Evaluation of Electrochemical Stability of Sulfonated Anthraquinone-Based Acidic Electrolyte for Redox Flow Battery Application

Abstract

Despite intense research in the field of aqueous organic redox flow batteries, low molecular stability of electroactive compounds limits further commercialization. Additionally, currently used methods typically cannot differentiate between individual capacity fade mechanisms, such as degradation of electroactive compound and its cross-over through the membrane. We present a more complex method for in situ evaluation of (electro)chemical stability of electrolytes using a flow electrolyser and a double half-cell including permeation measurements of electrolyte cross-over through a membrane by a UV–VIS spectrometer. The method is employed to study (electro)chemical stability of acidic negolyte based on an anthraquinone sulfonation mixture containing mainly 2,6- and 2,7-anthraquinone disulfonic acid isomers, which can be directly used as an RFB negolyte. The effect of electrolyte state of charge (SoC), current load and operating temperature on electrolyte stability is tested. The results show enhanced capacity decay for fully charged electrolyte (0.9 and 2.45% per day at 20 °C and 40 °C, respectively) while very good stability is observed at 50% SoC and lower, even at 40 °C and under current load (0.02% per day). HPLC analysis conformed deep degradation of AQ derivatives connected with the loss of aromaticity. The developed method can be adopted for stability evaluation of electrolytes of various organic and inorganic RFB chemistries.

Mixture of Anthraquinone Sulfo-Derivatives as an Inexpensive Organic Flow Battery Negolyte: Optimization of Battery Cell

“…The sulfonation of anthraquinone with oleum allows us to obtain the mixture of various sulfonated derivatives of anthraquinone (2,7-AQDS, 2,6-AQDS and AQS; see Scheme 1 ) and unreacted anthraquinone dissolved in weakly concentrated sulfuric acid [ 43 , 44 , 47 , 55 ]. The exact composition can be accomplished by controlling the conditions of synthesis: temperature, concentration of the starting reagents, time, and the presence of various additives.…”

“…Apart from widespread 2,7-AQDS, other ASM components (2,6-AQDS and 2-AQS) were tested separately as negolytes of RFB [ 22 , 23 ]. The only exception is recent research by Mazur et al focused mainly on chemical stability of ASM components during redox reactions [ 47 ] and confirming the fundamental possibility of using ASM as RFB negolyte. However, key parameters determining capabilities to use the ASM in RFB, namely discharge power, current density, energy efficiency, were not evaluated.…”

Multielectron Organic Redoxmers for Energy-Dense Redox Flow Batteries

“…A CO 2 -assisted dimerization mechanism was proposed but the predicted result of 1.5 usable electrons per AQDS is in contradiction with the observed >90% of 2e – capacity in flow cells . More recent studies also suggest desulfonation or disproportionation-based degradation pathways for AQDS, which however remain inconclusive due to a lack of detection of the decomposition products. , …”

Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

“…This is the reason why most flow batteries perform rather poorly under cold climate conditions. On the other hand, increased temperatures may facilitate unwanted phenomena, such as cross-over of active species through the ion exchange membrane, redox compound decomposition [29,74], and water splitting on the electrodes. Capacity fade mechanisms are explored in more detail in Section 2.5.…”

“…However, the relatively low cell voltage (below 0.9 V), capacity losses due to bromine crossover to negolyte [104], and toxicological aspects of bromine represent the challenges to the technology. Additionally, insufficient stability of AQDS in its reduced form and at elevated temperatures has been reported, most probably due to anthrone formation [74]. AQDS-FeSO 4 RFB has also been reported [105], providing low material costs but also very low cell OCV of 0.6 V. Recently, the hybridization of an AQDS-based negative half-cell with an oxygen electrode in the concept of a fuel cell and flow electrolyzer has been reported for stationary energy storage application, showing comparable performance to its vanadium analogue [106].…”