Kinetics of electrochemical reduction of copper from a choline chloride:ethylene glycol (1:2) medium on a glassy carbon electrode

Abstract

This study investigates the electrochemical reduction of copper in a deep eutectic solvent based on choline chloride and ethylene glycol using cyclic voltammetry and electrochemical impedance spectroscopy. The reduction process is shown to proceed via two consecutive steps: Cu2+/Cu+ and Cu+/Cu0. The stability of Cu+ ions in a choline chloride:ethylene glycol (1:2) medium was demonstrated using a rotating ring-disk electrode. The results indicate that both stages are governed by diffusion-controlled kinetics. Diffusion coefficients were determined using the Randles–Ševčík equation and found to be: DCu2+  = 2.7 × 10-7 cm2 s-1 and  DCu+ = 9.8 × 10-7 cm2 s-1. The charge transfer rate constants, evaluated from the semi-logarithmic relationship (ln jp vs. (Ep – E0′)), were calculated as 5.6 × 10-7 cm s⁻¹ and 3.5 × 10-4 cm s-1. Electrochemical impedance spectroscopy revealed that increasing temperature leads to a decrease in charge transfer resistance from 6.3 to 2.1 Ω cm2, indicating enhanced kinetics of the electrochemical processes. These findings provide deeper insight into the mechanism of copper reduction in deep eutectic solvents and may be useful for developing copper electrodeposition strategies aimed at producing efficient catalysts for electrochemical CO2 reduction.