Anthraquinone-2,7-disulfonic acid (2,7-AQDS) is a promising organic compound, which is considered as a negolyte for redox flow batteries as well as for other applications. In this work we carried out a well-known reaction of anthraquinone sulfonation to synthesize 2,7-AQDS in mixture with other sulfo-derivatives, namely 2,6-AQDS and 2-AQS. Redox behavior of this mixture was evaluated with cyclic voltammetry and was almost identical to 2,7-AQDS. Mixture was then assessed as a potential negolyte of anthraquinone-bromine redox flow battery. After adjusting membrane-electrode assembly composition (membrane material and flow field)), the cell demonstrated peak power density of 335 mW cm⁻² (at SOC 90%) and capacity utilization, capacity retention and energy efficiency of 87.9, 99.6 and 64.2%, respectively. These values are almost identical or even higher than similar values for flow battery with 2,7-AQDS as a negolyte, while the price of mixture is significantly lower. Therefore, this work unveils the promising possibility of using a mixture of crude sulfonated anthraquinone derivatives mixture as an inexpensive negolyte of RFB.

The proposed anthraquinone-bromate cell combines the advantages of anthraquinone-bromine redox flow batteries and novel hybrid hydrogen-bromate flow batteries. The anthraquinone-2,7-disulfonic acid is of interest as a promising organic negolyte due its high solubility, rapid kinetics of electrode reactions and suitable redox potentials combined with a high chemical stability during redox reactions. Lithium or sodium bromates as posolytes provide an anomalously high discharge current density of order ~A cm⁻² due to a novel autocatalytic mechanism. Combining these two systems, we developed a single cell of novel anthraquinone-bromate flow battery, which showed a power density of 1.08 W cm⁻², energy density of 16.1 W h L⁻¹ and energy efficiency of 72% after 10 charge–discharge cycles.

Due to its high solubility and fast kinetics of redox reactions, anthraquinone-2,7-disulfonic acid is a promising electroactive molecule for redox-flow-battery electrolytes and other energy applications. However, its widespread use is currently limited, primarily due to its tendency to chemical side-reactions and the formation of quinhydrone complexes between the molecule’s different redox-forms. The possibility of overcoming these shortcomings by using a simple anthraquinone-2,7-disulfonic acid functionalization with the poly(diallyldimethylammonium) polycation is studied. The ionic complexes are shown to be formed in this mixture, which leads to the suppression of the quinhydrone compound formation. At the same time, the poly(diallyldimethylammonium)/anthraquinone-2,7-disulfonic acid mixtures retain their redox activity and can be used as a negolyte in anthraquinone–bromine redox flow batteries, while all key characteristics of such a battery are comparable with those of anthraquinone–bromine redox flow batteries which used anthraquinone-2,7-disulfonic acid without any additives. The poly(diallyldimethylammonium)/anthraquinone- 2,7‑disulfonic acid-based battery (0.1 M anthraquinone-2,7-disulfonic acid) has the power density of 105 and 65 mW/cm² for the battery state-of-charge values 100% and 50%, respectively; the energy efficiency for five charging–discharging cycles, 57.4%. In the future, the composition of the poly(diallyldimethylammonium)/anthraquinone-2,7-disulfonic acid ionic complexes can be optimized, in order to maintain good kinetics and solubility of anthraquinone-2,7-disulfonic acid and at the same time reduce the intensity of chemical side-reactions, including quinhydrone-complexes formation.