Desymmetrized hexasubstituted radialene anions as aqueous organic catholytes for redox flow batteries†
Negatively substituted trimethylenecyclopropane dianions, a subclass of hexasubstituted radialenes, are candidates for use as active species in redox flow batteries (RFBs) due to their stability in water, reversible electrochemistry, and tailorable synthesis. Hexacyanoradialene 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-tetracyanoradialene catholyte, sulfonated-methyl viologen as the anolyte, and a cation exchange membrane provides an operating Vcell = 0.9 V, 99.609% coulombic efficiency, and minimum capacity fade over 50 cycles.