Chemical Synthesis And Improvements Of Stability And Discharge Performance Of Potassium Ferrate (â…¥) For Alkaline Super Iron Battery

Ferrate(VI) battery is so-called super-iron battery. It becomes a new focus on the research field of battery, because of its relatively high redox, large electrochemical capacity, flat discharge platform, environmentally friendly discharge product and abundantly available starting materials. Unfortunately, despite of their potentially superior performance, the development of ferrate-based battery in industrialization producing has been restricted by their high cost of synthesis, complex synthesis process and their chemical instability. To resolve the bottleneck of synthesis of potassium ferrate and to improve the stability of potassium ferrate, the following works were carried out in this paper:(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) In this paper, an improved method for synthesis of K2FeO4was adopted. And the factors, such as the temperature and the concentration of KOH of collecting chlorine, the concentration and quantity of reactant, the temperature and time of reaction, the process of separation and purification, which affected the synthesis of K2Fe04were studied. The characterization of potassium ferrate was presented by the chromite methode, X-ray diffraction (XRD) spectrum, Fourier transform infrared spectroscopy (FT-IR) and Scanning electronic microscopy (SEM). The purity of the synthesized K2Fe04was determined by chromite method to be90%-99.2%. XRD and FT-IR showed that K2FeO4has an orthorhombic structure with a space group of D2h (Pnma) and the synthesized K2FeO4was a pure phase. SEM shows K2Fe04is a thin flake and a smooth, highly crystalline surface.(3) In this paper, the electrochemical performance of the K2FeO4electrodes, such as the cyclic voltammetry performance, the discharge performance was systematacially studied. And the influence of the concentration of the electrolyte, the discharge rate, the electrical conductors, the discharge temperature, the purity of K2FeO4on the discharge performance of K2FeO4cathode was discussed. The results showed that the discharge capacity is the highest when discharged at25℃-35℃, the rate of0.25C to a cutoff of0.8V in10mol/L KOH, with K2FeO4:acetylene black mass ratio of9:1. And the higher purity of potassium ferrate possesses a higher capacity.(4) To improve the stability of K2Fe04electrodes, tetraphenylporphyrin (TPP) was used to coat K2Fe04in dichloromethane medium. Examinations such as scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and Fourier transform infrared spectroscopy (FT-IR) showed that K2FeO4cathodes were coated by TPP overlayer successfully. The cyclic voltammetry performance of coated K2Fe04, the stability in moist air and in10mol/L KOH, the Electrochemical impedance spectroscopy (EIS) performance and the Open-circuit potential performance were studied. The cyclic voltammetry showed that the cathode current peak for the coated K2Fe04was less than that of the uncoated material. Moreover, the potential of the coated K2FeO4cathode has a small positive excursion compared to the uncoated one. The test for the stability in moist air showed that tetraphenylporphyrin coated K2FeO4was more stable than the uncoated K2FeO4. The galvanostatic discharge curves indicated that K2FeO4was instable in KOH. After soaking in KOH electrolyte for6h, the capacity of K2Fe04electrode decreased by75%. When used tetraphenylporphyrin as coating overlay, the stability of K2FeO4electrode was improved largely, especially8%tetraphenylporphyrin was used. The discharge capacity of8%TPP coated K2Fe04cathodes increased to160mAh g-1from75mAh g-1for the uncoated electrodes after soaking in a10M KOH electrolyte solution for6h. The electrochemical impedance spectroscopy (EIS) results reveal that tetraphenylporphyrin coating increased the transfer resistance in the electrode and blocked K2Fe04contact with the electrolyte, which are responsible for the enhancement of K2Fe04stability. Open-circuit potential curves indicated that the stability of K2Fe04decreased with increasing soaking time. And it also indicated that the stability of K2Fe04increased with increasing content of tetraphenylporphyrin.(5) The tetraphenylporphyrin coating acts as an insulating protectionlayer that reduces the discharge capacity of the K2FeO4cell when freshly assembled. To overcome this shortcoming and improve the tetraphenylporphyrin coated K2Fe04discharge performance, potassium periodate (KIO4), potassium chlorate (KC103), sodium bismuthate (NaBiO3) and potassium persulfate (K2S2O8) were added to the electrode as additives. The discharge curve indicated that sodium bismuthate (NaBiO3) and potassium persulfate could increase the discharge capacity of tetraphenylporphyrin coated K2FeO4electrode. To investigate the effects of NaBiO3, the XRD spectrum of both the before-and after-discharge products was collected.