Study On The Preparation And Hydrogen Storage Properties Of High Capacity Mg-based Hydrogen Storage Materials

Mg-based hydrogen storage alloys have many advantages such as high hydrogen storage capacity and abundant resources,but their application and development are seriously hindered by the disadvantages such as slow kinetic performance and poor cyclic stability.In this paper,MgH₂ was combined with AB₅ hydrogen storage alloy,Li BH₄ and other additives by high-energy ball milling to prepare the composite with different components,aiming at improving the dehydrogenation kinetic properties and decrease the dehydrogenation temperature of Mg/MgH₂ system.In this paper,the effects of additives such as AB₅ and Li BH₄ on hydrogen storage performance of MgH₂ were elucidated,and the mechanism of their action in the process of hydrogen absorption and desorption was analyzed.Considering the effect of rare earth elements(RE)and Ni elements in AB₅ alloy on improving MgH₂ hydrogen storage properties,a series of MgH₂-ANi₅(A=Ce,Nd,Pr,Sm and Y)composites with different compositions were prepared by high-energy ball milling in H₂atmosphere.The hydrogen storage properties of the composite were measured by DSC and Pro2000,then through the analysis,the results show that the composite obtained from ball-milling MgH₂ and 5 wt.%Ce Ni₅ for 6 h had the highest dehydrogenation capacity and the best comprehensive hydrogen storage performance.The results show that the MgH₂-YNi₅composite has the fastest dehydrogenation rate,which can reach 99.2%of the saturated dehydrogenation capacity in 148 s at 320°C.On the other hand,through analysis of XRD,SEM and JMAK model fitting,the results show that the dehydrogenation reaction control step of the composite is one-dimensional diffusion control.At the same time,the defects introduced by ball milling,rare earth hydride and Mg-Ni hydride catalysts generated in situ can effectively reduce the dehydrogenation temperature of the Mg/MgH₂ system,more importantly,improve the adsorption/desorption kinetic properties.In order to further improve the hydrogen storage capacity and cycling stability of the Mg/MgH₂ system,the MgH₂-Li BH₄-YNi₅ composite with excellent hydrogen storage properties was successfully synthesized by ball milling MgH₂ with YNi₅ and Li BH₄ in hydrogen atmosphere.Measured by DSC and Pro2000,the composite of MgH₂-4 wt.%Li BH₄-1 wt.%YNi₅ has the lowest dehydrogenation temperature of 309.8°C,its total dehydrogenation capacity reached 7.10 wt.%at 300°C,and the comprehensive kinetic properties were also the best.In addition,the cyclic test of MgH₂-1 wt.%Li BH₄-1 wt.%YNi₅ composite showed that its capacity was only reduced to 6.31 wt.%after 200 cycles the capacity retention rate was up to 89.4%,showing excellent cycling stability.Combined with XRD,SEM and other phase analysis,it shows that under the synergistic action of YNi₅ and Li BH₄,effective catalytic active substances such as Mg Ni₃B₂ and YH₃ are formed,which effectively improves the hydrogen storage performance of Mg/MgH₂ composite.The kinetic properties of Mg/MgH₂ system can be effectively improved by nanocrystallization and catalyst.It is found that AB₅ hydrogen storage alloy can effectively reduce the dehydrogenation temperature of Mg/MgH₂ system,and greatly improve its kinetic properties.Under the combined action of YNi₅ and Li BH₄,the dehydrogenation capacity of the Mg/MgH₂ system is greatly increased and the capacity of the Mg/MgH₂ system can still maintain 89.4%after 200 cycles of absorption/desorption of hydrogen which has a good reversible cycling performance.