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

Significance 

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.

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

About the author

Grant Charlton received his B.Sc. with a in Medicinal Chemistry from the University of New Brunswick, Canada in 2014. After researching the use of iminophosphorano-substituted pyridines in catalysis as a summer research student, he turned his focus to investigating symmetrical, all-organic redox flow-type batteries for his Honours research project. He now works in industry in the area of quality control.

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About the author

Dr. Stephanie Barbon studied chemistry at the University of Western Ontario, and obtained her PhD in 2017 after working on novel boron-containing dye chemistry in the lab of Professor Joe Gilroy. She then moved to the University of California, Santa Barbara where she held a Banting postdoctoral fellowship and worked on sequence controlled polymer synthesis and block copolymer self-assembly in the lab of Professor Craig Hawker. She is currently a Senior Chemist at Dow Chemical.

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About the author

Joe Gilroy is an Associate Professor in the Department of Chemistry at The University of Western Ontario (aka Western University). Originally from the West Coast of Canada, he completed both his BSc and PhD at the University of Victoria where he conducted research in stable radical chemistry under the supervision of Prof. Robin Hicks.  He moved to the University of Bristol for his postdoctoral studies where he worked in various areas of metallopolymer chemistry with Prof. Ian Manners as an NSERC and EU Marie Curie PDF. At Western, Joe leads a talented team of students and postdocs who work in many different areas of synthetic materials chemistry, with major focuses including the development of luminescent and semiconducting molecular and polymeric materials.

He has received several awards, including the Thieme Chemistry Journal Award, Western’s Petro-Canada Young Innovator and Faculty Scholar Awards, the CNC-IUPAC travel award, and an Ontario Early Researcher Award

About the author

C. Adam Dyker obtained his BSc from the University of New Brunswick in 2002. He completed his PhD in the area catena-phosphorus cations under the supervision of Dr. Neil Burford at Dalhousie University in 2007. As an NSERC Postdoctoral Fellow, he studied under Dr. Guy Bertrand at the University of California, Riverside where he developed stable bent allenes. In January of 2010, Adam joined the Department of Chemistry at the University of New Brunswick as an Assistant Professor.

His current research interests include the development of organic redox flow batteries and the exploitation of ylidic pi-donor substituents for new advances in organic and inorganic chemistry.

Reference

Grant D. Charlton, Stephanie M. Barbon, Joe B. Gilroy, C. Adam Dyker. A bipolar verdazyl radical for a symmetric all-organic redox flow-type battery. Journal of Energy Chemistry, volume 34 (2019) page 52–56.

Go To Journal of Energy Chemistry

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