Design And Performance Of Fe²⁺-I₃^-flow Battery Directly Charged By Solar

As one of the important clean energy,the efficient utilization of solar energy can not only alleviate the increasingly serious energy crisis,but also promote the sustainable development of human society.However,the utilization of solar energy is limited by natural factors such as day and night alternation,weather change and so on.As a novel and efficient solar energy storage technology,the solar rechargeable battery system composed of solar cells and secondary batteries can realize the simultaneous conversion and storage of solar energy,which has attracted extensive attention of researchers.Regarding the problem of the poor matching of the working voltage between the traditional secondary cells and solar cells,herein we focuses on the aqueous Fe²⁺-I₃^-redox flow battery system,which could obtain high activity,low cost and safety,based on understanding of dye-sensitized solar cells.As a result,the synchronous conversion and storage of solar energy,electric energy and chemical energy can be realized.The main research contents and results are as follows:（1）It is expected to build a high-capacity,low-cost,safe and eco-friendly flow battery system with Fe³⁺/Fe²⁺and I₃^-/I^-redox pair,,due to its high theoretical capacity,natural abundance,well compatibility with aqueous electrolytes and high solubility.However,the narrow potential gap between iron ions/ferrous ions(Fe³⁺/Fe²⁺,E=0.77V vs SHE)and polyiodide ions/iodide ions（I₃^-/I^-,E=0.53 V vs SHE）results in a voltage mismatch with solar cell.In order to control the redox potential of the electrode materials,highly water-soluble,multidentate ligands that have good chelating properties with iron ions were employed,and the redox potential of Fe³⁺/Fe²⁺redox pairs under different coordination environments was further studied by means of electrochemical testing methods.Finally,the citrate ligand(citrate^3-)was optimized because the electrode potential of[Fe（citrate）]/[Fe（citrate）]^-redox pair was reduced to～0 V vs SCE.An aqueous Fe²⁺-I₃^-redox flow battery was further constructed with I₃^-/I^-redox pair.The battery system demonstrated an steady working potential of 0.39 V and could be cycled for 10 times.（2）By adjusting the electrode potential of Fe³⁺/Fe²⁺redox pair,a good match between the working voltage of the Fe²⁺-I₃^-redox flow battery and the output voltage of the dye sensitized solar cell could be achieved.A solar rechargeable system was designed by combining Fe²⁺-I₃^-redox flow battery and dye sensitized solar cells.During photo-charging,I^-in the anode electrolyte was oxidized to I₃^-,and[Fe（citrate）]in the anode electrolyte was synchronously reduced to[Fe（citrate）]^-,solar energy was thus stored as chemical energy.In the process of discharge,I₃^-in the positive electrolyte was reduced to I^-,and[Fe（citrate）]^-in the negative electrolyte was oxidized to[Fe（citrate）].The electrochemical reaction was similar to that of conventional redox flow battery.The first cycle capacity conversion efficiency of the obtained solar rechargeable flow battery could reach 59.9%,additionally,the battery could cycled steadily for more than 10 times.