Biomimetic Design of Amide–Phosphate Functionalized TEMPO Catholyte for High-Performance Nonaqueous Organic Redox Flow Batteries

Nonaqueous organic redox flow batteries (NAORFBs) are desirable candidates for large-scale energy storage applications. 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) derivatives exhibit favorable electrochemical activity, but their practical applications are severely constrained by their instability. Functional group modifications are essential for regulating the redox potential, solubility, and chemical and electrochemical stability. Herein, we propose a biomimetic molecular engineering strategy to synthesize 4-[2-(diethoxyphosphoryl)acetamino]-2,2,6,6-tetramethylpiperidine-1-oxyl (DEPAcNH-TEMPO) modified with amide bonds and diethyl phosphate groups in a one-step route. DEPAcNH-TEMPO has high solubility (1.8 M), excellent electrokinetic properties, and impressive stability. When paired with 1,1'-dibenzyl-4,4'-bipyridinediium dihexafluoro-phosphate (DBV), the DEPAcNH-TEMPO/DBV NAORFB exhibits superior rate performance and improves cycling stability by nearly 20 times compared to unmodified 4-NH₂-TEMPO. These results highlight the importance of rational biomimetic design in suppressing molecular degradation and provide useful guidance for the development of stable, high-performance catholytes for next-generation NAORFBs.