Insights into Molecular Oxidation Mechanism to Achieve Highly Stable Aqueous Organic Flow Batteries

Aqueous organic redox flow batteries (AOFBs) have emerged as promising candidates for large-scale energy storage systems. However, due to the high sensitivity of organic molecules to O₂, most AOFBs have to be operated in glove boxes, which limits their practical applications. In this study, the reversible and irreversible oxidation behaviors of 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS) are investigated through in situ UV-vis and ex situ transient absorption spectra. It turned out that the high alkaline concentrations can effectively reduce the reversible auto-oxidation rate, however, it would also accelerate the irreversible oxidation reaction triggered by the photoexcited singlet oxygen (¹O₂) when under irradiation. As a result, combining the reducing agents and employing light-proof storage tanks can improve the cycle life of AOFBs effectively. Batteries encapsulated and operated outside the glove box can achieve a stable operation with a Coulombic efficiency of over 99% and a capacity decay rate of 0.0054%/cycle for 2500 cycles. The work proves that stable AOFBs under ambient conditions are possible, further enabling their application in large-scale energy storage systems.