全钒氧化还原液流电池电极过程动力学特性及其对容量衰减的影响

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	全钒氧化还原液流电池电极过程动力学特性及其对容量衰减的影响
	陈富于
学位类型	博士
导师	严川伟
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	腐蚀科学与防护
关键词	钒氧化还原液流电池容量衰减动力学副反应杂质离子 Vanadium Redox Flow Battery Capacity Attenuation Kinetic Side Reaction Impurity Ions
摘要	"钒电池作为一种新型能量存储系统，具有可快速和大容量充放电、功率和容量可调等特性，是解决风能、太阳能存储的理想储能电源。钒电池正负极电解液为含VO2+/VO2+与V2+/V3+氧化还原电对的硫酸水溶液，其中不同价态钒离子是钒电池能量存储于转换的核心，硫酸不仅为导电介质，且参与正极电极反应。电池运行中，正负极活性物质分别参与电极反应，导电介质氢离子透过隔膜形成通路。因为电池正负极活性物质为离子形态，电极材料为惰性碳类材料，电极活性物质在充放电过程中没有消耗，电池的理论容量是不变的。但是，实验中发现随着充放电次数的增加，电池的容量逐次衰减。因此，研究钒电池运行中导致电池容量衰减的原因，对于提高活性物质利用率、延长电池寿命、推广电池应用至关重要。本文首先利用紫外可见分光光度法定性和定量分析电解液中各价态钒离子浓度，便于实时监测电池的荷电状态和电解液的稳定性。通过正负极动力学过程的研究，找到导致正负极容量失衡的原因。通过对负极析氢副反应的研究，获得负极析氢副反应发生的条件、对电极材料及钒离子反应的影响。这是导致正负极容量失衡的又一重要原因。电解液中杂质离子的存在可能导致电池容量的衰减，分析了电解液中几种主要杂质离对电池性能及容量衰减的影响。利用紫外可见分光光度法定性和定量分析电解液中不同价态钒离子浓度，不同价态钒离子标准曲线线性相关系数都大于0.9990，线性范围分别为0.326~ 2.445，0.326~ 2.445，0.720~ 5.403，1.784~ 13.437 g L-1，测定样品相对标准偏差(n=6)为0.594%~ 3.535%，满足钒电池电解液中钒离子价态分析的需要。利用电化学方法（循环伏安法、电化学交流阻抗法）研究了正负极反应历程、控制步骤及典型条件下的动力学参数。正极电极过程是准可逆过程，且VO2+离子在电子转移之前存在前置的化学转化过程。负极是只涉及一个电子转移的准可逆过程。阻抗的高频段存在电化学极化，低频段表现为扩散控制电极过程。通过动力学参数的计算表明，同样条件下，正极的反应速率常数较大，离子的扩散速度较快。导致电池多次循环后，正负极反应速度失衡，电池容量衰减。氢气在石墨电极上析出速率与极化电位、硫酸浓度及钒离子浓度有关。发生析氢反应后氢原子吸附在石墨表面增大石墨电子传递电阻且电阻值的增大与氢原子吸附量成正比。石墨表面含氧官能团C-OH、C-O-C与氢原子反应，导致含氧官能团数量减少和电极表面的腐蚀。含氧官能团数量减少抑制了V2+/V3+电对电化学活性。析氢副反应消耗电池电流，导致电池正负极容量失衡，容量衰减。考察电解液原材料中几种典型杂质离子对电极电化学活性、析氢过电位及电池库伦效率、电压效率和能量效率的影响。结果表明Na+对电池性能没有影响，不会加速电池容量衰减速度。Fe2+加入电解液后，电池容量提高。SiO32-降低了电极反应活性、减小析氢过电位，降低了电池电压效率和能量效率，但是增大了电池的容量，且减慢了容量衰减的速度。Ca2+加速电池容量衰减。Cu2+在电池负极发生不可逆反应，生成的Cu单质吸附在电极表面，降低了析氢过电位值，负极发生析氢反应的电位增大，电池的性能下降、容量衰减。"
其他摘要	"The vanadium battery, as a new type of energy storage system with large capacity, fast charge and discharge, is the ideal form to store the wind energy, solar energy. The cell employs the VO2+/VO2+ and V2+/V3+ redox couples in each positive and negative half-cell electrolytes, with sulfuric acid as supporting electrolyte. Besides, sulfuric acid also participates in the cathode electrode reaction. During the process of charge and discharge, the active materials react in each positive and negative half-cell, and the hydrogen ions go through the membrane to form a conductive passage. Because the materials are not consumed and carbon electrode does not participate in the reaction, theoretically, the battery capacity will not decay. But in fact, it attenuates gradually along with the cycles of charge and discharge. So, study on reason of capacity attenuation is important to improve the battery efficiency and prolong its cycle life. The main work of this paper is as follows: First, spectrophotometry analysis methods is used to qualitative and quantitative analysis different valence state of vanadium, so as to monitor the stage of charge. Second, kinetics of VO2+/VO2+ and V2+/V3+ redox couples is studied to obtain reasons of capacity attenuation. Third, hydrogen evolution reaction in the negative half-cell at a graphite electrode is investigated to find out the principle of hydrogen evolution. Finally, the influence of several main impurity ions is studied on which may cause capacity attenuation. The method of spectrophotometry analysis is successfully used to qualitative and quantitative analysis different valence states