Knowledge Management System Of Institute of process engineering,CAS
|Thesis Advisor||曹宏斌, 李玉平|
高效、节能、环保的脱盐技术是缓解全球水危机最有效的方法。与其他脱盐技术相比，电容脱盐（CDI）的能耗更低。CDI借助电极材料储存离子以达到降低盐度的目的。CDI中强化离子去除以提高脱盐容量是重要发展方向。学者们从负载阴、阳离子交换膜、材料改性、开发复合电极等方面展开研究。但分别存在成本高、脱盐提升率有限和材料制备程序复杂等缺点。本论文在不对材料进行改性和复合的基础上，从负载单种离子交换膜和使用两种不同正负极材料的角度出发，构建非对称结构来强化离子去除。主要研究内容和结论如下：（1）组装只添加阴离子交换膜的非对称膜电容脱盐装置（AMCDI-AEM），以聚2，6-二甲基-1，4-苯醚为原料合成一系列阴离子交换膜，探究阴离子交换膜的离子交换容量、膜电阻和含水率等性能对脱盐的影响机制。结果表明，阴离子交换膜对有膜辅助的电容脱盐至关重要。相比常规CDI，AMCDI-AEM强化了离子去除，使脱盐容量和电荷效率增大，电极的长期循环稳定性增强。当离子交换膜具有较高的离子交换容量、较低的膜电阻和含水率时，对脱盐最有利。同时使用阴、阳离子交换膜的对称膜电容脱盐装置相比非对称膜电容脱盐装置电荷效率提高（95.0 vs. 54.7%），但脱盐容量几乎不变（7.2 vs. 7.4 mg g−1)。（2）组装正极材料为铋（Bi）、负极材料为活性炭（AC）的非对称电容脱盐装置（CDI-Bi-AC），主要研究氯离子的去除效果和机理。结果表明，相同条件下非对称电容脱盐装置的氯离子去除容量是对称电容脱盐装置（CDI-AC-AC，正、负极材料均为活性炭）的2~3倍，同时可通过控制电压大小和加压时间控制出水的pH值，但装置的脱盐循环稳定性有待进一步提升。证实氯离子的去除是通过与Bi电极发生氧化反应生成氯氧化铋（BiOCl）实现的。（3）基于以上非对称电容脱盐装置优异的氯离子去除效果和铋电极在纯硫酸钠（Na2SO4）溶液中的电化学惰性，进一步研究CDI-Bi-AC非对称装置在NaCl和Na2SO4混合溶液中对氯离子的选择性去除效果。研究发现，虽然硫酸根离子的存在会限制铋对氯离子的去除率，但铋电极仍然对氯离子表现出选择性。氯离子与硫酸根离子的摩尔浓度比是影响铋电极选择性去除氯离子的关键因素。摩尔浓度比越大，选择系数越高。在摩尔比为8的混合溶液中，当电压为1.6和2.0 V时，选择系数最高可达4.5。由此，提出针对阴离子选择性去除的转化反应选择机理，即目标离子与电极材料发生转化反应而被去除，其他大部分离子仍留在溶液中。 （4）构建以活性炭为正极、插层材料（六氰铁酸铜）为负极的非对称流动电极对，将其应用于流动电极电容脱盐模块中，同时设计非对称流动电极电容脱盐系统（FCDI-AC-CuHCF）。该系统包含一个脱盐模块和一个浓缩模块，分别施加正向电压和反向电压同时产生脱盐液和浓缩液，可实现半连续化操作。研究结果显示，适当的电极液流速是FCDI-AC-CuHCF高效且稳定运行的基础。高电压对脱盐有利。当电压从1.2 V增大到2.8 V时，FCDI-CuHCF-AC的除盐速率从0.01 mg cm−2 min−1增大到0.11 mg cm−2 min−1，除盐率从11%提升至91%，同时电荷效率均保持在90%以上。与常规对称流动电极电容脱盐系统（FCDI-AC-AC，以活性炭为流动电极对）相比，非对称型的FCDI-AC-CuHCF系统在高电压下（2.0~2.8 V）有脱盐优势，具有更高的除盐速率、除盐率和电荷效率。;Efficient, energy-saving and environmentally friendly desalination technology is the most effective way to alleviate the global water crisis. Compared with other technologies, capacitive deionization (CDI) is more energy-efficient which relies on the storage of ions in electrode materials to reduce salinity. Enhancement of ion removal in CDI to improve desalination capacity is an important development direction. Many researches have been carried out from the aspects of loading anion and cation exchange membranes, modification of materials and development of composite electrodes. However, there are some shortcomings, such as high cost, limited enhancement rate of desalination and complex material preparation. Here, without modification of materials and development of composite electrodes, the asymmetrical configuration is constructed for enhancement of ion removal from the perspective of packing only one ion exchange membrane and utilizing two different kinds of electrode materials. The main research contents and conclusions are as follows:(1) An asymmetric membrane capacitive deionization device with anion exchange membrane packed alone (AMCDI-AEM) was constructed. A series of anion exchange membrane was lab-synthesized from poly (2, 6-dimethyl-1, 4-phenylene oxide). The effect of the main membrane properties, such as ion exchange capacity, water uptake and membrane resistance on the desalination performance of AMCDI-AEM was systematically discussed. The results indicate that anion exchange membrane is necessary in membrane-assisted CDI. Compared with the conventional CDI, AMCDI-AEM enhances ion removal, with both the salt adsorption capacity and charge efficiency increased and the long-time stability of the electrodes improved. An anion exchange membrane with high ion exchange capacity, low membrane resistance and low water uptake is beneficial for AMCDI-AEM. In comparison with the asymmetric membrane capacitive desalination device, the symmetric membrane capacitive device integrated with both anion and cation exchange membrane) has higher charge efficiency (95.0 vs. 54.7%), but the desalination capacity is almost unchanged (7.2 vs. 7.4 mg g−1).