Structure And Electrochemical Property Of Composite Electrode Alloy In Ni/MH Battery

A single-sort of hydrogen storage alloy has inherent advantages and disadvantages, and it is very difficult to meet the request of performance and expanding applied field. In order to make their advantages complement each other and overcome their disadvantages, it is optional that a second phase with good electrocatalytic properties can be added into a certain matrix alloy, and also it is possible to bring synergetic effect within better characteristics of the constituent alloys, and therefore it is our aim to prepare composite electrode alloy with excellent comprehensive performances in Ni/MH battery.In this paper, composite electrode alloy with multi-phase is prepared by two-step re-melting. Microstructure, thermodynamic characteristics, electrochemical properties and kinetic characteristics of the composite electrode alloys have been investigated systematically by means of XRD, FESEM (SEM), EDS, ICP-OES analyses and electrochemical test methods such as galvanostatic charge-discharge, potentialstatic discharge and EIS etc., and the significant results have been achieved.Effect of additional alloy including AB5 type alloy of LaNi5, LaNi₄Al_0.4Mn_0.3Co_0.3 and La-Mg-based alloy of La_0.85Mg_0.25Ni_4.5Co_0.35Al_0.15 on the structure and electrochemical characteristics of Ti0.10Zr0.15V0.35Cr0.10Ni0.30 solid solution alloy has been investigated, respectively. The results show that though the main phase of the matrix alloy remains unchanged after a small quantity of the additive alloys is added, a unknown phase differing from constituent alloys is formed in the composite electrode alloys, and it is mainly composed of Zr, V, Ti and Ni. The amount of the unknown phase increases with rising addition. The additive alloys are decomposed during composite process, and La can be detected only in the new specimen. The real discharge capacity of the composite alloy electrodes is not equal to the sum of all the discharge capacity of the constituent alloys, and synergetic effect appears distinctly during the composite process. With increasing the additive, the thermodynamic characteristics, the electrochemical performances and the kinetic properties of the composite electrode alloy are significantly improved. At 303 K, with increasing LaNi5 additive, the electrochemical P-C-T plateau pressure of the composite alloy electrode is elevated, and the hysteresis effect decreases dramatically, when La_0.85Mg_0.25Ni_4.5Co_0.35Al_0.15 alloy is added, the maximum discharge capacity of the composite alloy electrode is 361.8 mAh g^-1 at additive amount of 5 %,and the activation number, the high rate dischargeability at discharge current density of 1500 mA g^-1, the exchange current density and the hydrogen diffusion coefficient are 3 cycles, 75.96 %,452.64 mA g^-1 and 26.29×10-10 cm2 s-1 at additive amount of 20 %, respectively. The low temperature discharge ability is 73.47 % at 233 K. The high rate dischargeability of all composite alloy electrodes is essentially controlled by both the charge-transfer reaction of hydrogen at the electrode/electrolyte interface and the hydrogen diffusion in the bulk of the alloy at discharge current density of 1500 mA g^-1.The structure and electrochemical characteristics of TiV_1.1Mn_0.9Ni_0.5 + x % ZrCr2 (x = 0, 1, 5, 10, 20) alloys have been studied systematically. The results show that though the composite alloys is of two-phase structure which is the same as that of the TiV_1.1Mn_0.9Ni_0.5 matrix alloy under the present experimental condition, the composition, the content, the lattice parameters and the cell volumes of the two phases in the composite electrode alloy change with increasing the additive. The plateau pressure of hydrogen desorption first rises and then falls, and it gets its minimum at x = 5, and both the maximum hydrogen absorption capacity [H/M]max and the maximum discharge capacity first increases and then decreases, and they get their maximum of 1.92 mass% and 457.2 mAh g^-1, respectively. Synergetic effect appears distinctly during the composite process when a small amount of ZrCr2 alloy is added. The cyclic durability and the charge retention of the alloy electrode increase notably with increasing x. The maximum discharge capacity of the composite alloy electrodes increases with increasing temperature. The dynamic characteristics such as the high rate dischargeability, the exchange current density and the hydrogen diffusion coefficient et al. are improved evidently with increasing x, and the optimum composition is found to lie in x = 5, The HRD is controlled by both the charge-transfer reaction of hydrogen at the electrode/electrolyte interface and the hydrogen diffusion in the bulk of the alloy at discharge current density of 1000 mA g^-1.The structure and electrochemical characteristics of LaNi5 + x % Ti_0.10Zr_0.16V_0.34Cr_0.10Ni_0.30 (x = 0, 1, 5, 10) alloys have been systematically investigated. The reasults indicate that the matrix phase of LaNi5 alloy with CaCu5 structure remains unchanged after Ti_0.10Zr_0.16V_0.34Cr_0.10Ni_0.30 alloy is added, the second phase of the C14 Laves phase with hexagonal structure is formed in the composite electrode alloys. The amount of the second phase increases with increasing x. The synergetic effect within constituent alloys appears during the composite process. With increasing x, The plateau pressure of hydrogen desorption gradually increases, and the maximum hydrogen absorption capacity [H/M]max first increases and then decreases, and it gets 1.54 mass% at x = 5. The width of the pressure plateau changes in the same trend, and the slope factor changes in the opposite trend simultaneously. The electrochemical characteristics of the composite alloy electrodes are greatly improved, and the optimum composition is found to lie in x = 5, at which the low temperature dischargeability at 233K is 87.37 %, and the maximum discharge capacity, the high rate dischargeability at discharge current density of 1800 mA g^-1, the exchange current density and the hydrogen diffusion coefficient of the composite alloy electrode are 326.1 mAh g^-1, 71.98 %, 174.99 mA g^-1 and 8.544×10-10 cm2 s-1 at 303 K, respectively. The HRD is essentially controlled by the charge-transfer reaction of hydrogen on the electrode/electrolyte interface at discharge current density of 1800 mA g^-1.The study of the cycling degradation mechanism of TiV_1.1Mn_0.9Ni_0.5 + 5 % ZrCr2 composite alloy electrode shows that the degradation of the discharge capacity is mainly caused by the continuous corrosion and the dissolution of the main constituent elements, and the pulverization and the oxidation of the alloys during cycling in KOH alkaline electrolyte.