Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study the electrolytes containing CuCl2 and CuCl salts in choline chloride (ChCl) and ethylene glycol (EG) mixtures. The study focused on the concentration of 100 mM of both CuCl2 and CuCl with the ratio of ChCl/EG varied from 1:2, 1:3, 1:4 to 1:5. It was found that the Cu2+ and Cu+ have different solvation environments in their first solvation shell. Cu2+ is coordinated by both Cl- anions and EG molecules whereas Cu+ is only solvated by EG. However, both Cu2+ and Cu+ show strong interactions with their second solvation shells which includes both Cl- anion and EG molecules. Considering both the first and second solvation shells, the concentrations of Cu2+ and Cu+ that are free of Cl- coordination in each solution were calculated, which were found to correlate with the exchange current density reported in previous experiments of Cu2+ reduction to Cu+. This finding made a direct connection between atomic solvation structure observed in MD simulations and redox reaction kinetics measured in electrochemical experiments, thus revealing the significance of the solvation environment of reduced and oxidized species in deep eutectic solvents (DESs) for electrokinetics.

Solvation structure, dynamics and charge transfer kinetics of Cu2+ and Cu+ in choline chloride ethylene glycol eutectic electrolytes

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