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 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 exhibit 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,...