【作者】 李锐；

【导师】 潘洪革；

【作者基本信息】 浙江大学 ， 材料学， 2007， 博士

【摘要】 本文全面综述了钛钒基贮氢电极合金的研究进展以及各种贮氢合金电极衰退机理的研究进展。在此基础上，确定以钛钒基贮氢电极合金为研究对象，采用XRD／Rietveld、SEM、TEM和AES等材料分析方法以及恒电流充放电、电化学阻抗谱、线性极化、阳极极化和恒电位阶跃等电化学测试技术，对钛钒基合金的成分、相结构与电化学性能三者之间的关系进行了系统的研究，提出贮氢电极合金的“本征／非本征衰退机理”，并以钛钒基合金为实例对该机理进行了验证。对Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x（x=0～2.0）贮氢电极合金的结构及电化学性能进行了研究。结果表明：当x在0到1.50之间时，合金由C14型Laves相和钒基固溶体相两相组成。C14型Laves相的丰度随着Ni含量的增加而先增加后减少，在x=0.50时取得极大值（52.9、wt.％），钒基固溶体相的丰度则相应地先减少后增加。当x增加到1.75之后，合金中出现了少量的杂相。所有的合金中，C14型Laves相均形成连续的三维网络结构，而钒基固溶体相则以等轴晶或树枝晶的形态嵌入于C14型Laves相基体中。随着Ni含量的增加，钒基固溶体相逐渐由等轴晶向树枝晶过渡。此外，当合金中不含Ni元素时，由于缺乏必要的电催化活性，合金电极几乎没有电化学容量。随着合金中Ni元素含量的逐渐增加，合金电极的电催化活性得到了提高。在上述相结构及电催化活性两方面因素的综合作用下，合金电极的电化学性能随着Ni含量的增加而显著变化：最大放电容量先增大后减小，当x=0.75时达到最大值373.7 mAh／g；循环稳定性不断提高，x=2.00时，合金电极经200次循环后的容量保持率达96.2％；高倍率放电性能先大幅提高而后略有降低，适中的Ni元素含量有利于降低合金电极的表面反应阻抗和氢扩散阻抗，从而提高合金电极的高倍率放电性能。研究了Cr元素含量对Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_xNi_1.75（x=0.0～0.7）贮氢电极合金的结构及电化学性能的影响规律。所有合金均由C14型Laves相和钒基固溶体相组成，随着Cr含量的增加，合金中钒基固溶体相不断增加，而C14型Laves相则相应地逐渐减少。钒基固溶体相均以树枝晶的形态分布于C14型Laves相基体中。随着合金中Cr含量的增加，合金电极的最大放电容量和高倍率放电性能均先增加而后下降。而循环稳定性则随着合金中Cr含量的增加而得到大幅度的提高，180次循环后合金电极的容量保持率C₁₈₀／C_max由x=0时的25.4％提高到了x=0.7时的93.8％。对Mn替代Ni时Ti_0.8Zr_0.2V_2.7Mn_0.5+xCr_0.8Ni_1.5-x（x=0.0～0.4）贮氢电极合金的相结构及电化学性能进行了研究。结果表明：所有合金均由C14型Laves相和钒基固溶体相两相组成。随着Mn替代量的增加，合金中钒基固溶体相逐渐减少，而C14型Laves相的丰度则相应地逐渐增加。适中的Mn替代量可提高合金电极的放电容量，但同时会导致合金电极循环稳定性的降低。电化学阻抗谱、线性极化、阳极极化以及恒电位阶跃放电的研究结果表明，少量地用Mn替代Ni可提高合金电极表面的电化学反应速率和氢在合金中的扩散速率，从而改善合金电极的高倍率放电性能。选用Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x（x=0.75，1.25，1.75）三种成分的合金（分别记为样品A，B，C）作为研究对象。对其本征衰退行为进行了系统的分析。采用ICP对不同循环次数时合金电极系统中的电解液进行成分分析。结果表明，以V为代表的几种主要吸氢元素的溶出量在前数十次循环内迅速增加，而后渐趋饱和。对照各合金电极的循环寿命曲线得知，活性吸氢元素的溶出不是造成各合金电极循环寿命之间差异的原因，也不是造成合金电极容量持续衰退的决定性因素。对不同循环次数后的合金进行SEM形貌分析和表面AES分析。发现覆盖在样品A合金表面的是以Ti、Ni的氧化物为主体的氧化层；而样品C合金的表层则是Ni含量70％以上的富Ni层。对合金电极经不同循环次数后的XRD图谱进行分析，发现各个样品中均不同程度地出现了氢化物的衍射峰，不可逆氢化物的产生和不断增多，是造成电极放电容量下降的另一个重要原因。综合分析认为，活性吸氢物质的腐蚀溶出、形成氧化层、不可逆氢化物的产生等几种典型的本征衰退行为对Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x（x=0.75，1.25，1.75）合金的容量衰退有着较大的影响，但并非决定性因素。尤其是在循环的后期，合金电极的容量衰退另有原因。以Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_x（x=0.75，1.25，1.75）合金为研究对象，对贮氢电极合金的非本征衰退机理进行了分析探讨。发现颗粒尺寸对贮氢合金电极的最大放电容量和循环稳定性有着显著的影响，但对于不同成分的合金而言，其影响规律有所差异。吸放氢循环后的形貌观测显示，样品A合金会随着吸放氢循环的进行而严重粉化；而样品C合金多次循环后的粉化非常轻微。抗粉化性能的显著差异，是造成两种合金电极循环稳定性悬殊的另一个重要原因。对经120次充放电循环后的电极合金进行TEM分析，结果显示，样品A合金和样品C合金的表面腐蚀情况具有显著的不同，前者被一层疏松絮状的氧化物所包覆，而后者被一层形态致密的富Ni层所包覆。对不同循环次数后的合金电极进行电化学阻抗谱分析，发现经多次循环后，样品A的电化学反应阻抗显著大于样品C。这主要归因于两种合金表层性质的不同，样品C表面富Ni层的导电性和催化活性均要远远高于样品A表面的氧化物层。对Ti_0.8Zr_0.2V_2.7Mn_0.5Cr_0.8Ni_0.75合金电极不同循环次数时的放电曲线进行了分析，发现由非本征因素导致的电化学极化增大、活性物质利用率下降是该合金电极放电容量迅速衰退的关键原因。本征衰退主要是活性物质减少导致的衰退，非本征衰退是活性物质的利用率下降导致的衰退，综合两者的影响，可以得到对合金电极在循环过程中容量衰退的圆满解释，这就是我们提出的“贮氢电极合金本征／非本征衰退机理”。更多还原

【Abstract】 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.更多还原