A Resonance Hybrid Design for Stable Aqueous Organic Redox Flow Batteries

Organic molecules are promising candidates for aqueous organic redox flow batteries (AORFBs) due to their structural versatility, tunability, and reliance on earth-abundant elements for sustainable energy storage solutions. However, achieving stability for organic molecules in aqueous batteries remains challenging due to water-induced degradation. The conjugated systems with enhanced rigidity, such as anthraquinone or phenazine derivatives, have shown improved stability, yet often exhibit unwanted restructuring reactions in the reduced state due to overly high electron densities, leading to the loss of redox activity. In the present work, we introduce a resonance hybrid, 2,3-dihydroxyl substituted phenazine dication (HSPC), whose redox center is distributed among different resonance forms between phenazine and quinone to enable efficient reversible electron transfer in aqueous environments. The electrochemical synthesis, resonance structure, and key properties of HSPC were investigated through a framework of combined electrochemical measurements, spectroscopic analysis, and theoretical studies. A 1.4 M electron concentration cell based on HSPC exhibits a low capacity decay rate of 0.0009% per cycle, 0.006% per day. These results indicate the significant potential of the resonance hybrid design to build stable redox materials for AORFBs and other energy storage applications.