Molecular Design of Fused-Ring Phenazine Derivatives for Long-Cycling Alkaline Redox Flow Batteries

The utilization of redox-active organic species in aqueous redox flow batteries holds great promise for large-scale and sustainable energy storage. Herein, we report the low-temperature green synthesis of three different phenazine derivatives and investigate their performances in alkaline organic redox flow batteries. Electrochemical characterizations reveal that the ortho-substituents of the hydroxyl groups in phenazine derivatives have significant influences on the battery performances. By introducing an additional phenyl group adjacent to the hydroxyl group in the phenazine motif and a carboxyl group with strong solubilizing effect, the redox flow battery based on fused-ring benzo­[a]­hydroxyphenazine-7/8-carboxylic acid with 1.0 M electron concentration exhibits greatly improved capacity retention rate of 99.986% cycle–1 (99.92% day–1) and stable average energy efficiency of ∼80% for over 1300 cycles. Moreover, a combinatorial library of hydroxyphenazine derivatives with varying substituent groups was built, and their redox properties were simulated to guide further molecular structure design of phenazine-derived electroactive compounds.