Study On The Negative Electrolyte For All-vanadium Redox Flow Battery

Abstract:All-vanadium redox flow battery shows great prospect in the field of large-scale energy storage because of its attractive advantages including the decoupling of power and energy requirements, quick response, long service life and safety quality. As the storage medium of ion conductor and active species, electrolyte is considered as the key component of the redox flow battery system. The research is focused on the negative electrolyte of all-vanadium redox flow battery. Different additives are employed in the negative electrolyte to improve its stability and electrochemical performance. The main contents are as follows.The influences of factors such as temperature and sulfuric acid concentration on the basic performance of negative electrolyte are investigated by stability test, UV-vis spectrometry, viscosity measurement and cyclic voltammetry. The stability of V(Ⅲ) electrolyte decreases at low temperatures or with high sulfuric acid concentrations. Raising sulfuric acid concentration will promote the formation of sulfate complexed V(Ⅲ) molecules and increase the viscosity, which could affect the mass transfer process in electrolyte. The electrochemical activity of V(Ⅲ)/V(Ⅱ) redox reaction thus would be reduced.Tetradecyltrimethylammonium bromide(TTAB), sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDS’) are introduced as additives for the negative electrolyte. The effects of different surfactants on the stability and electrochemical performance of electrolyte are investigated. The addition of TTAB and SDS has no positive effect on the stability of negative electrolyte. The cyclic voltammetry tests show that the reversibility and activity of electrode reaction are reduced after adding TTAB and SDS. However, the addition of SDS’can effectively enhance the stability of V(Ⅲ) electrolyte with high sulfuric acid concentrations. The vanadium retention of V(Ⅲ) electrolyte with SDS’rises from80.2%to87.8%. Considering its overall impact on stability and electrochemical performance,0.2-0.4%is determined as the best addition amount of SDS’. Appropriate amount of SDS’wouldn’t affect the electrochemical activity of V(Ⅲ) electrolyte as well as the charge-discharge performance of the battery.DL-malic acid and L-aspartic acid are investigated as additives for the negative electrolyte to study the influence of organic additives with functional groups such as-COOH and-NH2on the stability and electrochemical performance of negative electrolyte. The stability experiments indicate that the addition of L-aspartic acid into the2M V(III) electrolyte can stabilize the electrolyte by delaying its precipitation. The results of cyclic voltammetry and electrochemical impedance spectroscopy show that the introduction of DL-malic acid and L-aspartic acid can improve the electrochemical performance of the negative electrolyte by promoting the diffusion of active species and facilitating the charge transfer of V(Ⅲ)/V(Ⅱ) redox reaction. The improvement resulted from L-aspartic acid is more remarkable. Moreover, the cell employing negative electrolyte with L-aspartic acid exhibits a higher discharge capacity and excellent cycling stability. A Higher average energy efficiency (76.4%) compared to the pristine cell (73.8%) is also achieved. The comparison of charge-discharge performance and analysis of element contents on the electrode surface confirm that L-aspartic acid in the electrolyte can modify the electrode by constantly providing oxygen-and nitrogen-containing groups, leading to the enhancement for electrochemical performance of the cell.