Differential pH as a method for increasing cell potential in organic aqueous flow batteries

In aqueous flow batteries, high battery cell potentials close to the electrochemical window of water are paramount for high cycle efficiency, power and energy density. The standard potential of many organic redox species has a strong dependence of pH and opens the possibility for increasing the cell potential by pH tuning. In the present work, a differential pH aqueous flow battery is demonstrated. The positive side is based on bromine operated at pH similar to 2, while the negative side is based on anthraquinone-2,7-disulfonate disodium operated at pH similar to 8. This results in a cell potential of 1.3 V which is a significant increase from 0.86 V when operated under acidic conditions on both sides. Furthermore, the differential pH of the flow battery is remarkably stable and shows no significant change over a period of at least 14 days (similar to 100 cycles). The high stability is attributed to electrochemical buffer effects of the redox solutions on each side.