A PEGylated Viologen for Crossover‐Free and High‐Capacity pH‐Neutral Aqueous Organic Redox Flow Batteries

Abstract Aqueous organic redox flow batteries (AORFBs), which exploit the reversible electrochemical reactions of water soluble organic electrolytes to store electricity, gain increasing momentum for the grid‐scale integration of renewable electricity. However, the crossover of organic electrolytes across the membrane and the limited capacity significantly impede the widespread adoption of AORFBs. Here we report a PEGylated viologen anolyte bearing four positive charges, namely BTMAE−Vi, for crossover‐free and high capacity pH‐neutral AORFBs. This anolyte is rationally designed by appending positively charged ethylene glycol side chains to raise water solubility and increase the molecular size. The solubility of BTMAE−Vi is increased to 3.4 M because of an improved molecular polarity and the existence of four positive charges, while membrane crossover rate is significantly reduced. A BTMAE−Vi‐based flow cell demonstrates an extremely high capacity retention rate of 99.996 % per cycle or 99.979 % per hour when the first reversible redox reaction is utilized, but the irreversible molecular decomposition of the doubly reduced BTMAE−Vi results in severe capacity fade. Our results and the proposed concept in mitigating membrane crossover and elevating capacity will pave the way for developing high‐performance AORFBs.