An Investigation On The Structure, Properties And Degradation Mechanism Of The TiV-Based Hydrogen Storage Electrode Alloys

Based on a overall review of the research and development of the Ti-V-based hydrogen storage electrode alloys, and the degradation mechanism of the hydrogen storage alloy electrodes, the Ti-V-based alloys were selected as the study objects of this thesis. By means of XRD/Rietveld, SEM, TEM, AES analyses and 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 of the Ti-V-based hydrogen storage alloys was systematically studied. An "intrinsic/extrinsic degradation mechanism" of hydrogen storage electrode alloys was developed and validated.The structural and electrochemical properties of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x (x = 0 ～ 2.0) hydrogen storage alloys were systematically studied. When x varies between 0 and 1.50, the alloys mainly consist of a C14-type Laves phase with hexagonal structure and a V-based solid solution phase with b.c.c. structure. The abundance of the C14-type Laves phase increases first and then decreases with the increasing Ni content, reaches its maximum (52.9 wt.%) at x = 0.50, while the abundance of the V-based solid solution phase varies reversely. When x reaches 1.75, a little impurity phase appears in the alloy. In all the studied alloys, the C14-type Laves phase forms a continuous network structure, while the equiaxed or dendritic V-based phase was embedded in the C14-type Laves phase matrix. With the increase of Ni content, the V-based phase transforms from equiaxed to dendritic gradually. Furthermore, when there is no Ni element in the alloy, the alloy electrode shows little discharge capacity for its lack of electrocatalytic activity. With the increase of Ni content, the electrocatalytic activity increases gradually. As the result of the variations of both the phase structure and electrocatalytic activity, the electrochemical properties of the alloy electrodes vary markedly: The discharge capacity increases first and then decreases, reaches its maximum of 373.7 mAh/g at x = 0.75; The cyclic stability improves remarkably, when x = 2.00, the ratio of remaining capacity after 200 cycles is 96.2%; The high rate dischargeability (HRD) increases markedly first and then decreases slightly. It is found that a moderate content of Ni is favorable for decreasing both the reaction resistance at the surface of the electrodes and thediffusion resistance in the electrodes, and consequently improves the HRD of the alloy electrodes.The study of the influence of Cr content on the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_xNi_1.75 (x = 0.0 ～ 0.7) alloys shows that all the alloys consist of a C14-type Laves phase and a V-based solid solution phase. The abundance of the V-based phase increases continuously with the Cr content. The discharge capacity and the HRD both increase first and then decrease, while the cyclic stability improves remarkably with the Cr content, the ratio of remaining capacity after 180 cycles increases from 25.4% (x = 0) to 93.8%.The study of Mn substitution for Ni on the structural and electrochemical properties of the Ti_0.8Zr_0.2V_2.7Mn_0.5+xCr_0.8Ni_1.5-x (x = 0.0 ～ 0.4) alloys shows that all the alloys consist of a C14-type Laves phase and a V-based solid solution phase. With the increase of Mn content, the abundance of the V-based phase decreases continuously. Moreover, it is found that an moderate substitution of Mn for Ni can increase the discharge capacity of the alloy electrodes, but decrease the cyclic stability at the same time. The results of the EIS, linear polarization, anodic polarization and potentialstatic discharge tests reveal that a little substitution of Mn for Ni can improve both the electrochemical reaction rate and the hydrogen diffusion rate of the alloy electrodes, and then leads to the increase of the HRD of the electrodes.The Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x (x = 0.75, 1.25, 1.75) alloys (noted as sample A, B, C) were selected and their intrinsic degradation behaviors were systematically studied. The results of ICP analysis on the electrolyte indicate that the dissolution of hydrogen absorbing elements increases rapidly in the first tens of cycles and then gradually becomes saturated. Compared to the cycling curves of the alloy electrodes, it can be concluded that the dissolution of hydrogen absorbing elements is responsible to neither the difference of the alloy electrodes' cyclic stability, nor the continuous degradation of alloy electrodes. SEM and AES analyses on the alloys underwent tens of electrochemical cycles indicate that on the surface of sample A alloy there's an oxidation layer consist mainly of Ti and Ni oxides, while on the surface of sample C there's a Ni-rich layer with a Ni content above 70%. XRD analysis on the alloy electrodes underwent tens of cycles shows that diffraction peaks of metal hydride appear in all the sample on different degrees. The formation and continuous increase of irreversible metal hydrides is another reason of the declination of discharge capacities of alloy electrodes. Synthetically analyzing the above results, it is suggested that intrinsic degradation behaviors, such as the dissolution of active hydrogen absorbing elements, the formation of the oxidation layer and the irreversible hydrides are important, but not decisive factors for thedegradation of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x (x = 0.75, 1.25, 1.75) alloys. Especially for the later period of the cycling, there're other reasons responsible for the continuous degradation. We call them extrinsic degradation behaviors.The extrinsic degradation behaviors of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x (x = 0.75, 1.25, 1.75) alloys were systematically studied. It was found that the particle size of the alloys has remarkable effects on the discharge capacity and cyclic stability of the hydrogen storage alloy electrodes. And the effects differ from the alloy composition. Observation on the alloys underwent hydrogen absorbing-desorbing cycles indicate that the sample A alloy pulverize seriously after hydriding-dehydriding cycles, while the pulverization of the sample B is quite slight. The difference on the anti-pulverization ability is another important reason for the large disparity of the alloys' cyclic stability. TEM analysis found that after 120 cycles the sample A alloy is covered by a layer of loose flocculent oxides, while the sample B alloy is covered by a layer of dense Ni-rich compounds. EIS analyses show that after certain cycles the electrochemical reaction resistance of the sample C is markedly smaller than that of the sample A. This can be attributed to the much better conductivity and electrocatalytic activity of the Ni-rich layer of the sample C than those of the oxidation layer of the sample A. Analyses of the discharge curves of the Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_0.75 alloy indicate that the increase of electrochemical polarization and the decrease of the utility ratio of active materials caused by extrinsic factors are the key reasons for the rapid declination of the discharge capacity. Intrinsic degradation is the degradation caused by the lost of active materials, while extrinsic degradation is the degradation caused by the utility ratio decrease of active materials, an integration of the both can give a satisfactory explanation for the discharge capacity declination of the electrode alloys. This is our "intrinsic/extrinsic degradation mechanism" of hydrogen storage electrode alloys.