Mixed Solvents And Supporting Electrolytes For All-organic Redox Flow Batteries

The development of flexible and stable large-scale energy storage technology has become a research hotspot due to the expansion of renewable energy sources.Redox flow battery(RFB)is expected to meet the need for the efficient utilization of energy sources.In this paper,a new non-aqueous all-organic RFB system is developed,which involves the solvent,supporting electrolyte and active species in the electrolyte.In chapter 2,the electrochemical performance of the novel active species,benzophenone(BP)and 1,4-di-tert-butyl-2,5-dimethoxybenzene(DBB),is tested using cyclic voltammetry(CV).With tetraethylammonium hexafluorophosphate(TEAPFs)as the supporting electrolyte and acetonitrile(AN)as the solvent,the reactions of active species are quasi-reversible and both practically diffusion-controlled.The theoretical open circuit voltage(OCV)is 2.92 V.The solubility of DBB in solvent is improved by the addition of co-solvent.The effects of co-solvents,Tetrahydrofuran(THF),Dichloromethane(DCM),N,N-Dimethylformamide(DMF),1,3-Dioxolane(DOL),Dimethyl carbonate(DMC)and Diethyl carbonate(DEC)in AN on the physical and electrochemical properties of the electrolyte are analyzed.The co-solvent changes the electrochemical window of electrolyte and the solubility of active species.The decline in the conductivity and the significantly rise in the viscosity and the solubility of DBB can be observed as the ratio of co-solvent increases.The largest diffusion coefficients of DBB and BP are in the range of 1.53-1.98×10-5 and 1.75-2.53×10-5 cm2·s-1 respectively when DOL/AN is 1:1.The highest measured OCV is 3.12V when DOL/AN is4:1.The electrochemical performances of two BP derivatives 4,4'-dimethylbenzophenone(DMBP)and 4,4'-dimethoxybenzophenone(DMOBP)are tested as the negative active species.As electron-donating group methyl-and methoxy-are introduced,the OCVs increase to 3.02 V and 3.12V,respectively,while the diffusion coefficients of the active species decrease.In chapter 3,an ionic liquid tetraethylammonium bis(trifluoromethylsulfonyl)imide(TEATFSI)is synthesized through a two-step method and applied as a new supporting electrolyte in BP|DBB all-organic RFB.The influences of several common supporting electrolytes on the conductivity of electrolyte and the electrode kinetics are analyzed.The charge-discharge performance is test on a self-assem bled single RFB with dif ferent supporting electrolytes.The cations of the supporting electrolytes show a significant effect on the diffusion coefficients of both BP and DBB.With TEATFSI as the supporting electrolyte,the battery exhibits a high OCV of 2.95V.The coulombic,voltage and energy efficiencies of the battery are 97%,46%and 44%,respectively.The battery shows a stable p erformance during 50 cycles.The charge-discharge performa nces of DM BP|DBB and DMOBP|DBB all-organic RFBs are also tested with TEATFSI.The stability of DMBP is close to BP du ring the charge-discharge process,while that of DMOBP is poor though the discharge platform is relatively high.