A low-redox-potential phenazine-based negolyte with high stability for aqueous organic flow batteries

Phenazine-based negolytes for aqueous organic redox flow batteries (AOFBs) have attracted significant attention due to their structural tunability and highly reversible redox behaviour. However, phenazine derivatives often suffer from adverse tautomerization and side-chain cleavage and their stability would become inferior as redox potential decreases. Consequently, developing stable phenazine derivatives with low redox potential (especially ≤-0.8 V vs. SHE) remains a major challenge, yet it is essential for achieving high-voltage AORFBs. Herein, we report a new phenazine derivative, 4,4'-((7,8-dimethoxyphenazine-2,3-diyl)bis(oxy))dibutyric acid (dMeODBAP), which exhibits a favourable redox potential of -0.84 V (vs. SHE) along with exceptional stability, as validated by both spectroscopic analyses and theoretical simulations. Besides, a microporous blend membrane is fabricated, which effectively suppresses the crossover issue and exhibits a higher potassium ion conductivity than the commercial Nafion 212 membrane. Hence, the AOFB based on dMeODBAP and the blend membrane achieves a high voltage of 1.34 V, an impressive peak power density of 194 mW cm^-2 and a remarkable capacity retention of 99.95% over 1000 cycles at 100 mA cm^-2, corresponding to an ultra-low capacity fade rate of 0.007% per day.