| The main objective of this thesis is to identify the effect of electrolyte composition on the performance of zinc-bromine flow batteries,and provides necessary experimental basis for the development of high-performance zinc-bromine flow batteries.In first part of this thesis,the performances of four electrode materials and four diaphragms were compared by using cyclic voltammetry?CV?,electrochemical impedance spectroscopy?EIS?,and constant current charge-discharge.The experimental results show that the best performance can be achieved by using carbon felts treated at 500? as electrode;and the battery using BEST cation exchange membrane as separator has highest coulombic efficiency and energy efficiency during the charge/discharge cycles due to its low resistance.Therefore,the carbon felts treated at 500? were used as the electrode materials in the follow-up study,and the BEST cation exchange membrane was selected as the separator.Based on the above results,the effects of four additives,i.e.,KCl,MEP?N-ethyl-N-methyl bromide pyrrolidine?,Pb?NO3?2 and SnCl2?2H2O on the performance of the 1M ZnBr2 electrolyte were investigated by using the electrochemical methods,such as linear scan voltammetry?LSV?,CV,EIS,etc.The results show that the electrolyte solution containing 1 M ZnBr2,1 M KCl,1 M MEP,5×10-4 M Pb?NO3?2 and 5×10-4 M SnCl2?2H2O has the smallest solution resistance,and the positive electrode reaction has the smallest overpotential and the biggest reduction peak current corresponding to redox pair of Br2/Br-in the electrolyte.The deposition performance of zinc in different electrolytes was studied by using Hull cell electrodeposition experiments.The morphology of the deposited layer was observed by SEM.The results show that when the solution composition is 1 M ZnBr2+1 M KCl+1 M MEP+5×10-4 M Pb?NO3?2+5×10-4 M SnCl2?2H2O,the zinc deposition layer on the cathode has the most uniform morphology,which means that the solution has a better dispersion ability.However,when the battery is charged and discharged at a constant current of 13 mA/cm2,the energy efficiency and Coulombic efficiency of the battery using the optimized electrolyte are not significantly different from those of batteries using other solutions.When the charge/discharge current density is raised to20 mA/cm2,the battery exhibits the best cycle performance,the Coulombic efficiency can reach 95%,and the energy efficiency can be stabilized at about 70%in 20 cycles,which is obviously better than that of other batteries.Therefore,it can be concluded that the electrolyte solution consisting of 1 M ZnBr2,1 M KCl,1 M MEP,5×10-4 M Pb?NO3?2 and 5×10-4 M SnCl2?2H2O is most suitable for zinc-bromine flow battery.Since the concentration of ZnBr2 in the electrolyte has a decisive influence on the specific energy of the zinc-bromine flow battery,the effect of the concentration of ZnBr2 on the electrochemical performance of the electrolyte and the battery performance and stability was studied further.Electrochemical performances of ZnBr2electrolytes with different concentrations were studied by electrochemical techniques such as LSV,CV,EIS and galvanostatic charge-discharge.The results showed that the electrolyte with ZnBr2 concentration of 2.5 M had the best electrochemical performance.The charge/discharge performance of the batteries with different concentrations of ZnBr2 electrolyte was tested at the current density of 13 mA/cm2and 20 mA/cm2,respectively.The battery with the ZnBr2 concentration of 2.5 M was found to have the highest coulombic efficiency and energy efficiency with the current density of 20 mA/cm2.Therefore,based on the cell structure used in this experiment,the battery can be charged and discharged under higher current density with better stability when the electrolyte contains 2.5 M ZnBr2,1 M KCl,1 M MEP,5×10-4 M Pb?NO3?2 and 5×10-4 M SnCl2?2H2O. |
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