Reversible Redox Chemistry in Pyrrolidinium‐Based TEMPO Radical and Extended Viologen for High‐Voltage and Long‐Life Aqueous Redox Flow Batteries

Abstract Aqueous organic redox flow batteries (AORFBs) are regarded as a promising candidate for grid‐scale, low‐cost and sustainable energy storage. However, their performance is restricted by low aqueous solubility and the narrow potential gap of the organic redox‐active species. Herein, a highly‐soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended viologen, namely Pyr‐TEMPO and [PyrPV]Cl 4 , which exhibits high cell voltage (1.57 V) and long cycling life (over 1000 cycles) in AORFBs is reported. The intrinsic hydrophilic nature of the pyrrolidinium group enables high aqueous solubilities (over 3.35 m for Pyr‐TEMPO and 1.13 m for [PyrPV]Cl 4 ). Furthermore, the interaction of nitroxyl radicals with water is observed, which may be helpful to prevent collision‐induced side reactions or structure decomposition. Notably, the assembled AORFBs realize a high energy density of 16.8 Wh L ‐1 and a peak power density of 317 mW cm ‐2 . The evidence is provided to clarify the capacity degradation mechanism of TEMPO/viologen AORFB systems by a series of comprehensive characterizations. Furthermore, the reversible consumption and re‐generation of the nitroxyl radicals upon charging and discharging are well understood. This work presents effective electrochemical and spectroscopic approaches to clarify the redox chemistry and capacity degradation mechanism of radical incorporating AORFB systems.