A dihydroxyethylamino-functionalized naphthoquinone derivative with highly stable and reversible two-electron reaction for aqueous organic redox flow batteries

The development of aqueous organic redox flow batteries (AORFBs) is currently limited by the poor stability and low energy density of organic active materials. To address these issues, this paper presents a novel 2-bis(dihydroxyethyl)amino naphthoquinone derivative (HEANQ, C₁₄H₁₅O₄N). This compound features a naphthoquinone backbone functionalized with a di(hydroxyethyl)amino (−N(CH₂CH₂OH)₂) group and undergoes a reversible, stepwise two-electron redox reaction involving a stable semiquinone intermediate as an anodic species in aqueous media. Theoretical analysis and experimental results clearly demonstrate its enhanced chemical and electrochemical characteristics, including a high kinetic constant (5.98 × 10⁻⁴ cm s⁻¹), a high redox potential (−0.73 V vs Ag/AgCl), all of which surpass those of conventional 1,4-naphthoquinone. Furthermore, the HEANQ/K₄Fe(CN)₆ flow cell exhibits reliable cycling performance over 200 cycles, with average coulombic and energy efficiencies of 97.74% and 79.75%, respectively. Remarkably, the cell retains 93.64% of its initial discharge capacity by the 200th cycle, attributed to the stabilizing effects of the diethanolamine moiety, which suppresses the degradation pathway of the active species. These findings indicate that the synthesis of HEANQ represents a promising molecular engineering strategy for developing highly efficient and stable AORFBs for large-scale energy storage.