Unveiling dominant impact of electrochemical stability on performance deterioration in alkaline redox flow batteries utilizing different benzoquinone derivatives

Aqueous organic redox flow batteries (AORFBs) hold potential for large-scale energy storage, which are limited by poor chemical stability of organic compounds. To create stable organic active materials, systematic molecular engineering approaches involving a range of substitutions of functional moieties become imperative. Here, we introduce varying numbers of hydroxyl groups onto p-benzoquinone (p-BQ) core structure as stabilizing agents and then integrate the series of hydroxyl-functionalized BQs into the anolyte of alkaline RFBs to investigate how these substitutions affect cycle stability in a full cell operation. Comprehensive full cell cycling tests followed by ex-situ spectroscopic analyses confirm that an increased quantity of hydroxyl groups plays a crucial role in enhancing the electrochemical stability of molecules. Nevertheless, when BQ is completely substituted, the molecular structure becomes less stable due to increased electrostatic repulsion around the benzene ring. This observation suggests the existence of optimal ranges for functional group quantities to fine-tune molecular stability. Furthermore, our findings indicate that the principal contributor to instability in BQ derivatives stems from electrochemically irreversible redox reactions, rather than the alkaline environment itself. This empirical insight into structure-stability relationships has the potential to provide a robust design toolkit for highly stable organic molecules tailored for AORFB electrolytes.