【作者】 王素琴；

【导师】 杨占红；

【作者基本信息】 中南大学 ， 应用化学， 2011， 博士

【摘要】 高铁酸盐(Fe(Ⅵ))是铁的高氧化态化合物,具有较高的氧化还原电位、较大的电化学理论容量、合成原料来源丰富,放电产物Fe2O3·nH2O对环境无污染,且可以用做废水处理的絮凝剂。高铁酸盐可以用作废水和生活用水的处理剂及有机合成反应中的氧化剂,还可以用作高铁电池中的正极材料。高铁酸盐(Fe(Ⅵ))的主要制备方法有三种：电化学氧化法、干化学氧化法和湿化学氧化法。湿化学氧化法在目前被认为是最切实可行的方法,具有合成工艺成熟、成本低、产品纯度较高等特点,但是目前的湿化学氧化法仍具有产率相对较低、操作繁琐、反应时间长、产品容量低等缺点。目前研究的高铁酸盐中K2FeO4最容易被合成,且通常作为合成其他高铁酸盐的中间体。且高铁酸盐均不稳定,容易分解,其中最稳定的高铁酸盐也是K2Fe04(固态时在干燥密封的条件下保存,每年分解0.1%)。由于K2FeO4的这些优点,使其受到更多关注。但是K2Fe04在水溶液介质特别是酸性介质中不稳定,在饱和KOH溶液中比较稳定,但是也会逐渐分解成Fe2O3·nH2O,从而降低其放电性能。针对合成K2Fe04过程出现的问题以及K2FeO4的不稳定性,本文作了如下研究：(1)概述了高铁酸盐的合成方法、纯度分析方法、结构、性质及其应用,并提出了本论文的研究目的和意义。(2)提出了超声波辅助合成K2FeO4的方法,这种方法具有产率较高(53-59%)、操作简单、反应时间短、产品容量高等优点。本文研究了影响制备K2FeO4纯度的因素(吸收氯气的温度、反应物的量、反应时间、分离和纯化工艺等)。通过铬法测定,合成出的产品纯度为95-96.8%。X-射线衍射(XRD)研究表明合成出的K2FeO4为四面体结构,属于D2h (Pnma)空间群。扫描电子显微镜(SEM)显示合成出的K2FeO4晶体为多面体柱状结构,约25-200μm长,1-10μm宽。红外光谱表明K2FeO4是以Fe原子为中心,与四个等价对称的O原子连接构成正四面体的结构。(3)研究了高铁酸钾电池中电极的组成、电解液浓度及放电倍率对其放电性能的影响。并研究了高铁酸钾电极的循环伏安性能。结果显示,高铁酸钾与乙炔黑按照80：20的质量比混合制备的电极,在10mol·L-1 KOH溶液中,以0.25C的放电倍率放电性能最好。且高铁酸钾的纯度越高,放电性能也越好。本工艺合成出的高铁酸钾电池的容量均高于以前文献报道的容量。(4)为了增加高铁酸钾的稳定性,我们介绍了用有机化合物2,3-萘酞菁作为高铁酸钾的保护膜包覆在高铁酸钾的表面,包覆后的材料通过扫描电子显微镜(SEM)、傅里叶红外转换光谱(FT-IR)、X-射线光电子能谱(XPS)表征。本章还研究了不同比例2,3-萘酞菁包覆K2Fe04后浸泡在电解液中不同时间的电化学性能。恒流放电测试显不K2FeO4的放电容量随浸泡时间的增加而降低,浸泡于10 mol.L-1KOH溶液中3 h,以0.25C的倍率放电到截止电压0.8 V,2,3-萘酞菁的包覆含量增加,K2Fe04的放电容量增大。但是浸泡时间为6 h后,2%的2,3-萘酞菁对于改善K2FeO4的容量更好。2,3-萘酞菁包覆的高铁酸钾浸泡于碱液中3-12 h,其容量大约可以增加26%-50%。开路电位曲线显示K2Fe04的稳定性随着浸泡时间的增加而减弱,随着2,3-萘酞菁含量的增加而增强。通过电化学阻抗谱研究了2,3-萘酞菁改善K2FeO4的稳定性的原因：2,3-萘酞菁包覆在高铁酸钾表面,对高铁酸钾电极起到保护膜的作用,在短时间的浸泡过程中,减慢了高铁酸钾的分解；2,3-萘酞菁在高铁酸钾电极中还增强了电极中电子在界面之间的转移能力。(5)我们选择了与之结构类似的有机化合物酞菁作为高铁酸钾的保护膜。不同比例的酞菁包覆后的K2Fe04通过扫描电子显微镜(SEM)研究,发现K2Fe04晶体的表面也被酞菁部分包覆住,且酞菁的包覆含量越高,包覆的面积越大也越厚。傅里叶红外转换光谱(FT-IR)也显示酞菁包覆在了K2Fe04的上面。本章还研究了不同比例酞菁包覆K2FeO4后浸泡在电解液中不同时间的电化学性能。恒流放电测试显示浸泡在电解液中相同时间后,酞菁包覆的高铁酸钾电池阴极容量比未包覆高铁酸钾的高得多。通过用很少量的酞菁包覆高铁酸钾,浸泡在碱液中3-12h,超铁碱性电池的阴极容量大约可以增加21%-35%,略低于同比例2,3-萘酞菁包覆高铁酸钾的容量。开路电位也显示包覆后的K2FeO4的稳定性随着浸泡时间的增加而减弱,随着酞菁包覆含量的增加而增强。通过电化学阻抗谱探讨了酞菁改善K2FeO4的稳定性的原因：酞菁包覆在高铁酸钾表面,对高铁酸钾电极起到保护膜的作用,在短时间的浸泡过程中,减慢了高铁酸钾的分解；酞菁在高铁酸钾电极中增强了电极中电子在界面之间的转移能力。(6)介绍了使用有机化合物萘作为保护膜,研究其对高铁酸钾稳定性的改善。不同比例的萘包覆后的K2FeO4通过扫描电子显微镜(SEM)研究,发现K2FeO4晶体的表面被萘部分包覆住,且萘的含量越高,包覆的面积越大越厚。傅里叶红外转换光谱(FT-IR)也显示萘包覆在了K2FeO4的上面。本章还研究了不同含量萘包覆K2FeO4后浸泡在电解液中不同时间的电化学性能。恒流放电测试结果显示通过用很少量的萘包覆高铁酸钾后浸泡在碱液中3-12 h,超铁碱性电池的阴极容量大约可以增加1.9%-49%,明显低于同比例2,3-萘酞菁及酞菁对高铁酸钾阴极容量的改善。开路电位显示K2FeO4的稳定性随着浸泡时间的增加而减弱,随着萘的含量的增加而增强。通过电化学阻抗谱探讨了萘包覆对高铁酸钾电极稳定性改善不大的原因：萘在高铁酸钾电极中没有增强电极中电子在界面之间的转移能力。萘只起到保护层的作用,部分阻碍了高铁酸钾和电解液的接触。更多还原

【Abstract】 Ferrate(Ⅵ) is the high-oxidation-state compound of iron which possesses relatively high redox potential, large electrochemical capacity, abundance of raw materials in nature and environmentally friendly discharge product Fe2O3·nH2O which could be used as coagulation in waste water. Ferrate(Ⅵ) compounds could be also used to be as strong oxidants in waste water treatment and as selective oxidants in organic synthesis, also could be as cathodic materials in super iron batteries.There are three main three preparation methods of ferrate(Ⅵ): electrochemical oxidation method, dry oxidation method and wet oxidation method. Among these preparation methods, the wet oxidation method is widely considered to be the most practical, because the approach possesses low ferrate(Ⅵ) generation yield, long reaction time, fussy operation process and low capacity product.In past several years, among these Fe(Ⅵ) compounds investigated, K2FeO4 was readily synthesized and usually used as the precursor