Microstructure And Electrochemical Properties Of V-Ti-Ni-Zr Based Hydrogen Storage Electrode Alloys

In the thesis, previous research and development of V-Ti-Ni based hydrogen storage electrode alloys with dual-phases have been reviewed. Based on this review, the microstructure and electrochemical performance of V_2.1TiNi_0.4Zr_0.06Mn_x(x=0～0.20) alloys and V_2.1TiNi_0.4Zr_0.06Cu_x(x=0～0.12) alloys were investigated by means of XRD, SEM, EDS analyses and electrochemical measurements. Whereafter, the influence of added elements (Cr, Co, Fe, Nb, Ta) on the phase structures and electrochemical properties were investigated for improving the overall performance of V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy.The study on the microstructure and electrochemical performance of V_2.1TiNi_0.4Zr_0.06Mn_x (x=0～0.20) alloys shows that all of these alloys consist of a V-based solid solution main phase with a bcc structure and a C14-type secondary phase in the form of three-dimensional network structure, and the secondary phase precipitates along the grain boundaries of the main phase. Moreover, the unit cell of both the main phase and secondary phase decrease with the increase of Mn content. The results show that as some Mn (x=0.05～0.10) added into V_2.1TiNi_0.4Zr_0.06 alloy, the maximum discharge capacity and high-rate dischargeability are improved significantly, and the activation behavior is invariable. However, the higher Mn content (x≥0.15) in the alloy improves the cycling stability, but decreases the maximum discharge capacity and high-rate discharge ability. Among the alloys studied, V_2.1TiNi_0.4Zr_0.06Mn_0.05 alloy has better overall electrochemical properties than others. This alloy is fully activated at the second cycle and reaches the highest discharge capacity of 462 mA·h·g^-1. Its high-rate dischargeability at the discharge current of 400 mA·g^-1 is 39.0%, and the capacity retention after 20 charging/discharging cycles is only 14.18%.The study on the microstructure and electrochemical properties of V_2.1TiNi_0.4Zr_0.06Cu_x (x=0⁰.12) alloys shows that all the alloys consist of a V-based solid solution main phase with a bcc structure and a C14-type secondary phase in the form of three-dimensional network structure, and the secondary phase precipitates along the grain boundaries of the main phase. Moreover, the unit cell of both the main phase and secondary phase expand with the increase of Cu content. The electrochemical measurements reveal that the results show that the maximum discharge capacity is improved and the activation behavior is invariable as some Cu (x=0.03～0.06) is added into the V_2.1TiNi_0.4Zr_0.06 alloy. However, the higher Cu content (x≥0.09) in the alloy impairs the discharge capacity. Furthermore, adding Cu into the V_2.1TiNi_0.4Zr_0.06 alloy can improve its cycling stability and high-rate dischargeability significantly. Among the alloys studied, V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy has better overall electrochemical properties than others. This alloy is fully activated at the second cycle and reaches the highest discharge capacity of 474.3 mA·h·g^-1. Its high-rate dischargeability at the discharge current of 400 mA·g^-1 is 60.8%, and the capacity retention after 20 charging/discharging cycles is only 29.18%.Based on the above work, in order to improve the cycling stability of V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy, the effects of Cr content on the microstructure and electrochemical properties of V_2.1TiNi_0.4Zr_0.06Cu_0.03Cr_x(x=0～0.15) alloys were investigated. It is found all of these alloys consist of a V-based solid solution main phase with a bcc structure and a C14-type secondary phase in the form of three-dimensional network, and the secondary phase precipitates along the grain boundaries of the main phase. Moreover, Cr predominantly exists in the main phase, and the unit cell of both the main phase and secondary phase decrease with the increase of Cr content. The electrochemical measurements reveal that the maximum discharge capacities of the Cr-added alloys are lower than V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy. However, adding Cr into the alloys can improve the capacity retention and high-rate dischargeability significantly. The results show that the alloy with x=0.10 has a good overall electrochemical properties. The study on the V_2.1TiNi_0.4Zr_0.06Cu_0.03M_0.10 (M=Cr, Co, Fe, Nb, Ta) alloys shows that each alloy has a V-based solid solution main phase with a bcc structure and a secondary phase with a three-dimensional network structure, where Cr, Nb or Ta predominantly exists in the main phase, and Co or Fe is mainly distributed in the secondary phase. The addition of Cr, Co, Fe, Nb or Ta leads to a lower maximum discharge capacities comparing with V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy. However, as a result of the effective restraint in the corrosion and dissolution of vanadium and titanium by adding Cr, Co, Fe, Nb or Ta into V_2.1TiNi_0.4Zr_0.06Cu_0.03 alloy, the cycling stability is improved. Moreover, Cr, Co, Nb or Ta enlarges the reaction rate of surface on the alloys and the high-rate dischargeability. The V_2.1TiNi_0.4Zr_0.04Cr_0.10 alloy obtains better overall electrochemical properties, namely a maximum discharge capacity of 412.8 mA·h·g^-1 at the 2nd cycle, a cycling stability S₃₀ of 67.73%, and a high-rate dischargeability of 85.49% at the discharge current of 400 mA·g(-1).