Characterization of Electrode-Electrolyte Interfaces in Deep Eutectic Solvents

Deep eutectic solvents (DESs), compromised of hydrogen bond donors and acceptors, are concentrated electrolytes with extremely low volatility and tunable physical properties, and therefore they are promising for large scale energy storage. The electrode-electrolyte interfaces of concentrated and complex electrolytes such as DESs have not been rigorously studied. Owing to the similarity of DESs to ionic liquids (ILs) where a camel shape potential-dependent capacitances have been reported with significant deviations from dilute electrolytes, it will be critical to understanding the DES behavior near polarized surfaces to inform electrochemical kinetics. In this work, the excess charge accumulation of mixtures of choline chloride and ethylene glycol with 1:2 (Ethaline DES composition), 1:4 and 1:6 molar ratios are probed on silver and glassy carbon electrodes via electrochemical impedance spectroscopy. To study the surface species and changes with applied potential, in-situ surface-enhanced Raman spectroscopy was employed on an electrochemically roughened silver substrate as the electrode. Results from these studies suggest Ethaline DES is different than most ILs and present u-shaped differential capacitance and more in line with regular electrolytes. We will present the results from our ongoing studies of the electrode-DES interfaces outlining the impact of composition and electrode material.