Study On Structure And Electrochemical Performance Of Ti-Zr-V-Mn-Ni Solid Solution Hydrogen Storage Alloy

In order to compete favorably with other advanced secondary batteries, the overall properties of hydrogen storage alloys used as negative materials in Ni/MH batteries should be substantially improved. The research and development of novel hydrogen storage alloys with high capacity, high rate dischargeability and high temperature dischargeability have obvious significance in both theoretical and practical application aspects.In this work, B, Mo, Cr, Si and rare earth elements La, Ce, Pr, Nd and Gd have been used as additive element to Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloy to improve the electrocatalytic activity and the kinetic performance of the alloy electrode. The microstructure and electrochemical properties of the alloy have been investigated by XRD, FESEM-EDS, TEM, EIS, potentiostatic discharge technique and ICP-MS measurements.The results show that all alloys are composed of V-based solid solution with body-centered-cube (BCC) structure and C14 Laves phase with hexagonal structure. The addition of B and Mo increases the discharge capacity of Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloy electrode and Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33B0.1 alloy delivers the discharge capacity of 476.7 mAh·g-1at discharge current of 60 mA·g-1. It is found that a small addition of Si, Mo and substitution of Cr for V greatly improve the cycle life of Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloy electrode. But addition of Si,Mo,B or substitution of Cr for V decreases the high-rate discharge ability of the alloy electrode in some extent. EIS indicates that addition of Si, Mo and B or the substitution of Cr for V decreases the dynamic performance, which makes the charge transfer resistance (Rct) increases and the exchange current density (I0) decreases markedly. Considerable improvement in the high-temperature dischargeability is observed by addition of B and Mo, and the discharge capacity is up to 633 mAh·g-1 at 343 K for Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33Mo0.075 alloy electrode.The effects of five kinds of rare earth element addition on microstructure and electrochemical properties of Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloys are studied in this paper. It is found that these alloys all consist of V-based solid solution with bcc structure as main phase and C14 Laves phase with hexagonal structure. The cell volumes of two phases are swelled after rare earth elements are added to the alloy. In the alloy, a new phase is found, which comprises of Ce and some other elements. Various kinds of rare earth elements can improve the activation characteristics of the electrodes alloy. However, the effect on the other performance of electrode alloy is different each other with the different rare earth elements addition. Cerium and praseodymium addition can enhance the maximum discharge capacity of electrode alloy, and neodymium and gadolinium addition improve the cycle stability of the electrode alloy. The discharge capacity of the alloys is quite sensitive to temperature, and the excessively high temperature makes the discharge capacity of the electrode alloys to degrade. The discharge capacity of the electrode alloys which contains of rare earth element is up to maximum at 333 K. The rare earth element has certain influence on the charge retention of the electrode alloy, and the addition with rare earth elements La, Ce, Pr can improve the high-rate dischargeability of the electrode alloy, respectively.Effects of rare earth elements Ce, Nd, Gd partial substitution for V on the microstructure and electrochemical properties of Ti0.26Zr0.07V0.23Mn0.1Ni0.33RE0.01 (RE = Ce, Nd, Gd) hydrogen storage alloy have been investigated. The partial substitution of Nd for V is beneficial for Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloy to improve the kinetic performance, which makes the charge transfer resistance (Rct) decreases and the exchange current density (I0) and the hydrogen diffusion coefficient (D) increase markedly. The partial substitution of rare earth element Ce for V can improve the discharge capacity of the electrode alloy, and its maximum discharge capacity can reach 403.9 mAh·g-1 with discharge current density of 60 mA·g-1.In present study, high-energy ball milling (HEBM) is used to improve the cyclic stability and other electrochemical properties of Ti0.26Zr0.07Mn0.1Ni0.33V0.24 alloy. The effect of the milling time on crystallographic and electrochemical characteristics of the alloy has been investigated systematically. Electrochemical studies show that the cyclic stability of the ball-milled alloys is noticeably improved. The capacity retention rate C40/Cmax after 40 cycles increases from 46.3% (t=30 min) to 78.3% (t=180 min), although the maximum discharge capacity of the ball-milled alloys decreases moderately. Both electrochemical impedance spectra and Potentiostatic discharge studies indicate that the electrochemical kinetics of the ball-milled alloys is improved with increasing the ball-milling time.The performances degradation of Ti_0.26Zr_0.07V_0.24Mn_0.1Ni_0.33 alloy electrode has been investigated. The decrease of hydrogen diffusion coefficient (D), the increase of charge transfer resistances (Rct), and the dissolution of V and Zr elements to KOH solution with charge/discharge cycling would be responsible for the performances degradation of the alloy electrode.