for other Fe(Ⅵ) synthesis. Among the super-iron cathodes, K2FeO4 has attracted the most attention during the past few years due to its higher solid-state stability (<0.1% decomposition/year in the dry and sealed condition). However, in aqueous solution, K2FeO4 is very unstable. And the stability of K2FeO4 in acid solution is lower than that in saturated KOH solution. In saturated KOH solution, the K2Fe04 could still decompose to be Fe2O3·nH2O, which would debase the discharge performance of K2Fe04.To resolve the questions about synthesis of potassium ferrate and improve the stability of potassium ferrate, the following works were carried out in this dissertation.(1) The preparation method, the purity analysis methods, the structure, the physicochemical properties and the application of ferrate(Ⅵ) were reviewed. Then the goal and ideas of the work were introduced.(2) An ultrasound-assisted convenient method was developed for the synthesis of battery grade K2FeO4 with high yield (53-59%). The use of ultrasonic simplified the preparation process and increased the practical discharged capacity of product. The factors (eg. the temperature of collecting chlorine, the quantity of reactant, the temperature and time of reaction, the process of separation and purification, etc.) which affected the purity of K2Fe04 were studied. The purity of the synthesized salt was determined by chromite method to be 95-96.8%. It was found that sample of the solid potassium ferrate has a tetrahedral structure with a space group of D2h (Pnma) from X-ray diffraction (XRD) spectrum. From the scanning electronic microscopy (SEM), the K2Fe04 powders were crystallized polyhedron-shaped stick, and the particles had dimensions on the order of 25-200μm in length and 1-10μm in width. Fourier transform infrared spectroscopy (FT-IR) showed K2FeO4 had a tetrahedral structure which centralizes with Fe atoms next to the four equivalent Fe-O bonds.(3) In this dissertation, the electrochemical performance of the K2Fe04 electrodes was studied by using cyclic voltammetry, galvanostatic discharge methods. The influence of the composition of the electrode, concentration of the electrolyte and discharge rate on the discharge performance of K2FeO4 cathode. The results showed that the discharge capacity is the highest when discharged at rate of 0.25C to a cutoff of 0.8 V in 10 mol/L KOH aqueous electrolyte. The higher purity of potassium ferrate possesses a higher capacity which is higer than that reported in previous literature.(4) To improve the stability of K2FeO4 electrodes, using 2,3-Naphthalocyanine (C48H26N8) as coating was introduced in the dissertation. The electrode material with the coating was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) and the results showed that the K2FeO4 was locally coated with 2,3-Naphthalocyanine. Electrochemical behavior of potassium ferrate (VI) (K2Fe04) coated with different weight level of 2,3-Naphthalocyanine is investigated as a function of exposure time to electrolyte. Galvanostatic discharge curves indicate that the electrochemical capacity of K2FeO4 decreases with increasing exposure time. It also indicates that the level of 2,3-Naphthalocyanine has a positive effect on the capacity of K2FeO4 within 3 h. But after storing in the electrolyte for 6 h, the effect of level of 2,3-Naphthalocyanine on the capacity is not obvious when the content of 2,3-Naphthalocyanine is more than 2%. The capacity