Recently, there has been an increasing interest in the electrodeposition of various metals from deep eutectic solvents (DESs) as an alternative to other non-aqueous electrolytes. Beneficial characteristics include low viscosity compared to ionic liquids, a broad electrochemical stability window, and the individual components can be purified separately.^[1] DESs consist of two components, an organic (type III) or inorganic (type IV) salt and a hydrogen bond donor (HBD).^[2,3] If mixed in their eutectic ratio, a liquid is typically formed at room temperature and the freezing-point depression is believed to result from strong interactions between salt and HBD. However, the effect of the different components on this phenomenon has not been studied in detail, yet.

In this study, the impact of various HBDs on the thermal properties is investigated as a function of their molar ratio in relation to the salt. Phase diagrams of DESs types III and IV are generated by differential scanning calorimetry (DSC), and subsequently, the eutectic composition and temperature of the respective mixtures are determined. As copper electrodeposition onto Au(111) single crystal electrodes has already been subject of numerous studies,^[4,5] in this work it has been chosen as reference system to investigate the electrochemical characteristics of DESs by cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance (EQCM). The impact of the salt, the HBD, and their molar ratio on Cu underpotential and bulk deposition and dissolution behavior are studied. We find that these reactions cannot be associated with the freezing-point depression of the DESs, but are instead strongly dependent on the electrolyte composition.

The mechanism of Cu electrodeposition from chloride- and nitrate-containing DESs type III in combination with Cu⁺ and Cu²⁺ salts is affected by the high concentration of ions. Regardless of whether ethylene glycol or trifluoroacetamide is used as HBD, both Cu⁺ and Cu²⁺ species are easily reduced and deposited. The two-electron reduction prevails with presence of ethylene glycol and higher salt content. In all media studied, deposition follows a nucleation and three-dimensional growth mechanism controlled by diffusion. The diffusion rate of the Cu species is strongly related to the molar ratio of salt and HBD.

DESs type IV exhibit a different behavior during electrodeposition and dissolution that is attributed to the HBD containing various functional groups. The HBD also influences the morphology of Cu deposits, as studied by atomic force microscopy (AFM). The crucial role of the anion is demonstrated in the examples of triflate and nitrate.

References

[1] E.L. Smith, A.P. Abbott, K.S. Ryder, Chem. Rev. 2014, 114, 11060-11082.

[2] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun. 2003, 70-71.

[3] A.P. Abbott, J.C. Barron, K.S. Ryder, D. Wilson, Chem. Eur. J. 2007, 13, 6495-6501.

[4] G.L. Borges, K.K. Kanazawa, J.G. Gordon, K. Ashley, J. Richer, J. Electroanal. Chem. 1994, 364, 281-284.

[5] P. Sebastián, E. Gómez, V. Climent, J.M. Feliu, Electrochem. Comm. 2017, 78, 51-55.