Electron‐Rich Para ‐Methylpyridine‐Functionalized TEMPO Empowers Long‐Lived Neutral Aqueous Organic Redox Flow Batteries

Abstract

2,2,6,6‐Tetramethyl‐1‐Piperidinyloxy (TEMPO), one of the most stable nitroxide radicals, has been widely used as a catholyte for aqueous organic redox flow batteries (AORFBs) owing to its high potential and reversible redox characteristics. In spite of improved stability and water‐solubility after several modification strategies, the limited conjugation of the molecular structure and low electron cloud density on the hydrophilic side still hinder its practical applications. Herein, an electron‐rich regulation strategy is employed to obtain three TEMPO catholytes, that is, ortho/meta/para‐ MePy‐TEMPO, which are prepared using a simple and efficient method on account of 2/3/4‐methyl‐pyridinium‐appendant N ‐acetylamino‐functionalization. In comparison to TEMPO analogues, para‐ MePy‐TEMPO has the highest reaction yield and the best battery stability. When paired with diol‐Vi anolyte, the 0.1 M system can maintain unprecedented capacity retention of 99.69% over 5310 cycles under 40 mA·cm −2 current density, and the 1.5 M system can also hold 99.84% capacity over 250 cycles (60 mA·cm −2 ), alongside with 62.3% energy efficiency. The study can not only enrich the promising candidates for TEMPO‐based catholytes but also lay a solid foundation for high‐performance AORFBs.