Studies On Fe(Ⅱ)/Fe(Ⅲ) Redox Couple In Cathodic Electrolyte For A Novel Redox Battery Application

Fe(Ⅱ)/Fe(Ⅲ) used in positive half cell has many advantages: such as iron is a very rich and low-cost active element, fast electrode reaction kinetics, safety, non-toxic and so on. which is mainly used for Fe/Cr redox flow battery. Meanwhile, zinc negetive electrode has advantages over high hydrogen overpotential, fast reaction, large negative potential and high energy density. Hence, a redox flow battery utilizing the Fe(II)/Fe(III) and Zn/Zn2+redox couples, would combine the advantages of the Fe/Cr systems and Zn/Zn2+as negative, which is a low-cost, safety, environmental and pollution-free battery. These characteristics render great potential for Fe/Zn redox flow battery for wide-range and large-scale energy storage. In this thesis, comprehensive investigation has focused on the characterization of Fe(Ⅱ)/Fe(Ⅲ) half cell and battery cycling performance. The main points can be summarized as follows:The electrochemical properties of Fe(Ⅱ)/Fe(Ⅲ) redox couple in various acidic solutions of iron salt were compared. The results showed that, Fe(Ⅱ)/Fe(Ⅲ) couple in hydrochloric acid possessed better reaction kinetics and reversibility then in methyl-sulfonic acid and in sulfuric acid solutions. The diffusion coefficients of Fe2+and Fe3+in a hydrochloric acid were3.732×10-6cm2·s-1,4.423×10-6cm2·s-1respectively. It is found that the discharge capacity of the iron chloride solution decreases rapidly during cycling. Iron methyl-sulfonic solutions display preferably stable charge-discharge performance and maximum average discharge capacity, with the charging plateau ranges from1.58V to1.68V, and the plateau of the discharge ranges from1.32V to1.18V. So methyl-sulfonic acid was selected as the best solution for Fe(II)/Fe(III) redox couple. The characteristics of Fe(Ⅱ)/Fe(Ⅲ) redox couple at a platinum in methane-sulfonic acid were further studied over a wide range of electrolyte compositions:methane-sulfonic acid (1.0mol·dm-3-3.0mol·dm-3), iron(Ⅱ) methane-sulfonic (0.5mol·dm-3-1.0mol·dm3) and iron (Ⅲ) methane-sulfonic (0.5mol·dm-3-1.0mol·dm-3). The cyclic voltammetry curves indicated that the diffusion coefficients of Fe2+and Fe3+were3.689×10-6cm·s-1,4.333×10-6cm2·s’-, respectively. The reversibility of the redox reaction depended on the electrolyte composition and improved at a lower methane-sulfonic acid concentrations.The effects of added chloride (ammonium chloride, potassium chloride and sodium chloride) in the positive solution, on the electrolyte conductivity, viscosity and electrochemical properties were investigated. With the addition of different chloride electrolyte viscosity, conductivity were increased. The cyclic voltammetry curves and charge-discharge tests showed that the reversibility of the electrode reaction and the reaction kinetics were improved, the cell efficiency was also enhanced by the addition of chloride. At40mA·cm2, added1.0mol·dm-3NH4CI the energy efficiency increased from76.71%of pristine to80.15%.