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

Aqueous organic redox flow batteries (AORFBs) have become increasing attractive for scalable energy storage. However, it remains challenging to develop high voltage, powerful AORFBs because of the lack of catholytes with high redox potential. Herein, we report methyl viologen dibromide ([MV]Br₂) as a facile self-trapping, bipolar redox electrolyte material for pH neutral redox flow battery applications. 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^-) and addresses the toxicity concern of volatile bromine simultaneously. A 1.53 V bipolar MV/Br AORFB delivered outstanding battery performance at pH neutral conditions, specifically, 100% total capacity retention, 133 mW/cm² power density, and 60% energy efficiency at 40 mA/cm².