| Ferrate(?) 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(?) 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(?): 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(?) generation yield, long reaction time, fussy operation process and low capacity product.In past several years, among these Fe(?) compounds investigated, K2FeO4 was readily synthesized and usually used as the precursor for other Fe(?) 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(?) 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 25-200?m in length and 1-10?m 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. |
PDF Full Text Request