La-Mg-Ni-based Hydrogen Storage Alloy And Zr, Nb Replaced First-principles Study

Hydrogen storage is a key issue in the application process of hydrogenenergy. Metallic hydrogen storage materials have been extensively studied for itsunique advantages. La-Mg-Ni based hydrogen storage alloy is considered to be avery promising solid hydrogen material because of its excellent hydrogenstorage performance. In this paper, by means of the analysis of the formationenthalpy, density of states and distribution of charges, the La-Mg-Ni hydrogenstorage alloys and its element substitution have been investigated by thefirst-principles plane-wave pseudopotential method based on the densityfunctional theory. The CASTEP module of Materials Studio software is used inthis investigation. The main results are as follows:(1)The crystal structure of AB3-type LaNi3, La2MgNi9and LaMg2Ni9alloy has been built. The calculate results show that there exists more bondingelectrons near Fermi level and the interaction between La(s) and Ni(s) orbits isstronger in La2MgNi9. This leads La2MgNi9to be the most stable alloy in thethree alloys. The bond length between atoms in La2MgNi9does not shortenwhile the radius of Mg atom is smaller than La atom obviously, which caused Hatoms can move in the gap easily and the hydrogen absorption increased. Thebond length shortens obviously in LaMg2Ni9.(2)The crystal structure of Mg2Ni and its hydrides Mg2NiH4has been built.The most stable lattice site of Zr or Nb atom has been identified through the testin different locations. The influence to the hydrogen storage properties ofMg2NiH4by Zr or Nb element substitution has also been analyzed. The resultsshow that when Zr or Nb atom occupies the Mg(6i) lattice site, the structure ofMg2Ni has the highest structure stability. In which the Nb substitution decreasesthe stability of Mg2Ni alloy obviously, which is beneficial for the hydrogenabsorption. The further analysis of the hydrides shows that Zr or Nb alloyingsignificantly benefits the improvement of the dehydrogenating properties ofMg2Ni hydrides. Analysis of the electronic structures shows that the Ni-H bondis weakened because the interaction between Zr or Nb and H is stronger than the interaction between Mg and H. This may be one of the reasons of decreasingstructure stability of the hydrides.(3)The crystal structure of LaMg2Ni and its hydrides LaMg2NiH7havebeen built. The calculate results show that LaMg2Ni system has a betterhydrogen absorption and desorption properties than Mg2Ni system. The H atomsexists in two states in LaMg2NiH7alloy, one part of which show strong covalentbonds with Ni atoms while the other part format covalent bonds with La atoms.The investigation also show that there exists a concomitant transition betweenmetallic(alloy) and non-metallic (hydride) properties in this system.