Highly Soluble and Crossover-Free All-Organic Redox Pair Using N-Heterocycle-Linked Tempo and Two-Electron-Capable Bipyridinium Towards High Performance Aqueous Flow Batteries

Abstract

Aqueous organic redox flow batteries (AORFBs) are the brilliant technologies for safe and sustainable stationary energy storage. However, the cross-contamination, limited cell voltage, and inferior cycling stability remain challenges. Herein, the N-heterocycle-substituted TEMPO (TMP-TEMPO) and pyrrolidinium-/ammonium-grafted bipyridinium ([PyrTMAV]Cl4) redox pair with multiple charges are designed for high-performance AORFBs. The whole material preparation is relatively simple and only needs two or three synthetic steps. The TMP-TEMPO and [PyrTMAV]Cl4 show high aqueous solubility of 2.4 M and 1.71 M, respectively, excellent electrochemical reversibility, and fast redox kinetics. Notably, the introduction of multiple charges into the active materials can produce a strong Gibbs-Donnan effect with ion exchange membranes, thus preventing the permeability of molecules through the membrane and avoiding the cross-contamination of active molecules. By using the crossover-free TMP-TEMPO and two-electron [PyrTMAV]Cl4 redox pair, the assembled two-electron AORFBs at an electron concentration of 0.2 M exhibit stable battery performance in environments with O2 contents of both 10 ppm and 100 ppm with a capacity retention of over 99% per day for hundreds of cycles (268 and 310 cycles), and a high energy efficiency of ~89% and Coulombic efficiency of ~100%. Furthermore, the AORFB achieves a high capacity of ~37.4 Ah L-1 at an ultra-high electron concentration of 1.5 M, while maintaining a capacity retention rate of ~99.7% over 138 cycles and a theoretical capacity utilization rate of 93%. The versatile molecular design for TMP-TEMPO and [PyrTMAV]Cl4 can be easily extended to the synthesis of various amino-functionalized TEMPO and asymmetric bipyridinium derivatives.

Keywords: N-heterocycle functionalized redox-active materials, Crossover-free, high voltage, Aqueous organic redox flow batteries