Unraveling the Electrochemistry of Verdazyl Species in Acidic Electrolytes for the Application in Redox Flow Batteries

Verdazyl radicals are a promising class of materials for the potential application in organic redox flow batteries. Although studies on the redox behavior of verdazyl radicals in organic solvents exist, only the disproportionation in aqueous acidic electrolytes is known. Herein, we address this knowledge gap by analyzing the verdazyl species after disproportionation with a series of electrochemical measurements, quantum-chemical calculations, and chemical characterizations to evaluate the redox chemistry. We thereby focused on the parent 1,3,5-triphenylverdazyl radical to potentially expand the findings to other verdazyl derivatives. Two electrons participate in the investigated electrochemical reaction in acidic electrolytes, compared to the two distinguishable single-electron transfers in organic electrolytes. Participation of two protons in the reaction leads to a pH-dependent potential in the investigated pH window. The oxidized and reduced forms were investigated separately and exhibit reaction rate constants in the range of 10–2 cm/s. Based on the obtained results, a detailed description of the redox behavior of verdazyl radicals is proposed. The high rate constants for the reaction connected with the utilization of two electrons pave the way to apply this material class in aqueous redox flow batteries, pending further structural modifications of the verdazyl radical.