A (verdazyl) radical approach to a symmetric all-organic redox flow-type battery

A redox flow battery (RFB) is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange occurs through the membrane while both liquids circulate in their own respective space. RFBs are a promising large-scale energy storage solution for integrating renewable energy, such as from solar, wind, or other sources, with electrical grids. Presently, the most well studied class of RFBs is that based on aqueous electrolytes, including the vanadium redox flow battery which employs the VO²⁺/VO²⁺ couple as catholyte and the V³⁺/V²⁺ couple as anolyte. More recently, studies have been aimed at increasing energy densities by employing organic solvents to accommodate redox couples separated by larger gaps in potential. These include metal-based active materials and all-organic systems. Organic active materials have the advantage of being tunable (to increase solubility, stability and/or redox potential) and offer the potential for low cost and environmentally friendly scalability compared to metal-based systems which require environmentally intensive mining and processing.

Among the systems involving organic active materials, there have been a few recent studies employing a single active material, with three stable oxidation states, as both catholyte and anolyte. Like vanadium RFBs, these symmetrical RFB designs offer the advantage that crossover of active species does not irreversibly damage the cell, which is a major issue in designs involving distinct catholyte and anolyte materials. To further advance this, a group of researchers from the Department of Chemistry at University of New Brunswick in Canada: Grant Charlton and Dr. C. Adam Dyker in collaboration with Dr. Stephanie M. Barbon and Dr. Joe Gilroy at the Department of Chemistry at The University of Western Ontario designed symmetric all-organic non-aqueous redox flow-type battery using the neutral small molecule radical 3-phenyl-1,5-di-p-tolylverdazyl, as the sole charge storage material. In their experiments, they focused their research efforts on assessing the suitability of a verdazyl radical as the anode- and cathode-active species in a symmetrical redox flow-type battery. Their work is currently published in Journal of Energy Chemistry.

To begin with, cyclic voltammetry of the verdazyl radical in 0.5 M tetrabutylammonium hexafluorophosphate in acetonitrile revealed redox couples at −0.17 V (anolyte couple: anion/radical) and −1.15 V (catholyte couple: radical/cation) vs. Ag⁺/Ag, leading to a theoretical cell voltage of 0.98 V. Moreover, 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.

The authors then investigated a symmetrical, static battery employing acetonitrile solutions of 3-phenyl-1,5-di-p-tolylverdazyl as both catholyte- and anolyte-active species. Charge/discharge experiments revealed high utilization of active materials during initial charge (98%) and discharge (93%), but cycle life was limited. Post-cycling analysis of the electrolytes by cyclic voltammetry suggested that the decomposition of the anionic species in the anolyte was likely limiting the lifetime of the cell. In their study, voltage and energy efficiencies of 68% and 65%, respectively, were reported.

In summary, Charlton and colleagues demonstrated the first symmetric flow-type battery based on a verdazyl radical. The authors highlight that the exceptional results of the first cycle warrant further investigation on verdazyl systems for energy storage, where molecular design in combination with careful choice of supporting solvent and salt should allow for increasing cycle life. This work should provide impetus for further development and refinement of such verdazyl radical RFBs.