Study Of The Effect Of Graphene On Gaseous Hydrogen Storage Properties Of La_2-xPr_xMg₁₆Ni （x=0.1～0.4） Hydrogen Storage Alloys

China has introduced a series of hydrogen energy development policies.Developing cost-efficient hydrogen storage technology is an important guarantee for achieving the"double carbon goal".In this thesis,La_2-xPr_xMg₁₆Ni（x=0.1～0.4）alloy was prepared by replacing the La element in La₂Mg₁₆Ni with the Pr element using the medium frequency vacuum induction method.The composite hydrogen storage alloy was prepared by ball milling with the addition of a graphene catalyst and investigated the hydrogen storage properties of the composite hydrogen storage alloy.Firstly,La_2-xPr_xMg₁₆Ni（x=0.1～0.4）alloy was ball-milled with 4 wt.%graphenes for10 h to prepare a composite hydrogen storage alloy powder to investigate the effect of Pr element substitution on the hydrogen absorption and release properties of the hydrogen storage alloy.The microstructure and phase composition of the alloy was analysed and examined using XRD and SEM.The experimental results show that after ball milling,the composite hydrogen storage alloys are composed of the primary phase La₂Mg₁₇and the secondary phases Mg₂Ni and La₂Ni₃.After hydrogen release,the alloy consists of the La H₃and Mg phases.Pr element substitution and the addition of graphene catalyst ball milling can significantly contribute to the refinement of alloy particles.The hydrogen uptake and release performance tests show that elemental substitution and the addition of graphene ball mills can significantly improve the alloy’s activation and hydrogen uptake and release performance and increase the rate of hydrogen uptake and release of the hydrogen storage alloy.At 573 k,the replacement amount x=0.4 shortens the hydrogen uptake and release time to 1/7 compared to x=0.1.The activation energy of the composite hydrogen storage alloy decreases and then increases as the Pr element substitution x increases from 0.1 to 0.4by JMAK fitting the hydrogen release data.The composite hydrogen storage alloy with La_1.7Pr_0.3Mg₁₆Ni+4 wt.%graphene has the lowest activation energy of 51.22 KJ/mol when the Pr substitution x=0.3.Secondly,we investigated the microstructure,phase composition,gaseous hydrogen storage activation and hydrogen absorption and release properties of the composite hydrogen storage alloy powder prepared by ball milling La_1.7Pr_0.3Mg₁₆Ni+y wt.%（y=0,2,4,6）graphene for 10 h.The XRD results show that the phase composition of the alloy does not change with the addition of graphene,indicating that the addition of graphene does not modify the phase composition of the hydrogen storage alloy.After ball milling,the diffraction peaks of the La₂Mg₁₇phase of the alloy broadened,and after fitting the XRD data,the average particle size of the La₂Mg₁₇phase of each composite alloy was around0.16-0.19 nm,indicating that the addition of graphene catalyst ball milling promotes the amorphization and nanosizing of the hydrogen storage alloy.After hydrogen absorption,the La H₃phase content of the alloy increased with the addition of graphene,which may be related to the fact that the addition of graphene can promote a faster rate of hydrogen absorption in the alloy.After analysis by SEM and EDS,it was found that graphene could induce a more uniform ball milling of the alloy particles and that graphene would adhere to the surface of the alloy particles.After fitting the hydrogen release data by the JMAK method,it was found that the activation energy of the alloy decreased and then increased with the addition of graphene,and the composite hydrogen storage alloy had the lowest activation energy of 51.22 k J/mol when the addition of graphene was 4 wt.%.Finally,we performed gaseous hydrogen absorption and discharge cycle life testing and analysis of composite hydrogen storage alloy powders prepared by ball milling of La_1.7Pr_0.3Mg₁₆Ni+y wt.%（y=0,2,4,6）graphene for 10 h.The SEM study found that the surface morphology of the alloy did not change significantly under less than 150 cycles of hydrogen absorption and discharge,but after 150 cycles of hydrogen absorption and discharge,more serious pulverisation occurred on the surface of the alloy particles and flocculation appeared on the surface of the particles.The amount of graphene added affects the uniformity of the graphene adhesion layer.At y=4 wt.%,the adherent layer on the surface of the alloy particles is more uniform and protects the alloy particles from severe pulverisation after several cycles,thus enabling the alloy to maintain a high capacity retention rate.After 150 cycles,the composite hydrogen storage alloy had a hydrogen absorption retention rate of 90.6%and a hydrogen release retention rate of 89.1%.The more significant agglomeration and spontaneous combustion of the composite hydrogen storage alloy after 150 hydrogen absorption and discharge cycles may be related to the re-agglomeration of fine particles at high temperatures on the one hand,and the precipitation of Mg nanocrystals in the alloy after hydrogen release on the other.