text: Quinones and their derivatives are the basis of many organic redox systems. In one study, 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) and 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) were employed as cathodes, and conventional Pb/PbSO was the anolyte in a hybrid acid AORFB. Quinones accept two units of electrical charge, compared with one in conventional catholyte, implying that such a battery could store twice as much energy in a given volume.
  Another quinone 9,10-Anthraquinone-2,7-disulfonic acid (AQDS), has been evaluated. AQDS undergoes rapid, reversible two-electron/two-proton reduction on a glassy carbon electrode in sulfuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the / redox couple, yields a peak galvanic power density exceeding 6,000 W/m at 13,000 A/m. Cycling showed > 99% storage capacity retention per cycle. Volumetric energy density was over 20 Wh/L. Anthraquinone-2-sulfonic acid and anthraquinone-2,6-disulfonic acid on the negative side and 1,2-dihydrobenzoquinone- 3,5-disulfonic acid on the positive side avoids the use of hazardous Br. The battery was claimed to last for 1,000 cycles without degradation. While this system appears robust, it has a low cell voltage (ca. 0.55V) and a low energy density (< 4Wh/L).
  Hydrobromic acid used as an electrolyte has been replaced with a less toxic alkaline solution (1M KOH) and ferrocyanide. The higher pH is less corrosive, allowing the use of inexpensive polymer tanks. The increased electrical resistance in the membrane was compensated increased voltage. The cell voltage was 1.2V. The cells efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery offered an expected lifetime of at least 1,000 cycles. Its theoretic energy density was 19Wh/L. Ferrocyanides chemical stability in high pH KOH solution without forming Fe(OH) or Fe(OH) needs to be verified before scale-up.
  Integrating both anolyte and catholyte in the same molecule has been examined. Such bifunctional analytes or combi-molecules allow the same material to be used in both tanks. In one tank it is an electron donor, while in the other it is an electron recipient. This has relevant advantages such as diminishing the effect of crossover. Thus, quinone diaminoanthraquinone and indigo-based molecules as well as TEMPO/phenazine combining molecules are potential electrolytes for the development of symmetric redox-flow batteries (SRFB).
  Another approach adopted a Blatter radical as the donor/recipient. It endured 275 charge and discharge cycles in tests, although it was not water-soluble.