Solvation Structure, Dynamics, and Charge Transfer Kinetics of Cu²⁺ and Cu⁺ in Choline Chloride Ethylene Glycol...

Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study electrolytes containing CuCl₂ and CuCl salts in mixtures of choline chloride (ChCl) and ethylene glycol (EG). 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 include both Cl^- anions and EG molecules. Considering both the first and second solvation shells, the concentrations of Cu²⁺ and Cu⁺ that have various coordination numbers in each solution were calculated and were found to correlate qualitatively with the exchange current density trends reported in previous experiments of Cu²⁺ reduction to Cu⁺. This finding makes a 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 for electrokinetics in deep eutectic solvents.