A Self-Trapping, Bipolar Viologen Bromide Electrolyte for Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) have become increasingly attractive for scalable energy storage. However, it remains challenging to develop high-voltage, powerful AORFBs because of the lack of a high redox potential catholyte. Herein, we report methyl viologen dibromide ([MV]Br2) as a facile self-trapping, bipolar redox electrolyte material for pH-neutral redox flow battery applications, representing the first report of methyl viologen as a highly efficient bromine complexing reagent. The formation of the [MV](Br3)2 complex was computationally predicted and experimentally confirmed. The low-solubility [MV](Br3)2 complex in the catholyte during the battery charge process not only mitigates the crossover of charged tribromide species (Br3–) but also addresses the toxicity concern of volatile bromine simultaneously. A 1.53 V bipolar MV/Br AORFB (10.2 Wh/L) delivered stable battery performance at pH-neutral conditions, specifically, 100% total capacity retention, 133 mW/cm2 power density, and 56% energy efficiency at 60 mA/cm2.