Study On The Electrolyte Of Nonaqueous Redox Flow Battery

With the increasing proportion of clean energy in the energy structure,flow batteries are considered as one of the most promising large-scale energy storage technologies.In recent years,non-aqueous system flow batteries have attracted more attention because of their advantages such as wide electrochemical window,wide application temperature range and flexible selection of active materials.The electrolyte solution with the active substance determines the performance of the flow battery greatly.In this work,we focus on the active materials of non-aqueous flow batteries,fabricate a number of flow battery systems,and explore solutions to increase battery energy density.Ferrocene and iron acetylacetonate are first used as the active species in a nonaqueous all-iron flow battery system in this work.The electrochemical properties of the active materials are investigated with cyclic voltammetry method.The electrode reactions are quasi-reversible and show stable electrochemical stability.Different supporting electrolytes have a significant effect on the electrode potential of the active materials.Through the flow battery test,the cyclic charge and discharge performance of the battery is examined.There is no irreversible reaction of the active material in 20 charge and discharge cycles.The coulomb efficiency is 82%,and the voltage efficiency is 40%.The solutions to increase the energy density of non-aqueous system flow batteries are discussed in this work.Increase of the energy density of a flow cell can be achieved by increasing the open circuit voltage of the cell and increasing the number of electrons transferred per unit of molecule.By introducing electron-donating substituents such as a methyl group,a methoxy group and an amino group into benzophenone,the electrode potential of benzophenone moves to the negative direction,achieving an increase in the open circuit voltage of the battery.2,5-Dimethyl-1,4-phenylenediamine undergoes a two-electron transfer reaction in a small potential range,and its chemical stability and potential application as active substance with two-electron transfer reaction are investigated through cyclic voltammetry and flow battery tests.The results show that2,5-dimethyl-1,4-phenylenediamine loses electrons to form stable radical cations,which is a electrochemically reversible oxidation reaction.However,the second oxidation process,the monovalent cations lose electrons to form divalent cation,is less reversible.An all-organic liquid flow battery system based on organic active substance4,4’-dimethylbenzophenone and 2,5-di-tert-butyl-1,4-dimethoxybenzene was constructed in this work.The open circuit voltage of this battery system is as high as2.97V.The flow battery system undergoes a reversible electrochemical reaction in an electrolyte solution in which tetraethylammonium hexafluorophosphate is used as supporting electrolyte and acetonitrile is used as solvent.In the 95 charge and discharge cycles,the average discharge capacity of the flow battery system was 52.1m Ah·L^-1 and the coulombic efficiency was 72%.