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

Abstract

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 1,3,5triphenylverdazyl 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.

Static theoretical investigations of organic redox active materials for redox flow batteries

“…As a result, oxidation potentials of around 0.75 V were predicted, but unreliably high potentials (⩾3.5 V vs. SHE) were obtained for several electron-deficient systems with strong withdrawing substituents such as -NO 2 . The unreliable results may be due to the [120,121], CO, CH2 for (a) [118]). incorrect description of nitro compounds at the DFT level of theory with a corresponding overestimation of the free energies of the reactions.…”

Coordination-Induced Bond Weakening and Electrocatalytic Proton-Coupled Electron Transfer of a Ruthenium Verdazyl Complex

“…Verdazyl radicals and their metalated counterparts provide just such an opportunity. Verdazyls and metalloverdazyl complexes have principally been studied for their unique magnetic and electronic properties, but the weak N–H bonds of the corresponding leucoverdazyls make them interesting candidates as PCET reagents. − Furthermore, coordination-induced changes in the verdazyl reactivity have already been reported. An N-benzylated leucoverdazyl exhibits a 3.8 kcal/mol decrease in N–C bond dissociation enthalpy (BDE) upon coordination to Ru­(hfac) 2 (Figure ).…”

Recent Developments in Materials and Chemistries for Redox Flow Batteries

“…8 Another aspect of molecular design is to explore new structures of molecular core, such as the parent 1,3,5-triphenylverdazyl radical that can show a twoelectron transfer reaction associated with a high rate constant in acidic electrolytes. 7 Studying redox chemistries in nonaqueous electrolytes is another appealing direction in redox flow batteries considering their potentially wider voltage windows for high-energy storage systems. To enable high-voltage flow batteries, the major focus is to design redox-active materials that can enable an extremely low or high redox potential in organic solvents as the anolyte or catholyte, respectively.…”

“…Aqueous redox flow batteries typically offer the promising characteristics of high safety, high power density, and economic sustainability, but the limited energy density and cycling stability remain as key challenges . These original research articles and timely reviews (refs − ) that are highlighted report redox-active organic materials that can show reversible multiredox reactions, reasonable solubility, appropriate redox potential in aqueous electrolytes, including viologen derivatives, verdazyl radicals, and phenylene-bridged bipyridine species . For rational molecular design, it is essential to understand the fundamental relationship between the chemical structures and relevant physiochemical properties (e.g., solubility, potential, redox reversibility, stability).…”

“…In addition, the viologen-based structure modified with a bridging phenylene and quaternary ammonium terminals can enable high solubility, low redox potential (−0.77 V vs SHE), and electrochemical stability, resulting in a promising anolyte . Another aspect of molecular design is to explore new structures of molecular core, such as the parent 1,3,5-triphenylverdazyl radical that can show a two-electron transfer reaction associated with a high rate constant in acidic electrolytes …”