgH₂ and MgH₂-10 wt%TiMgVNi₃ composites were prepared by mechanical ball milling.The hydrogen storage properties of the two samples were compared.Compared with MgH₂,MgH₂-10 wt%TiMgVNi₃ composites still need two cycles to complete activation,but the dehydrogenation activation energy after activation decreases from 152.7 k J mol^-1 H₂to 94.4 k J mol^-1 H₂.The subsequent 300℃cycle experiment shows that the first dehydrogenation of MgH₂-10 wt%TiMgVNi₃ composite takes 60 min and can realize complete dehydrogenation with a dehydrogenation amount of 5.39 wt%;After 50 cycles,the composite needs 90 minutes to realize complete hydrogen release,and the hydrogen release amount is 4.92 wt%.Based on this calculation,the final capacity retention rate is 92%.However,during the first dehydrogenation of MgH₂,the amount of hydrogen released in 90min is 4.04 wt%（hydrogen can be completely released in 4 h,and the amount of hydrogen released is 6.3 wt%）;After 50 cycles,the amount of hydrogen released in 90 min is only0.87 wt%（hydrogen can be completely released in 7 h,and the amount of hydrogen released is 3.3 wt%）.In this case,the capacity retention rate is only 22%.If the calculation is based on complete dehydrogenation,the final capacity retention rate is 52%.Obviously,compared with MgH₂ samples,the dynamic properties and cycle stability of MgH₂-10 wt%TiMgVNi₃composites are significantly improved.This is mainly because FCC type（Ti,Mg,V,Ni）H_x is formed in MgH₂-10 wt%TiMgVNi₃ composites after ball milling.The FCC type（Ti,Mg,V,Ni）H_x forms highly dispersed and structurally stable Mg₂Ni/Mg₂Ni H₄ and（Ti,V）H_1.8/（Ti,V）H₂ in situ in the subsequent hydrogen absorption and desorption process,on the one hand,provides more active sites and hydrogen diffusion channels,which hinders the agglomeration of MgH₂ powder;On the other hand,the growth of Mg grains is inhibited during the cycle.（3）MgH₂-10 wt%TiMgVNi₃-5 wt%Pr H₂ composites were prepared by mechanical ball milling.It was found that compared with MgH₂-10 wt%TiMgVNi₃ composites,the sample also needs two hydrogen absorption and desorption cycles to be activated,but the hydrogen evolution rate is significantly faster.The hydrogen evolution activation energy of the activated sample is 86.4 k J mol^-1 H₂.The cycle experiment shows that the hydrogen release is 5.70 wt%and complete within 12 minutes at 300℃.After 50 cycles,the hydrogen release is complete within 17 minutes,the hydrogen release amount is 5.29 wt%,and the capacity retention rate is 93%.Although the cycle stability has not been improved,the kinetic performance has been significantly improved.This is because the introduction of Pr H₂causes the synergistic catalytic effect of（Ti,V）H_1.8/（Ti,V）H₂,Mg₂Ni/Mg₂Ni H₄and Pr H₂/Pr H₃ formed in situ,which provides more active sites and H diffusion channels on the one hand,and inhibits the growth of Mg grains and the formation of MgO on the other hand.