A Study On The Novel V-Ti-Ni-Zr Based Dual-phase 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. On this basis, the microstructure and electrochemical performance of V_2.1TiNi_0.4Zr_x (x=0～0.08) alloys were investigated systematically by means of XRD, SEM, EDS, ICP analyses and electrochemical measurements. Furthermore, The mechanism for cycling capacity degradation of such alloys was analyzed by means of XRD-Rietveld analysis, XPS and AES investigations. Whereafter, the influence of added elements (Co, Cr, Cu, Ta, Nb) on the phase structure and electrochemical properties were investigated for improving the overall electrochemical performance of V_2.1TiNi_0.4Zr_0.06 alloy.The study on the microstructure and electrochemical properties of V_2.1TiNi_0.4Zr_x(x= 0～0.08) alloys shows that all alloys consist of a V-based solid solution main phase with a bcc structure and a secondary phase with a three-dimensional network structure, and the secondary phase precipitates along the grain boundaries of the main phase. For the alloy with x≤<0.02, the secondary phase is the TiNi-based phase. However, the secondary phase in the alloy changes into the C14-type Laves phase with an hcp structure as x>0.02. Moreover, both the unit cell of the main phase and secondary phase expand with the increase of Zr content. The electrochemical measurements reveal that the activation behavior and the maximum discharge capacities except for x=0.02 of the Zr-added alloys are better than those of the V_2.1TiNi_0.4 alloy. As Zr content increases, the high-rate dischargeability is improved significantly, but the cycling stability is degraded gradually. Among the alloys studied, V_2.1TiNi_0.4Zr_0.06 alloy has better overall electrochemical properties than others. This alloy is fully activated at the second cycle and reaches the highest discharge capacity of 468.5mAh/g. Its high-rate dischargeability at the discharge current of 300mA/g is 70.13%. However, its capacity retention after 30 charging/discharging cycles is only 22.24%.Based on the above work, the mechanism for cycling capacity degradation of V_2.1TiNi_0.4Zr_0.06 alloy was investigated by analyzing the relation between its microstructure and electrochemical performance after a certain cycles. It is found that the corrosion and dissolution of vanadium, which leads to the destruction of the secondary phase and the oxidation on the surface of alloy, are induced during the initial five charge/discharge cycles. When the cycling goes on, a little amount of NiO is generated onthe surface, and titanium element begins to dissolve into the KOH electrolyte. The corrosion and dissolution of large vanadium in the subsurface layer of alloy generates a thick oxide layer (about 2μm) which slower the dissolution of vanadium and titanium. The result brings the deterioration of the maximum discharge capacity with a slower reaction rate. Nevertheless, the thick oxide layer degrades electrochemical reaction rate of the electrode surface and namely heightening electrochemical reaction resistance. After 30 cycles, the C14 Laves phase disappears gradually, however, V₂H phase with tetragonal or monoclinic lattice, VH₂ phase with cubic lattice and a TiO₂ new phase appear. The oxide products on the surface of the alloy are composed of V₂O₅, TiO₂ and Ni (OH) 2. Therefore, the stability of the dual-phase structure becomes increasingly bad due to the corrosion and dissolution of vanadium and titanium, especially the C14 Laves secondary phase in possession of electrocatalytic activity gradually disappears, and the oxide layer becomes increasingly thick. Both two factors chiefly induce the deterioration of the discharge capacity.In order to improve the cycling stability of V_2.1TiNi_0.4Zr_0.06 alloy, Co, Cr, Cu, Ta and Nb were adopted as an alloying element. The study on the V_2.1TiNi_0.4Zr_0.06M_0.152(M=Co, Cr, Cu) 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 predominantly exists in the main phase, and Co or Cu is mainly distributed in the secondary phase. The addition of Co, Cr or Cu leads to a unit cell contraction of both main and secondary phase, a difficult activation and a lower maximum discharge capacities comparing with V2.1TiNio.4Zro.06 alloy. However, as a result of the effective restraint in the corrosion and dissolution of vanadium and titanium by adding Co, Cr or Cu into V_2.1TiNi_0.4Zr_0.06 alloy, the cycling stability is improved. Moreover, Cr enlarges the reaction rate of surface and the high-rate dischargeability. The V_2.1TiNi_0.4Zr_0.04Cr_0.152 alloy obtains better overall electrochemical properties, especially in the cycling stability, namely a maximum discharge capacity of 397.13mAh/g at the fourth cycle, a higher cycling stability S₃₀ of 77.96%, and a high-rate dischargeability HRD₄₀₀ of 62.80% at the discharge current of 400mA/g. The investigation on V_2.1TiNi_0.4Zr_0.06M_0.037(M=Ta,Nb) alloys shows that all alloys possess the similar three-dimensional network structure which is formed by a V-based solid solution main phase and a C14 type Laves secondary phase,and Ta and Nb are distributed predominantly into the main phase. After adding Ta and Nb, the unit cell of the main phase expands and that of secondary phase contract. The Ta-contained or Nb-contained alloy restricts the dissolution of vanadium and titanium into the KOH electrolyte, thus the cycling stability is improved, but the maximum discharge capacity decreases without change of activation behavior. Nb and Ta also enhance the high-rate dischargeability. Among the alloys studied above, V2.1TiNio.4Zro.06Tao.037 alloy shows a maximum discharge capacity of 411.74mAh/g at the second cycle, a higher capacity retention S₃₀ of 54.83%, and a high-rate dischargeability HRD₄₀₀ of 52.36% at the discharge current of 400mA/g.