Biomimetic Naphthoquinone Zwitterion Derivative with Water-Solubilizing Amino Acid Side Chain for High-Stability Aqueous Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) have gained attention as an alternative to transition metal ion-based flow batteries. Upgrading the functional groups and modification positions is crucial to regulating the redox potential, solubility, and stability of organic species. Herein, we introduce high-performance alkaline AORFBs using 3-(2-chloro-1,4-naphthoquinon-3-ylamino)propanoic acid (3-AFNQ), a biomimetic naphthoquinone zwitterion with a water-solubilizing amino acid group at the 3-position. Similar to natural amino acids, 3-AFNQ is a zwitterion with an asymmetric charge distribution. The hydrogen bonds between the amino acid group and water molecules enhance the aqueous solubility. The rational molecular design of 3-AFNQ prevents enol–ketone tautomerization, ensuring high reversibility, good redox kinetics, and stability when it is used as a negolyte. The AORFBs based on 3-AFNQ negolyte exhibit a 0.39% capacity fade rate per day at high concentration. These results underscore the significance of rational molecular design in avoiding molecular degradation and expanding naphthoquinone derivatives' potential applications in AORFBs.