Structure,Hydrogen Storage Properties And Degradation Mechanism Of Capacity Of La-Mg-Ni System AB₂ Type Alloys

La-Mg-Ni system AB2-type hydrogen storage alloys are considered to be the most promising materials in view of their high hydrogen storage capacity and excellent activation properties.The as-cast,annealed and as-spun Re-Mg-Ni-based AB2-type alloys were prepared by vacuum induction,vacuum annealing and melt spinning technology.The phase structure,composition and microstructure of the alloys are analyzed by X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electron microscope(TEM).The electrochemical capacity and cycle properties of the alloys are tested by the electrochemical testing system.The gaseous hydrogen storage capacity and cycle properties of the alloys are measured by the seviet testing equipment.The kinetic and thermodynamic properties of the alloys are analyzed.The mechanism of rapid degradation of the hydrogen storage capacity of the La-Mg-Ni system AB2-type alloy is studied and the solution to capacity recovery is proposed.The investigation results will provide a theoretical understanding and reliable experimental basis for studying the hydrogen storage properties of La-Mg-Ni system alloy and will be helpful guides to solve the degradation of cycle stability of La-Mg-Ni system AB2-type alloys.In this dissertation,the influences of Pr and Sm substitution on phase structure,phase composition and hydrogen storage properties were investigated.La1-xPrxMgNi3.6C0.4(x = 0-0.4)and La1-xSmxMgNi3.6Co0.4(x = 0-0.4)alloys consist of two phases of LaMgNi4 and LaNis.The substitution of Pr and Sm for La does not change the phase composition of the alloys,while the lattice constant a and the cell volume V of the LaMgNi4 decreased with increasing Pr content.With Pr and Sm substitution growing,the phase abundance of LaMgNi4 phase increases while the abundance of LaNi5 phase decreases.All the alloys electrodes exhibit the superior activation capability,reaching to the maximum discharge capacity at the first hydrogen absorption/desorption cycle,the maximum discharge capacity is 347 mAh/g(x = 0)and hydrogen absorption capacity isl.77 wt.%(x = 0).The discharge capacity and hydrogen absorption of the alloys decreases with Pr and Sm content increasing.With Pr and Sm substitution increasing,the pressure of the plateau of hydrogen absorption/desorption increases.The cyclic stability of the alloys gets improved with the increasing Pr and Sm content.The effects of annealing treatment and melt spinning on the phase structure and hydrogen storage properties were studied systemically.The single LaMgNi4 phase can be obtained by annealing treatment.The maximum discharge capacity of annealed La1-xPrxMgNi3.6Co0.4(x = 0-0.4)alloys is 354.2 mAh/g(x = 0.2)and the maximum hydrogen absorption capacity is 1.66 wt.%(x = 0.2).The pressure of hydrogen absorption/desorption plateau widen and flatten by annealing treatment.Melt spinning does not change the phase composition of the La1-xPrxMgNi3.6Co0.4(x = 0-0.4)alloys,and the alloys are composed of LaMgNi4 and LaNi5.The discharge capacity and hydrogen absorption capacity of the as-spun alloys are reduced as the lattice stress induced by melt spinning.With the speed of melt spinning increasing,the discharge capacity and cyclic stability of the as-spun alloys are enhanced.The kinetics characteristic and thermodynamics properties of the as-cast and as-annealed La1-xPrxMgNi3.6Co0.4(x = 0-0.4)were investigated.The high rate discharge properties(HRD)of the as-cast La1-xPrxMgNi3.6Co0.4(x = 0-0.4)alloys increase with the Pr content growing.The electrochemical kinetics of the as-cast alloys are co-affected by charge-transfer on the alloy surface and hydrogen diffusion in the alloy internal.The HRD of the as-annealed alloys increases with the Pr content growing when the discharge current density<900 mA/g,while the HRD first increases then decreases when the discharge current density>900 mA/g,which is highest at Pr content x = 0.2.The main factor affecting electrochemical kinetics of as-annealed alloy at higher discharge current density is hydrogen diffusion in alloy internal.The absolute values of enthalpy change AH and entropy change AS of the as-cast La1-xPrxMgNi3.6Co0.4(x = 0-0.4)alloys decrease with Pr content increasing,meaning that the stability of the metal hydride decrease,the plateau pressure of hydrogen absorption/desorption gets higher.With hydrogenation temperature increasing,the plateau pressure rised.The absolute value of enthalpy change AH and entropy change AS of the as-annealed La1-xPrxMgNi3.6C0.4(x = 0-0.4)alloys is minimum when Pr content x = 0.2.The smaller the value of enthalpy change ?H,the more stable the metal hydride hence lower plateau pressure,which is facilitate hydrogen absorption.The gaseous hydrogen absorption/desorption cycle capacity degradation mechanism of La1-xPrxMgNi3.6C0.4 alloys is explored.The phase structure and microstructure of the La1-xPrxMgNi3.6C0.4(x = 0-0.4)alloys after hydrogen absorption/desorption cycles were characterized by XRD,SEM and HRTEM.The main phase LaMgNi4 is amorphized to some extent after hydrogen absorption/desorption cycles.And the degree of amorphization increases with cycle number increasing.It is hydrogen-induced amorphization that leads the hydrogen absorption capacity rapid degradation.The structure and hydrogen absorption capacity can recovered at some extent by annealing treatment.