Deep eutectic solvents (DESs) are promising candidate electrolytes in electrochemical fields due to their excellent properties. Compared with their analogues ionic liquids, DESs possess extra merits including simple synthesis process and low cost, which benefit large-scale production in the field of electrochemistry. Current issues of electrodeposition in aqueous solution, such as toxicity of reagents and low current efficiencies, could be overcome by using DESs. Moreover, metal electrodeposition in DESs is of interest because DESs are featured by wider electrochemical windows, sufficiently large diffusion coefficient and proper metal ion solvation. However, information about electrical double layer, particularly the potential-dependent innermost layer structure and electrode surface structure, is still lacking. In this work, we have investigated electrode surface structure and adsorption behaviors of DES components on Au single crystal electrodes in DESs by in situ scanning tunneling microscopy (STM), cyclic voltammetry, electrochemical impedance spectroscopy and Raman spectroscopy. The effect of electrode crystallographic orientation and neutral molecule component has been explored. Cu electrodeposition in a DES has also been studied.
The most commonly used DESs in electrochemistry are mixture of quaternary ammonium salts and hydrogen bond donor molecules (such as glycol, urea, etc.). In DES ethaline, potential-dependent innermost layer structure in electrical double layer at Au(111) electrode has been revealed. It was found that chloride ion adsorption on Au(111) surface results in the lifting of Au(111) surface reconstruction and the abrupt increase of differential capacitance. When the potential shifts positively, a disorder-order phase transition of chloride ion adsorbate is revealed. When the potential shifts negatively, choline cations and ethylene glycol molecules adsorb on Au(111) surface with weak interaction detected by Raman spectroscopy.
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