Investigation On The Properties Of La-Mg Based Hydrogen Storage Alloys And The Electronic Structures Of Mg-Based Hydrogen Storage Alloys

In this paper, the electronic structures, the heat of formation of Mg-basedhydrogen storage alloys and the hydrogen storage properties of La-Mg basedalloys have been investigated. The electronic structures and heat of formation ofLT-Mg₂NiH₄ and Mg_2-xMNiH₄(M=Ag, Al, Ti or Zr) were calculated byfirst-principle plane wave pseudopotential (PW-PP) method which is based ondensity functional theory, and the influences of alloying elements(M=Ag, Al, Tior Zr) on the stability of Mg₂Ni hydride were investigated. Adsorption anddissociation of H₂ on the Mg₂Ni(010) face was studied in detail using the samemethod, and the most favorable adsorption sites for H₂ are identified.La_0.7Mg_0.3Ni_2.7Mn_0.3Co_0.5-xAl_x(x=0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storagealloys were prepared by induction melting under Ar atmosphere, thecharacteristics of alloy phases and the influences of Co and Al element on thehydrogen storage and electrochemical properties were studied.All results are shown in the following:The heat of formation and the increment of total energy betweenMg_2-xM_xNiH₄(M=Ag, Al, Ti or Zr) and Mg₂NiH₄ as well as their electronicstructures were calculated using first-principle plane wave pseudopotential(PW-PP) method. More negative value for formation heat ofMg_2-xM_xNiH₄(M=Ag, Al, Ti or Zr) compared with that of Mg₂NiH₄ indicates that the alloying atoms(M=Ag, Al, Ti or Zr) befit to improve thedehydrogenating properties of Mg₂NiH₄. And the more electronegative of thealloying elements, the more unstable of the hydride system. Based on theanalysis of the density of states (DOS) andcharge population of Mg₂NiH₄ andMg_2-xM_xNiH₄(M=Ag, Al, Yi or Zr). it is found that the improvement of thedehydrogenating properties of Mg₂NiH₄ caused by alloying atoms(M=Ag, Al, Tior Zr) mainly originates from the increasing of the valence electrons at Fermilevel (E_F) and the weakened bonding between Ni and H.The adsorption energy and reaction barrier of H₂ dissociation on theMg₂Ni(010) face were studied using first-principle plane wave pseudopotential(PW-PP) method. The top site of Ni(Horl) was found to be the most favorablemolecular adsorption site, the distances (r_d) between the Mg₂Ni(010) face andthe H₂ molecule are 1.6286(?), and the H-H bond length (r_H) was stretched up to0.9174(?) from 0.738(?), with the highest adsorption energy of 0.6769eV. It wasfound that the activation energy for dissociation of H₂ is 0.2778eV, withconsiderably higher adsorption energy than that of the molecular cases. Thebonding between the hydrogen atom and Mg₂Ni(010) face is very stronger, andthe charges transfer to hydrogen atoms mainly from the Ni.XRD analysis shows that the La_0.7Mg_0.3Ni_2.7Mn_0.3Co_0.5-xAl_x(x=0, 0.1, 0.2,0.3, 0.4, 0.5) hydrogen storage alloys consist of hexagonal CaCu₅-type structure,LaNi₃ phase and La₂Mg₁₇ phase. In the alloys, the La(Ni, Mn, Co, Al)₅ phasereplaces the La(Ni, Mn, Co)₅ phase with A1 substituting for Co and the La(Ni,Mn, Al)₅ phase replaces the La(Ni, Mn, Co, Al)₅ phase with x=0.5. The cellvolume of primary phase increases with Al substituting for Co. The plateaupressure declines gradually and the hysteresis between the hydrogen absorptionand desorption decreases with the amount of Al. The electrochemical measures indicate that the maximum discharge capacity is 333.8mAh/g(x=0.1) and thendecreases to 263.5mAh/g(x=0.5). The appropriate amount of Co inLa_0.7Mg_0.3Ni_2.7Mn_0.3Co_(0.5-x)Al_x alloys can extend the cycle life of the alloyelectrodes.