Electrochemical Behavior of Redox Active Organic Molecules in Ethaline Deep Eutectic Solvent

Abstract

Deep eutectic solvents (DES) are a relatively new class of electrolytes with potential application in electrochemical systems. DES are mixtures of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) that engage in a unique hydrogen-bonding network. Like traditional organic solvents, DES have the advantage of a wide electrochemical window compared to aqueous electrolytes, but with additional advantages over organic solvents such as low volatility and high energy density due to solvation strength towards redox active species. DES have shown promise for application in redox flow battery systems currently limited by lack of nonvolatile, high energy density electrolytes. While DES offer a potential solution to these issues, high viscosity and low conductivity have thus far limited their practical application. Catechol and 1,4-benzoquinone were introduced as redox active species and hydrogen bond disruptors to ethaline DES which is a 1:2 molar mixture of choline chloride and ethylene glycol. These redox active organic molecules are of particular interest due to their high solubility in DES. Furthermore, the reduction mechanism of these molecules does not involve electrodeposition and subsequent electrode morphology changes. This addresses issues often associated with traditional redox active species such as metal halides used in redox flow batteries. The physical and electrochemical properties of ethaline with catechol and 1,4-benzoquinone were investigated. Effects of the DES hydrogen-bonding network on the redox potentials, electrochemical reversibility, and diffusivity was investigated by voltammetry. Temperature dependent viscosity, conductivity, and density were measured to investigate the impact of hydrogen bond disruptors on macroscopic properties. Finally, attenuated total reflectance infrared spectroscopy was used to investigate the molecular interactions between the redox active species and the hydrogen-bonding network in DES to relate observed macroscopic properties to molecular level features. This work provides insight into the solvation of organic redox active species in DES and its impact on physical and electrochemical properties.