Design And Performance Of Zinc-organic Redox Flow Batteries Based On Deep Eutectic Solvent (DES)

The rapid development of the national economy in recent years has led to the rapid consumption of traditional non-renewable fossil energy,thus promoting the popularization of clean and renewable energy such as solar energy and wind energy.As one of the most promising electrochemical energy storage systems at present,redox flow batteries（RFBs）are characterized by their adjustable energy storage capacity,high power density,long cycle life,good rate performance,low-cost and environmental compatibility.It can effectively alleviate the impact of the unsteady characteristics of renewable energy on the power grid system.The cost of traditional organic solvents in non-aqueous flow batteries is high,and their low vapor pressure（high volatility）poses a threat to the safe operation of the battery.Deep eutectic solvents（DES）are conductive liquid media that can be obtained by mixing two（or more）components which can self-associate through intermolecular interactions.Due to the advantages of facile preparation,low-cost,green,and wide electrochemical stability window,DES has recently been developed as an electrolyte system for non-aqueous RFBs.In this dissertation,a zinc-organic RFB system based on the DES electrolyte is designed and developed.The main contents of this dissertation are as follows:（1）A deep eutectic solvents（DES）system was designed and developed based on choline chloride（Ch Cl）and trifluoromethanesulfonate{（Tf O）n M,M=Zn or Na},acetic acid and water as additives,tetramethylaminemethylene biphenol（TABP）is a deep eutectic electrolyte system of cathodic redox active species,and the solubility of electroactive species TABP in the above deep eutectic electrolyte system were investigated.The redox potential,electrochemical activity and stability of TABP in the above DES system were further investigated by cyclic voltammetry（CV）.On this basis,considering the basic requirements of RFBs for active electrolytes,two optimized DES systems were obtained（10#DES:molar ratio of Ch Cl/Zn（Tf O）₂/Ac OH/H₂O=4/2/0.56/7.94 and24#DES:molarratioof Ch Cl/Zn（Tf O）₂/Tf ONa/Ac OH/H₂O=4/1.5/1/2.45/7.78）.Using 10#DES as the anolyte,10#DES containing 0.1 mol/L TABP as the catholyte,and Nafion 112 membrane as a separator,a Zn//TABP cell was constructed.The100-cycle cycle test shows that the performance of the battery is relatively stable with less attenuation.The discharge capacity retention reaches87.5%,the Coulombic efficiency is greater than 99%,and the energy efficiency is 65.2%.Another Zn//TABP cell was constructed by using 24#DES as the anolyte,24#DES containing 0.1 mol/L TABP as the catholyte,and Nafion 112 membrane as the separator.The cell displays a discharge capacity retention of 91.8%after 60 charge-discharge cycles at a constant current of 20 m A.The Coulombic efficiency is close to 100%while the energy efficiency is 70.3%.（2）A DES system based on ethylene glycol（EG）and（Tf O）n M,acetic acid and water as additives,and TABP as the cathodic redox active species was designed and developed,and the effect of TABP in the above DES system was investigated.solubility.The redox potential,electrochemical activity and stability of TABP in the above DES system were investigated by CV technique.On this basis,an optimized DES system composition was obtained（12#DES:EG/Zn（Tf O）₂/Ac OH/H₂O molar ratio=6/1/0.33/10.22）.A Zn//TABP flow battery was constructed with 12#DES as anode active electrolyte,12#DES containing 0.1 mol/L TABP as cathode active electrolyte,and Nafion 112 membrane as separator.The experimental results of 100 charge-discharge cycles show that the battery has high material utilization and less attenuation.The discharge capacity retention rate is higher than 70%,the Coulombic efficiency is higher than 99%,and the energy efficiency is about 70%.（3）The electrochemical performance and half-life of redox-active organic molecules,violuric acid（VA）and zinc violurate（Zn（VA）₂）,in Na Cl system were also explored.The solubility and electrochemical properties of VA and(Zn（VA）₂ in different DES systems were also studied.The research shows that the redox potential of Zn（VA）₂ in Ch Cl/Zn（Tf O）₂DES system can reach 1.75 V(relative to Zn/Zn²⁺).After 200 CV cycles,it still has a high peak current retention rate,revealing good electrochemical stability.