A bipolar verdazyl radical for a symmetric all-organic redox flow-type battery

A symmetric all-organic non-aqueous redox flow-type battery was investigated employing the neutral small molecule radical 3-phenyl-1,5-di-p-tolylverdazyl, which can be reversibly oxidized and reduced in one-electron processes, as the sole charge storage material. Cyclic voltammetry of the verdazyl radical in 0.5 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile revealed redox couples at − 0.17 V and − 1.15 V vs. Ag+/Ag, leading to a theoretical cell voltage of 0.98 V. From the dependence of peak currents on the square root of the scan rate, diffusion coefficients on the order of 4 × 10−6 cm2 s−1 were demonstrated. Cycling performance was assessed in a static cell employing a Tokoyuma AHA anion exchange membrane, with 0.04 M verdazyl as catholyte and anolyte in 0.5 M TBAPF6 in acetonitrile at a current density of 0.12 mA cm−2. Although coulombic efficiencies were good (94%–97%) throughout the experiment, the capacity faded gradually from high initial values of 93% of the theoretical discharge capacity to 35% by the 50th cycle. Voltage and energy efficiencies were 68% and 65%, respectively. Post-cycling analysis by cyclic voltammetry revealed that decomposition of the active material with cycling is a leading cause of cell degradation.