Preparation And Electrochemical Performance Of Ferrate (VI) Compounds

The preparation and application of ferrate (VI) compounds, which could be used as strong oxidants in water treatment and organic synthesis, and cathodic materials in "super-iron" batteries, have got much attention in the world, due to their high redox potentials, large theoretic capacity, abundance of raw materials in nature and non-toxicity to the environment.In the main three preparation methods of ferrate (VI), the electrochemical preparation is widely investigated recently, due to its simple processe and non-pollution. However, the problem is that the current efficiency of ferrate (VI) electrosynthesis is not very high. So far, solid K₂FeO₄ is found to be the most stable in all investigated ferrate (VI) compounds, but its discharge performance is not very ideal. Hence it is interesting to probe alternate ferrate (VI) salts with high intrinsic capacity, such as CaFeO₄.In this dissertation, the preparation and the purity analysis, the physicochemical properties of ferrate (VI), and the electrochemical behavior of iron electrode in alkaline solutions, as well as the application in water treatment, organic synthesis and super-iron batteries were firstly reviewed. Then, the goal and ideas of our work were introduced.In the third chapter of this dissertation, the influences of anode materials, current density, temperature on the current efficiency, concentration of produced ferrate (VI), and the ratio of Fe (VI) /Fe_tol. were investigated. It was found that the variation trend of the related parameters in the ferrate (VI) electrosynthesis using iron wire gauze was similar to those using grey cast iron or nodular cast iron. Compared with the two latter anodes, the iron wire gauze anode presented the highest current efficiency at relatively small current density. After 2 h electrolysis, the current efficiency of ferrate (VI) electrosynthesis was 73.6% when current density was 1.02 mA/cm² using the iron wire gauze, while it was 58.8% and 61.7% using grey cast iron and nodular cast iron, respectively, when the current density was 8 mA/cm². The differences were presumably related to the composition and structure of the anodes. The influences of the addition of different anions in 14 mol/dm³ NaOH on the ferrate (VI) electrosynthesis were also investigated using iron wire gauze anode. The results showed that at the conditions of lower concentration of anions, such as, PO₄^3-, SO₄^2- and SiO₃^2-, etc. and lower temperatures, the current efficiency of ferrate (VI) electrosynthesis increased to some extent.In the fourth chapter, the effects of ultrasound on the direct electrosynthesis of solid K₂FeO₄ and the anodic behaviors of pure iron were investigated, and the physical properties of the samples were characterized by means of XRD, FTIR and SEM. The experimental results showed that the existence of ultrasound could decrease the formation potential of ferrate (VI) and the passivation extent of iron anode, as well as O₂ evolution rate, which led to higher current efficiency for the direct electrosynthesis of solid ferrate (VI) at 65℃in 14 mol/dm³ KOH solution. It was also found that in the experimental scope the suitable ultrasonic power was 14.6 W. The largest current efficiency under 14.6 W of the ultrasonic power and small electrolysis current (0.8 mA/cm²) reached to 77.2% after 6.84 h electrolysis.In this chapter, the influences of different ratio of K⁺/(K⁺ + Na⁺ ) ([OH^-]=14 mol/dm³) on the direct electrosynthesis solid K₂FeO₄ were also investigated. The results showed that, at lower temperatures(60℃), the apparent current efficiency of electrosynthesis solid potassium ferrate (VI) could reach 64.9 % in 9 mol/dm³ KOH + 5 mol/dm³ NaOH solution after 2 h electrolysis, while it went to its maximal value, about 63.9% at 70℃in 14 mol/dm³ KOH. The solid potassium ferrate (VI) samples prepared from the two solutions exhibited similar SEM and infrared absorptions. Therefore, solid K₂FeO₄ could be obtained at lower temperature (≤65℃) through optimizing the ratio of K⁺/Na⁺.In the fifth chapter of this dissertation, K₂FeO₄ powders were synthesized by the ex-situ and in-situ electrochemical methods, respectively, and characterized by IR, SEM, XRD and BET. Their electrochemical performances were investigated by means of galvanostatic discharge and electrochemical impedance spectroscopy (EIS). The results of physical characterization showed that the two samples had similar structural features, but the orientation growth of the crystals, their surface morphologies and specific surface areas were different. The results of discharge experiments indicated that the ex-situ electrosythesized K₂FeO₄ electrode had much larger discharge capacity and lower electrode polarization (that is, smaller Rt value) than the in-situ electrosynthesized K₂FeO₄ electrode.In the following chapter, calcium ferrate (VI) powders prepared from potassium ferrate (VI), were characterized by titration analysis, elemental analyzer, SEM, XRD, IR, TG and DSC. The results showed that the synthesized sample mainly consists of calcium ferrate (VI), and calcium ferrate (VI) may exist as CaFeO₄·2H₂O with a highest obtained purity of 74.9%. The relatively higher Fe (III) impurity and crystalloid water might be responsible for the poor stability of the calcium ferrate (VI) sample. The results of galvanostatic discharge experiments indicated that the calcium ferrate (VI) sample displays better intrinsic rate discharge capability and larger discharge capacity at lower temperatures (≤15℃).In the last part of this dissertation, Ag₂FeO₄ was characterized by SEM and FTIR, and its electrochemical performances were investigated by constant current discharge. The results showed that, Ag₂FeO₄ consisted of nano or submicron particles; with the temperature increasing, Ag₂FeO₄ decomposed quicker. At the same discharge conditions, the discharge performance of Ag₂FeO₄ was worse than that of K₂FeO₄ in the alkaline solution, but it was far better in the non-aqueous solution.