of vanadium. The results show that the related coefficients of four standard curves are more than 0.9990, linearity ranges are 0.326~ 2.445, 0.326~ 2.445, 0.720~ 5.403, 1.784~ 13.437g·L-1, respectively. The measurements of samples suggest that the method is suitable for analyzing concentration of vanadium ion with the RSDs (n=6) in the range of 0.594%~ 3.535%. The electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy) are used to study on the reaction mechanism of the positive and negative poles at a graphite electrode. The result suggests that electrode process of positive is quasi-reversible and there is a prepositive chemical process before electron transferring of VO2+. The electrochemical process of negative is also quasi-reversible. Characteristic of EIS plots with the capacitive semicircle in high frequency and the Warburg tail in low frequency are observed for the two couples, implying that both the electrochemical and diffusion process play important roles. The diffusion coefficient (D) and the rate constant (k) of VO2+/VO2+ and V2+/V3+ redox couples for varying state of charge (SOC) are obtained by the cyclic voltammetry. It suggests that diffusion coefficient (D) and rate constant (k) of VO2+/VO2+ couple are larger than that of V2+/V3+ couple, so, after repetitious cycles, reaction of the two couples loses balance which causes the capacity attenuation of battery. The hydrogen evolution reaction in the negative half-cell of a vanadium redox battery is studied at a graphite electrode. The result indicates that the rate of hydrogen evolution reaction depends on polarization potential, concentration of sulfuric acid and vanadium ion. The hydrogen evolution reaction makes adsorption hydrogen on graphite surface, with a trend toward an increase in electrical resistance and a decrease in the active surface area. It is shown that hydrogen evolution reaction changes surface morphology and introduces some defect sites on the surface of graphite. The electrochemical activity of graphite towards V3+/V2+ couple becomes less easily after hydrogen evolution reaction. The reaction consumes battery current and causes the imbalances of battery capacity which result in the capacity attenuation of battery. Electrochemical activety of electrode and performance of battery are investigated before and after impurity ions added in electrolyte. The results suggest that Na+ has no influence on efficiency of battery and the capacity of battery does not attenuate after repetitious cycles. The capacity of battery is increased by adding Fe2+ in electrolyte. The activity of the electrode, the overpotential of hydrogen reaction, the voltage efficiency and the energy efficiency are reduced while capacity of battery is increased by adding SiO32- in electrolyte. Capacity attenuation of battery is accelerated by adding Ca2+ in electrolyte. In the negative of half cell, Cu2+ reacts on the electrode and the reaction product Cu adsorbs on the electrode surface which reduces the overpotential of hydrogen evolution, depresses the battery performance and accelerates capacity attenuation."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64483
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	陈富于. 全钒氧化还原液流电池电极过程动力学特性及其对容量衰减的影响[D]. 北京. 中国科学院金属研究所,2012.