Molecular Revealing the High-stable Polycyclic Azine Derivatives for Long-Lifetime Aqueous Organic Flow Batteries

Soluble organic molecules with diverse and highly tunable structures are receiving more and more attention as redox-active materials for aqueous organic flow batteries (AOFBs). However, the undesirable parasitic reactions of redox-active molecules restrict their practical application. Herein, a general molecular stabilization strategy is proposed by introducing amino groups on 3,7-positions of polycyclic azine derivatives (phenothiazine and phenoxazine compounds) to effectively enhance the redox activity and electrochemical stability of molecules. The theoretical calculation and a series of electrochemical analyses revealed that the 3,7-substituted amino groups serving as the redox active sites can stabilize the intermediate radicals by delocalizing the electron density in the acidic solution, preventing nucleophilic/electrophilic attack. As a result, the designed 3,7-bis-((2-hydroxyethyl)(methyl)amino)phenothiazin-5-ium bromide not only achieved the remarkable electrochemical cycling performance in the flow cell with an ultra-low-capacity fade rate of 0.00029% per cycle for 18000 cycles (62 days) and but also delivered a stable high capacity of 47 Ah L-1 (with capacity fade 0.0056% per cycle, 0.077% per day) under ambient condition. Such finding undoubtedly provides a guidance to design stable redox-active molecules for AOFBs. A general molecular stabilization strategy is proposed by introducing amino groups on 3,7-positions of polycyclic azine derivatives to effectively enhance the redox activity and electrochemical stability of molecule. Based on this, the synthesized BHAP molecule achieves high solubility and remarkable electrochemical stability, providing a guidance to design stable redox-active molecules for aqueous organic flow battery.image