Aromatic Diester Anolytes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries require highly‐soluble and easy‐to‐synthesize organic redox‐active molecules that offer deep reduction or high oxidation potentials and that are stable in each state of charge. Herein, aromatic diesters for use as anolytes are investigated. Dimethyl terephthalate exhibits two single‐electron reductions. Compared to its radical anion, the dianion is unstable. Introducing 1,1′‐biphenyl and 2,2′‐bipyridyl aromatic units, stabilizes the diesters in their doubly‐reduced state via extension of the π ‐conjugation and less deep reduction potentials. Solubility is enhanced by using oligo(ethylene glycol) esters. The most promising anolyte that emerges from this work, bis(2‐(2‐ethoxyethoxy)ethyl) [2,2′‐bipyridine]−5,5′‐dicarboxylate, is used in combination with 2,5‐di‐tert‐butyl‐1,4‐bis(2‐methoxyethoxy)benzene as a catholyte in redox flow cell batteries. The mixed system flow battery has a nominal battery voltage of 2.82 V and a capacity retention of 99.5% per cycle for 260 cycles.