of K2Fe04 coated with a few percent of 2,3-Naphthalocyanine increased 26%-50% after storing in 10 mol/L KOH for 3-12 h. Open-circuit potential curves indicate that the stability of K2FeO4 decreases with increasing exposure time. But it also indicates that the stability of K2Fe04 increases with increasing content of 2,3-Naphthalocyanine. Electrochemical impedance spectroscopy (EIS) results reveal that 2,3-Naphthalocyanine coating blocks K2Fe04 contact with the electrolyte and decreases the transfer resistance in the electrode, which are responsible for the enhancement of K2FeO4 stability.(5) Phthalocyanine (C32H18N8) with a close structure with 2,3-Naphthalocyanine was used as coating was introduced in the dissertation. The electrode material coated with different content of phthalocyanine was characterized by scanning electron microscopy (SEM), the result showed that the K2Fe04 was locally coated with phthalocyanine and the proportion and thickness of phthalocyanine increseased with increasing the ratio of phthalocyanine. Fourier transform infrared spectroscopy (FT-IR) also showed that K2Fe04 was coated with phthalocyanine. Electrochemical behavior of K2Fe04 coated with different weight level of phthalocyanine is investigated as a function of exposure time to electrolyte. Galvanostatic discharge curves indicate that the electrochemical capacity of K2Fe04 coated with phthalocyanine is higher than uncoated K2FeO4. The capacity of K2Fe04 coated with a few percent of phthalocyanine which is slightly lower than 2,3-Naphthalocyanine increased 21%-35%. Open-circuit potential curves indicate that the stability of K2FeO4 decreases with increasing exposure time. But it also indicates that the stability of K2Fe04 increases with increasing content of phthalocyanine. Electrochemical impedance spectroscopy (EIS) results reveal that phthalocyanine coating blocks K2Fe04 contact with the electrolyte and decreases the transfer resistance in the electrode, which are responsible for the enhancement of K2Fe04 stability.(6) Organic compound naphthalene was introduced to be the coatings to improve the stability of K2FeO4. The electrode material coated with different ratio of naphthalene was characterized by scanning electron microscopy (SEM). The result showed that the K2Fe04 was locally coated with naphthalene and the proportion and thickness of naphthalene also increseased with increasing the ratio of naphthalene. Fourier transform infrared spectroscopy (FT-IR) also showed that K2FeO4 was coated with naphthalene. Electrochemical behavior of K2Fe04 coated with different weight level of naphthalene is investigated as a function of exposure time to electrolyte. Galvanostatic discharge curves indicate that the electrochemical capacity of K2FeO4 coated with naphthalene is slightly higher than uncoated K2FeO4. The capacity of K2FeO4 coated with a few percent of naphthalene, which is lower than 2,3-Naphthalocyanine and naphthalene, increased 1.9%-49. Open-circuit potential curves indicate that the stability of K2FeO4 decreases with increasing exposure time. But it also indicates that the stability of K2FeO4 increases with increasing content of naphthalene. Electrochemical impedance spectroscopy (EIS) results reveal that naphthalene coating increases the transfer resistance in the electrode, but naphthalene coating blocks K2Fe04 contact with the electrolyte which are responsible for the enhancement of K2FeO4 stability.更多还原