Desymmetrized hexasubstituted [3]radialene anions as aqueous organic catholytes for redox flow batteries.

Negatively substituted trimethylenecyclopropane dianions, a subclass of hexasubstituted [3]radialenes, are candidates for use as active species in redox flow batteries (RFBs) due to their stability in water, reversible electrochemistry, and tailorable synthesis. Hexacyano[3]radialene disodium is investigated as a pH 7 aqueous organic catholyte. The dianion and radical anion are stable in air and aqueous solutions at neutral pH. Systematic introduction of asymmetry via step-wise synthesis leads to enhanced solubility and higher capacity retention during galvanostatic cycling. An aqueous flow cell comprising a diester-tetracyano[3]radialene catholyte, sulfonated-methyl viologen as the anolyte, and a cation exchange membrane provides an operating V = 0.9 V, 99.609% coulombic efficiency, and minimum capacity fade over 50 cycles.