Enhanced cyclability of organic redox flow batteries enabled by an artificial bipolar molecule in neutral aqueous electrolyte

Organic redox flow batteries receive increasing interest for large-scale electrical energy storage owing to the structural diversity and elemental abundance of organic compounds. However, their implementation is challenged by the cyclability, which is an order of magnitude lower than traditional redox flow batteries using inorganic redox couples. The unsatisfactory cyclability mainly arises from electrolyte contamination and irreversible losses of active material in the electrolyte. To address such issues, a highly water-soluble artificial bipolar molecule containing cathode-active ferrocene moiety and anode-active bipyridinium moiety is realized and used as the sole active material in a symmetric organic redox flow cell. Owing to the good reversibility, minimum electrolyte contamination, and stability of the synthesized molecule, the demonstrated cell boosts the cyclability to 4000 cycles with capacity retention of ca. 75% at 100% depth of discharge, which fills in the gap of cyclability between organic redox flow batteries and conventional redox flow batteries.