Structures And Hydrogen Storage Properties Of La-Mg-Ni-based AB₂Type Alloys

La–Mg–Ni system alloy is considered to be one of the most potential candidates ofhydrogen storage material due to its higher theoretical capacity and excellent activationcharacteristic. Among which the AB₂type hydrogen storage alloys possess a very hightheoretical capacity, however, the actual capacity and the cycle stability is veryunsatisfactory, how to improve the hydrogen storage capacity and the cycle stability ofthe alloys is the key to put the alloy into practical use. Therefore, the LaMgNi_4-xM_x（M=Co, Mn, Cu, Al; x=0,0.2,0.4,0.6,0.8） alloys were prepared by vacuum inductionmelting, the phase compositions and structures of the as-cast alloys were analyzed by X-ray diffraction and SEM, then the electrochemical and gaseous hydrogen storageperformances were investigated systematically.The phase compositions of the alloys were analyzed by XRD, the results show thatthe as-cast alloys prepared by vacuum induction melting consist of two main phase,LaMgNi₄phase and LaNi₅phase. The phase compositions of the Co, Mn, Cu addedalloys not have change, however, when the Al element content x≥0.4, the LaAlNi₄phase appeared in Al added alloys. SEM combined with EDS results show thatLaMgNi_4-xCo_x（x=0,0.2,0.4,0.6,0.8） alloys have a lamellar structure, in which thecenter is LaNi₅phase and outer wrapped LaMgNi₄phase. The addition of Co elementssignificantly changed the microstructure of the LaMgNi_4-xCo_x（x=0,0.2,0.4,0.6,0.8）alloys, making the regular flake structure of the alloys chaotic.The electrochemical hydrogen storage performances were studied systematically,and the results shown that all the alloys reach their maximum discharge capacity in thefirst cycle. The as-cast LaMgNi₄alloy has a high electrochemical capacity (318.6mAh·g^-1), the maximum discharge capacity of the Co added alloys first increase thendecrease, and the Co0.4alloy reach the maximum discharge capacity of352.1mAh·g^-1.However, the discharge capacity of the alloys significantly decline with the increasing of Mn, Cu and Al element substitution. What is more, element substitution evidentlyaffects the kinetics of the electrochemical performances of the alloy. With the increaseof Co and Al element content, the high rate discharge ability （HRD） of the alloys firstincreases then decreases, and achieve the maximum values of95.0%and95.6%when x=0.4, respectively. The main factors affecting the electrochemical kinetics of the alloysare the hydrogen diffusion rate in alloy.The gaseous hydrogen absorption/desorption properties of the LaMgNi_3.6M_0.4（M=Ni, Co, Mn, Cu, Al） alloys were carried out, and it is found that the activationperformances of all the alloys at100°C were obviously better than that of50°C. Al andCu element substitution can promote the activation performance of the alloy. The p-c-Tcurves of the LaMgNi₄alloy contain two hydrogen absorption/desorption equilibriumpressure platforms, the curve shape is roughly similar to Co_0.4alloy. However, the Mn_0.4and Cu_0.4alloys only appear a single platform for the LaNi₅hydrogen absorption due tothe hydrogen-induced amorphization of the LaMgNi₄phase in the alloys. Elementsubstitution significantly affected the hydrogen absorption capacity of the alloys, the Coelement replacement significantly promotes the hydrogen absorption capacity, and theMn, Cu and Al element substitution all makes the hydrogen absorption capacity of thealloys decreased in varying degrees. The addition of Co and Al elements significantlyimprove the hydrogen absorption/desorption cycling stability of the alloy, however, it israpid decayed by substitute Ni with Mn and Cu. Mn, Cu, Al elements substitutionsignificantly promote the hydrogen-induced amorphization of the LaMgNi₄phase of thealloys.