Potential regulation of bipolar redox-active molecules with fused conjugation for high-voltage symmetric organic redox flow batteries

Abstract

Bipolar redox-active organic molecules (ROMs) are emerging as promising active materials as both anolyte and catholyte for symmetric organic redox flow batteries (ORFBs) which moderating the detrimental electrolyte crossover. However, certain bipolar ROMs suffer seriously from irreversible redox reactions when their redox potential exceeds the electrochemical window of electrolyte, resulting in unpredictable side reactions and serious degradation of battery cycling stability. In this study, we propose that incorporating electron-withdrawing or electron-donating groups into the bipolar ROMs with fused conjugation is capable to tune their redox potentials simultaneously to make them compatible with different electrolytes without sacrificing the output voltage. As a demonstration, we have designed and synthesized a series of bipolar ROMs with fused conjugation based on phenoxazine derivatives (denoted as QPO, QPO-3OMe and DQPO). All the three molecules undergo highly reversible redox reactions in the given electrolyte systems with voltage gap of ca. 2.5 V between two half-reactions. Moreover, symmetric ORFBs based on the bipolar ROMs maintain decent electrochemical stability when cycled in a compatible electrolyte, which the electrochemical window fully covers the voltage gap of the bipolar ROMs. This study provides a strategy for the design of bipolar ROMs with fused conjugation and the regulation of compatibility between the ROMs and the electrolytes, thus contributing to developing symmetric ORFBs with higher voltage and better stability.