Understanding the Role of Complexation in the Charge-Transfer Kinetics of the Cu2+ + e ↔ Cu1+ Redox Reaction in Ethaline Deep Eutectic Solvent

Abstract

Deep eutectic solvent (DES) based electrolytes are gaining attention for electrochemical applications. As such, knowledge of the charge transfer kinetics in DES and its dependence on electrolyte composition and temperature is important. Using Cu2+ + e ↔ Cu1+ as a model system, we demonstrate that metal redox reactions in chloride-containing DES media suffer from sluggish charge transfer kinetics. The exchange current density, i0, displayed a peculiar inverse relationship with the bulk chloride concentration, [Cl−]b. The i0 decreased from 7.9 to 3.0 mA/cm2 when [Cl−]b increased from 2.8 to 4.6 M at 30°C. Such dependence is shown to originate from Cl− complexation with reactant and product species. Furthermore, i0 increased with an increase in Cun+ bulk concentration in the 50–200 mM range. Also, increase in temperature elevated i0. The charge transfer coefficient αc, however, remained constant (∼0.5) and was unaffected by Cun+ or Cl− concentrations or by temperature. To explain how charge transfer kinetics depend on the various system parameters, a model incorporating complexation phenomena was developed and its predictions were compared to experiments.