Cyclohexanedione as the negative electrode reaction for aqueous organic redox flow batteries

Electrochemical characterization indicates that the redox reaction of cyclohexanedione is a proton-coupled electron transfer process with quasi-reversible behavior in acidic media (pH < 3). Among three isomeric compounds (1,2-, 1,3- and 1,4-cyclohexanedione), the reduction of 1,3-cyclohexanedione exhibits the most negative electrode potential (c.a. −0.6 V vs. Ag|AgCl (c.a. −0.4 V vs. NHE)) as well as the widest pH operating range (pH 1–5) for relatively reversible reactions. The resulting electrode potential is the most negative of those to have been reported in neutral/acidic electrolytes. 1,3-cyclohexanedione is subsequently used as the active species in the negative electrode of a parallel plate flow cell, which is charge-discharge cycled at 3.4 mA cm−2for 100 cycles, yielding half-cell coulombic efficiencies of c.a. 99%. The organic molecules derived from this group are observed to have high solubilities (>2 M) and exhibit reduction process with up to 4 electrons transferred.