Structure And Electrochemical Properties Of Low Co Content La-Mg-Ni-based Hydrogen Storage Electrode Alloys

The thesis aims to develop La-Mg-Ni-based hydrogen storage alloys with low price and high electrochemical properties.A series of low Co and no Co content La-Mg-Ni-based hydrogen storage alloys were studied and element Al was introduced and annealing treatment was adopted on the purpose of getting high cycle stability.Combing the using of XRD,SEM,and XPS,the influence of the composition as well as the heat treatment on the structure and electrochemical properties of the alloys was studied.The effects of Al substitution for Ni on the phase structure and electrochemical properties of La_0.7Mg_0.3Ni_3.5-xAl_x(x=0-0.4) hydrogen storage electrode alloys were firstly studied.It is found that all alloys are composed mainly of the(La,Mg)Ni₃ phase and LaNi₅ phase.The substitution of Al results in an increase of the lattice parameters and the appearance of minor amount of AlNi₃ phase and MgNi₂ phase.Al addtion enhances the cycle stability of the alloy electrodes obviously.The capacity retention after 100 cycles(S₁₀₀) is improved from 26.3%(x=0) to 55.5%(x=0.4), while the maximum discharge capacity and the electrochemical kinetics decrease.The correlative effects of Al and Co on the cycle stability of a La-Mg-Ni-based hydrogen storage electrode alloy were then investigated.The study on the correlative impacts among component,structure and electrochemical properties of La_0.7Mg_0.3Ni₂₀₇₅Co_0.75-xAl_x(x=0-0.4) showed that all of the alloys mainly consist of a LaNi₅ phase and a(La,Mg)Ni₃ phase.The(La,Mg)Ni₃ phase decreases while LaNi₅ phase increases,and the cell volumes of the both phases are enlarged with rising substitution of Al.With the increase of substitution of Al for Co,the maximum discharge capacity decreases,but the capacity retention of the alloys is improved significantly and the values of S₁₀₀ are enhanced from 46%for x=0 to 82%for x= 0.4.The Al addition fortifies the pulverization and corrosion resistance of the electrode alloys,which contribute to the improvement of the cycle stability.By substitution Ni for Co to further reduce the Co content,the further study on the content,structure and electrochemical properties of La_0.7Mg_0.3Ni_2.75+xCo_0.45-xAl_0.3(x= 0-0.4) alloys was carried on.It is found that the phase composition of all alloys mainly compose of the LaNi₅ phase and(La,Mg)Ni₃ phase.With the increase of x, the maximum discharge capacity of the alloys is increased and the electrochemical kinetics is also promoted.However,the cycle stability of alloy electrodes is slightly decreased.By the annealing treatment of 1173K×8h,both of the maximum discharge capacity and cycle stability of alloy electrodes are improved.