Investigation Of The Effect And Mechanism Of Transition Metal Compounds To Hydroganation Properties Of Mg₂Ni Alloy

Mg2Ni alloy has been considered as the potential hydrogen storage material due toits fine reversible hydrogen storage performance in relatively mild conditions. However,the high temperature of reversible hydrogen storage and poor kinetic properties restrict itspractical application. To further develop the hydrogen storage properties of Mg2Ni alloy,additives including rare earth hydrides, Ti and Nb hydrides/nitrides and light hydrogenstorage materials are ball milled with Mg2Ni alloy, respectively. Microstructure analysisand hydrogen storage characteristics of the composites with additives are investigated.The hydriding/dehydriding reactions of the composites as well as the catalyticmechanisms of the additives in the process of hydrogen absorption/desorption arediscussed in detail.Ultilizing the hydrogenated decomposition characteristics of LaMg2Ni alloy, theeffect of in-situ generated LaH3on the hydrogen storage properties of Mg2Ni alloy isinvestigated. It is found that the Mg2Ni+20wt.%LaMg2Ni composite presentscompletely reversible hydrogenation properties, and its hydrogen storage capacity reaches3.22wt.%. Meanwhile, the composite presents modified hydrogen kinetic properties. Theimprovements are attributed to the fortified catalytic effect of in-situ generated LaH3onhydrogen storage properties of Mg2Ni alloy. Moreover, the decomposition of LaMg2Niprovides abundant phase interface, which is benificial to the absorption/desorption ofhydrogen.Compared with the pure Mg2Ni sample, the onset decomposition temperature ofMg2Ni+10wt.%TiN composite decreases by13K. The composite absorbs3.08wt.%hydrogen during100s at423K. It is caused by the weakened Ni-H and Mg-H bonds inthe Mg2NiH4by the interaction between N and H atoms, which decrease thethermodynamic stability of the composite hydride. Meanwhile, the addition of Ti and Nbhydrides (TiH2and NbH) dramatically improves the hydrogen absorption/desorptionproperties of Mg2Ni alloy. Owing to the stronger electronegativity of Nb than Ti, theMg2Ni+10wt.%NbH composite presents modified thermodynamic property. The onsetdecomposition temperature of the composite is10K lower than that of pure Mg2Ni sample.The LaH3-TiH2composite additive is prepared by ball milling and added into Mg2Nialloy to produce Mg2Ni+10wt.%(LaH3-TiH2) composite. Compared with pure Mg2Ni,the composite presents decreasd onset decomposition temperature by18K and0.44wt.%H desorption capacity in2h at523K. The improvements are attributed to that the LaH3transformes with LaH2by the catalytic effect of TiH2in the composite additive and showsimproved effect on hydrogen storage properties of Mg2Ni alloy. During the hydrogenabsorption/desorption cycles, the Mg3TiNi2is generated by the TiH2and Mg2Ni in thecomposite. The generation of the new phase increases the H absorption interface in thecomposite, leading to further improvement on the hydrogen kinetic properties of thecomposite. After72hydrogenation cycles, the Mg2Ni+10wt.%(LaH3-TiH2) compositeabsorbs1.26wt.%H more than that of the as-actived composite.With the addition of PrH2.92and CeH2.29, the activated4LiBH4+5Mg2NiH4composite particles are well covered with the generated flocculent Mg, which presentsuniformly spongy structure and increases the contact interface of the H atoms and Mg aswell as decreases the H diffusion distance in the composite. The onset decompositiontemperature of composite with rare earth hydrides (PrH2.92and CeH2.29) is25K and29Klower and the hydrogen desorption rate is1.95wt.%/h and0.73wt.%/h higher than thoseof pure4LiBH4+5Mg2NiH4composite, respectively.