N , N’ ‐Dialkylated Oxo‐Verdazyl Radicals as Bipolar Organic Molecules for Symmetric Redox Flow Batteries

The bipolar electrochemical properties of verdazyl radicals have long been known, and recently their application in symmetric redox flow batteries was investigated. However, the performance of aryl‐substituted Kuhn‐ and oxo‐verdazyl radicals in cycling experiments was unsatisfactory. In this work, N , N’ ‐dialkylated oxo‐verdazyl radicals were investigated as bipolar redox‐active organic molecules (ROM). With a higher theoretical cell voltage than that of Kuhn‐ and arylated oxo‐verdazyl radicals, these stable radicals revealed themselves a promising candidate for battery application. The initial tests of an N , N’ ‐diisopropyl‐verdazyl radical showed moderate cycling, stability and the formation of a new redox‐active species during the cycling between neutral and cationic states was observed. By postcycling analysis, the decomposition product could be identified as the radical lacking an isopropyl group. By subsequent molecular engineering, the performance of the N , N’ ‐dialkylated verdazyl radical could be improved by suppressing the decomposition pathway through variation of the N‐substituent. Overall, the optimized verdazyl radical possesses a theoretical cell voltage of greater than 1.6 V. H‐cell cycling experiments over 200 cycles showed an average Coulombic efficiency of 99.5% and a capacity fade of less than 0.1%/cycle for both oxidation and reduction. Finally, the best‐performing oxo‐verdazyl radicals were investigated under flow conditions.