Phase Structure And Hydrogen Storage Properties Of LaMg_8.52Ni_2.23M_0.15（M=Ni,Cu,Cr） Alloys

Magnesium rich Rare earth-Mg-Ni-based multiphase hydrogen storage alloys have alot of advantages, such as high hydrogen storage capacity, stable structure, abundance andso on. Therefore they become one of the most important research isseus of themagnesium-based hydrogen storage alloys. However, the dehydrogenation temperatureand hydriding kinetics of the alloys still fail to meet the practical application. This paperstudied the effect of partial substitution of Cu and Cr elements for Ni on hydrogen storageproperties of La-Mg-Ni-based multiphase alloys, and the LaMg_8.52Ni_2.23M_0.15（M=Ni, Cu,Cr） alloys were prepared by inductive melting. The microstructure and hydrogen storagecharacteristics of the alloys were investigated. In addition, the effect of hydrogenabsorption/desorption on hydrogen storage properties of the LaMg_8.52Ni_2.38alloy waspreliminarily investigated.X-ray Diffraction result shows that all the LaMg_8.52Ni_2.23M_0.15（M=Ni, Cu, Cr）alloys have multiphase structures, consisting of La2Mg17, LaMg₂Ni and Mg₂Ni phases.Energy Dispersive X-ray Spectrometer results reveal that most of Cu and Cr distribute inMg₂Ni phase. Hydriding/dehydriding measurements indicate that the reversible hydrogenstorage capacities of the Mg₂Ni phase in the LaMg_8.52Ni_2.23M_0.15（M=Cu, Cr） alloysincrease to1.05wt.%and0.97wt.%from0.79wt.%of the Mg₂Ni phase in theLaMg_8.52Ni_2.38alloy at523K. Partial substitution of Cu and Cr elements for Ni elementreduces the onset dehydrogenation temperature of the alloy hydrides and the temperaturelowers by18.20K and5.50K, respectively. The improvement in the dehydrogenationproperty of the alloys is attributed to that Cu and Cr decrease the stability of Mg₂NiH4phase.X-ray Diffraction result indicates that La2Mg17and LaMg₂Ni phases in theLaMg_8.52Ni_2.38alloy experience phase transition during the hydriding/dehydriding process.The alloy is composed of MgH2, Mg₂NiH₄and LaH3phases after hydrogenation and it isconsisted of Mg, Mg₂Ni and LaH_2.3phases after dehydrogenation. SEM shows that thereis obvious pulverization with increase of the cycle number. The P-C-T curves illustratethat the maximum hydrogen storage capacities after the20^thcycle are almost the same and are higher than that of the2^ndcycle. The reversible hydrogen storage capacity of theMg₂Ni phase in the alloy increases with increase of the cycle number. The DTA test showsthat the onset dehydrogenation temperature of the alloy hydrides reduces with increase ofthe cycle number, which may be attributed to that the pulverization of the alloy duringhydrogen absorption/desorption shortens the hydrogen diffusion distances in thedehydrogenation reaction, thus improving the dehydrogenation property of the alloyhydrides.