Water and Hydrogen Bond Donor Effects on Mass Transport and Copper Electrodeposition in Deep Eutectic Solvents

Choline chloride-based deep eutectic solvents (DESs) are promising for electrodeposition due to their high electrochemical stability and ionic conductivity, yet their application can be hindered by high viscosity and uncontrolled water uptake. Here, we systematically investigate how the hydrogen bond donor (HBD) - ethylene glycol (ethaline), glycerol (glyceline), or urea (reline) - and water content (0-16.5 M) affect physicochemical properties, Cu(I) ion diffusion, and Cu electrodeposition. Water addition drastically reduces viscosity of solutions and increases diffusion coefficients of Cu(I) ions up to two orders of magnitude, establishing a quantitative relationship between viscosity, diffusion, and deposition rates. Consequently, the Cu electrodeposition rate increases and the deposit morphology shifts from smooth to faceted crystallites upon water addition. Analysis of the experimental data within the framework of the Stokes-Einstein and Sutherland hydrodynamic models revealed significant deviations of Cu(I) diffusion from the classical dependences. In ethaline and glyceline, the Sutherland coefficient θ ≈ 4 (perfect sliding regime) holds across all water contents. In reline, however, θ falls below 4 under anhydrous conditions, indicating a possible contribution of a hole transport mechanism, whose role diminishes upon water addition. These findings provide a systematic framework for tuning HBD and water content to control metal electrodeposition in DESs.