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 (TBAPF(6)) 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 x 10(-6) cm(2) 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 TBAPF(6) 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. Postcycling analysis by cyclic voltammetry revealed that decomposition of the active material with cycling is a leading cause of cell degradation. (C) 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.