(2) The asymmetric capacitive desalination device (CDI-Bi-AC) with bismuth as the anode material and activated carbon as the cathode material is assembled. The removal performance and mechanism of chloride ions were mainly studied. It is found that under the same conditions, the chloride removal capacity of the asymmetric CDI-Bi-AC device is 2~3 times that of the symmetric capacitive desalination device (CDI-AC-AC, with activated carbon as both the anode and cathode materials. Moreover, the pH value of the treated solution can be controlled by changing the applied voltage and the charging time. However, the cycle stability of the asymmetric device needs to be further improved. It is confirmed that the removal of chloride ions is achieved by its oxidation reaction with Bi electrode, which is converted to bismuth oxychloride (BiOCl). (3) Based on the superiority of the asymmetric device for chloride ions removal and the electrochemical inertness of the Bi electrode in single sodium sulfate (Na2SO4) solution, CDI-Bi-AC was further used for selective removal of chloride ions from mixed NaCl and Na2SO4 solution. Results show that although the ability of bismuth electrode to capture chloride ions is suppressed by sulfate ions, bismuth electrode still shows selectivity for chloride ions. The mole ratio of chloride ions to sulfate ions is the key factor affecting selectivity for chloride ions. The selectivity coefficient increases when the mole ratio rises. The highest selectivity coefficient of 4.5 is achieved in mixed solution with the mole ratio of 8 at voltage of 1.6 and 2.0 V. Based on the results, conversion-reaction selective mechanism is proposed for selective removal of cations, that is, the target ions are removed by conversion reaction with electrode material, and most of other ions remain in solution.(4) An asymmetric flowable electrode pair configuration with AC as anode and intercalation material (copper hexacyanoferrate, CuHCF) as cathode was constructed and applied in flow-electrode capacitive desalination cell. An asymmetric flow-electrode capacitive desalination system (FCDI-AC-CuHCF) was also contrived, which contained a desalination cell and a concentration cell. A forward voltage and a reversed voltage were respectively applied and desalinated solution and concentrated solution were simultaneously obtained. And semi continuous operation was realized. Results show an appropriate flow rate of the electrode fluid is the basis of the efficient and stable FCDI-AC-CuHCF operation. High voltage favors desalination. Salt removal rate increases from 0.01 to 0.11 mg cm–2 min–1 and salt removal efficiency increases from 11 to 91% respectively when the voltage rises from 1.2 to 2.8 V, and the charge efficiency remains above 90%. Compared with the conventional symmetric system with activated carbon electrode pair (FCDI-AC-AC), the novel asymmetric FCDI-AC-CuHCF system shows superiority at high voltage (2.0~2.8 V) in terms of higher salt removal rate, salt removal efficiency and charge efficiency.
|常俊俊. 非对称型电容脱盐体系构建及强化离子去除研究[D]. 中国科学院大学,2020.|
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