Sulfonated Anthraquinone-Based Ionic Complexes as a Promising Organic Negolyte for Redox-Flow Batteries

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/cm 2 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.