Indolo[2,3-b]quinoxaline as a Low Reduction Potential and High Stability Anolyte Scaffold for Nonaqueous Redox Flow Batteries

Redox flow batteries(RFBs) are a promising stationaryenergy storagetechnology for leveling power supply from intermittent renewable energysources with demand. A central objective for the development of practical,scalable RFBs is to identify affordable and high-performance redox-activemolecules as storage materials. Herein, we report the design, synthesis,and evaluation of a new organic scaffold, indolo[2,3-b]quinoxaline, for highly stable, low-reduction potential, and high-solubilityanolytes for nonaqueous redox flow batteries (NARFBs). The mixtureof 2- and 3-(tert-butyl)-6-(2-methoxyethyl)-6H-indolo[2,3-b]quinoxaline exhibits a lowreduction potential (-2.01 V vs Fc/Fc(+)), high solubility(>2.7 M in acetonitrile), and remarkable stability (99.86% capacityretention over 49.5 h (202 cycles) of H-cell cycling). This anolytewas paired with N-(2-(2-methoxyethoxy)-ethyl)phenothiazine(MEEPT) to achieve a 2.3 V all-organic NARFB exhibiting 95.8% capacityretention over 75.1 h (120 cycles) of cycling.