Surveying the structural dependence of electrochemical properties and performance of verdazyl radicals in organic redox flow batteries Matthew J. Milner, Sebastian Fricke, Diddo Diddens, Elena S. Horst, Aleksandr Korshunov, Zhengshuo Huang, Mariano Grünebaum, Andreas Heuer, Martin Winter and Armido Studer Westfälische Wilhelms-Universitat Munster, Germany The quest for a grid powered entirely by renewable energy is driving the search for economic and scaleable energy storage, for which redox flow batteries (RFBs) offer a promising solution. We have reported the feasibility of using stable verdazyl radicals as the redox-active electrolyte in a symmetrical all-organic RFB. 1 The bipolar nature of the radical allows its use on both sides of the battery, limiting the destructive effects of membrane crossover. Meanwhile, the nature of the three appended substituents has dramatic effects on the redox potentials, electrochemical reversibility, solubility, and long-term cycling stability. Subsequent to our proof-of-principle work, we have studied the electrochemical behaviour of a wide variety of both known and newly synthesized verdazyl radicals (>80) using first cyclic voltammetry, and subsequently in-flow cycling experiments (with >15 verdazyls) in a 3D-printed RFB. Free energy relationships and extensive density functional theory calculations allow us to correlate verdazyl structure with performance and demonstrate the tuneability of the radicals. The best example shows significantly improved capacity retention over the first 200 cycles compared to the example from our previous report, as well as high solubility in acetonitrile, and the wide variation in cycling performance across the dataset shows the scope for significant future improvements beyond that already achieved. Finally, we demonstrate the ability to synthesize the verdazyl radicals from their precursors in high yields by an electrochemical oxidation, replacing the need for a stoichiometric chemical oxidant and elevated temperatures. References 1. A. Korshunov, M. J. Milner, M. Grünebaum, A. Studer, M. Winter and I. Cekic-Laskovic , J. Mater. Chem. A , 2020, 8 , 22280- 22291
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