Studies On High-performance Electrolytes For Redox Flow Batteries

Redox flow battery, for its high efficiency, theoretically infinite cycle life, low running cost and independent tunability of power and capacity, has attracted much concern as a promising large-scale energy storage device. In redox flow battery system, electrolyte serves as not only ion conductor, but also energy storage medium, which to a large extent determines the overall performance of a flow battery. In this paper, L-glutamic is employed as positive electrolyte additive for all-vanadium redox flow battery to improve the electrochemical performance and thermal stability. Besides, in combination of the advantage of non-aqueous system and flow battery, a novel organic redox flow battery is preliminarily explored. The main contents are as follows:L-glutamic is used as additive for positive electrolyte for VRFB, and its effects on the electrochemical performance and thermal stability are systematically investigated. It is found that the addition of L-glutamic can not only improve the electrochemical performance of positive electrolyte, increase the reversibility of VO2+/VO2+redox reaction, promote the mass transportation, but also significantly alleviate the precipitation of V2O5from positive electrolyte, and thus improve its thermal stability. Moreover, techniques including UV-vis spectra, Raman spectroscopy and X-ray photoelectron spectroscopy are used to discuss the mechanism for the improvement in electrochemical performance and thermal stability, which find out that the trace amount of additive does not react with vanadium ion and the combination of L-glutamic and V(V) ion is physical adsorption, rather than chemical coordination. The improved thermal stability should be due to the adsorption of L-glutamic amphoteric ion which hinders the aggregation of V2O5nucleus. Meanwhile, the added L-glutamic can introduce more nitrogen-containing and oxygen-containing functional groups onto the surface of carbon felt electrode, which are believed to provide more active sites for VO2+/VO2+electrochemical reaction and facilitate the electron transfer on electrode/electrolyte interface. The electrolyte with L-glutamic exhibits good charging-discharging performance. The coulombic and energy efficiency can be as high as96%and74%, respectively, with excellent cycling stability.On the basis of non-aqueous system and flow battery, a novel organic redox flow battery is constructed with1,4-Di-tert-butyl-2,5-dimethoxybenzene(DDB) and alizarin(QS) as active materials, N,N-dimethyl formamide as solvent and NaClO4as supporting electrolyte. Cyclic voltammetry tests show that both DDB and QS exhibit good electrochemical activity and reversibility. The capacity of constructed statistic battery stays stable after10charging-discharging cycles and the capacity conservation rate can be over70%after50cycles, exhibiting good cycling stability and the coulombic efficiency is over90%.