An Investigation On The Phase Structure And Electrochemical Properties Of The La-Mg-Ni-Co-based Hydrogen Storage Electrode Alloys

Based on the review of the research and development of the non-AB5 type rare earth-based hydrogen storage alloys, the La-Mg-Ni-Co-based hydrogen storage electrode alloys was selected as the study object of this work. By means of XRD (Rietveld), SEM, PCT, XPS and AES analyses, and the electrochemical test methods including the galvanostatic charge-discharge, EIS, linear polarization, anodic polarization and potentialstatic discharge etc., the relationship among compositions, phase structure and electrochemical properties was systemically studied for developing the new type rare earth-based hydrogen storage electrode alloys with high discharge capacity and long cycling life. The study includes the followings: the effect of non-stoichiometry on the structure, the hydrogen storage and electrochemical properties of La0.7Mg0.3(Ni0.85Co0.15)x (x=2.5-5.0) alloys, the structure change of La0.7Mg0.3(Ni0.85Co0.15)3.5 alloy during charge/discharge process, the influence of Mn substitution for Ni on the structure, the hydrogen storage and electrochemical properties of La0.7Mg0.3Ni2.975-xCo0.525Mnx (x=0~0.5) alloys, the effect of Co content on the structure, the hydrogen storage and electrochemical properties of La0.7Mg0.3Ni3.4-xCoxMn0.1 (x=0~1.6) alloys, the cycling capacity degradation mechanism of La0.7Mg0.3Ni3.4-xCoxMn0.1 (x=0, 0.75, 1.3) alloys and the influence of Al substitution for Ni on the structure, the hydrogen storage and electrochemical properties of La0.7Mg0.3Ni2.65-xCo0.75Mn0.1Alx (x=0~0.5) alloys.For the La0.7Mg0.3(Ni0.85Co0.15)x (x=2.5-5.0) alloys, as their stoichiometry or x changes, their phase structure, hydrogen storage and electrochemical properties vary accordingly. It is found that all the alloys mainly consist of the (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and the LaNi5 phase with the hexagonal CaCu5-type structure, and the metallic Mg existing only in the (La,Mg)Ni3 phase (occupying 6c site of PuNi3 structure) by Rietveld analyses. With increasing x, the lattice parameters of two main phases vary in a complex manner, but their cell volumes decrease monotonously. Moreover, the relative phase abundance of the (La,Mg)Ni3 phase and the LaNis phase varies and accordingly makes the hydrogen storage capacity to increase first from 0.86 wt.% (x=2.5) to 1.50 wt.% (x=3.5), and then to decrease to 1.19 wt.% (x=5.0). In the range of x=2.5~3.5, the plateau pressure remains almost constant, but the pressure hysteresis gradually increases. In the range of x=4.0~5.0, the plateau pressure increases and the pressure hysteresis decreases. However, the plateau slope monotonously decreases with increasing x, and the plateau region increases. To summarize the results of the investigation, as x increases, the overall electrochemical properties of the alloys are markedly improved, and the optimum composition is found to be x=3.5, at which the maximum discharge capacity, the activation cycles and the HRD at the discharge current density 1000 mA/g (HRD1000) are 359.6 mAh/g, 2 and 85.8%, respectively. However, the capacity retention after 60 charge/discharge cycles (S60) reduces to only 45.9%, so the cycling stability should be further improved.The study of the crystal structure of the La0.7Mg0.3(Ni0.85Co0. 15)3.5 alloy during charge/discharge (absorption/desorption hydrogen) process revealed that during initial charge process, the LaNi5 phase absorbs hydrogen preferentially and then the hydrogen atoms diffuse subsequently from the LaNis phase into the (La,Mg)Ni3 phase, so that, in the present case, the LaNis phase is not only a hydrogen absorption phasebut also a catalyst for activating the (La,Mg)Ni3 phase to absorb a large amount of hydrogen from the alkaline electrolyte. However, during discharge process, the hydrogen is desorbing from the LaNis phase and the (La,Mg)Ni3 phase almost simultaneously. For the (La,Mg)Ni3 phase and LaNi5 phase, the following phase transitions are found, e.g., a phase a+ phases p phase during charge/discharge process, and the cell volume expansion during phase transformation of h...