<br/>Ionic liquids (ILs) and deep eutectic solvents (DESs) often regarded as concentrated electrolytes due to the high molarity of the salt component. The possibility of high concentrations coupled with tunable solvation strength for a variety of solutes makes these solvents attractive for a variety of applications including energy storage, elecrodeposition processes, extractions and sensors. However, the interfacial characteristics are critical to understand in terms of accessing the capabilities, limitations and future prospects of these electrolytes. In particular, the polarization induced interfacial structure at the electrode-electrolyte interface has a significant role in electron transfer kinetics. We probe these buried interfaces by electrochemical impedance spectroscopy (EIS) and surface enhanced Raman spectroscopy (SERS). While EIS provides a measure of the charge density within the interface between the electrode and the bulk liquid, SERS make available the detection of surface species and suggests molecular orientations at the electrode surface.<br/>The impact of the alkyl chain length, ring vs. aliphatic, and aromatic vs non-aromatic cations, namely the ammonium, imidazolium and pyrrolidinium based ILs, on the interfacial structure are studied on glassy carbon and silver electrodes. In all of the studied systems, a camel shaped differential capacitance is observed as a function of electrode potential. However, the asymmetry and the rate of increase to global maxima from the point of zero charge depend on the ion size and the compacity of the liquid. SERS suggests that for cations with longer alkyl chains (C8 in comparison to C4) encourages suppression of anion-anion repulsions and with increased polarization buckling of chains leads better packing at the interface. For DESs, the hydrogen bonding is believed to control the bulk structure and little is known about the interfacial structure near a charged electrode. We employed the same techniques to a model system based on choline chloride –the hydrogen bond acceptor and ethylene glycol – the hydrogen bond donor. In this presentation, we will compare and contrast these electrolytes against molecular solvents from the view point of their interfacial structures and stabilities.<br/>

Charge density and structure of ionic liquids and deep eutectic solvents near an electrode surface

ACS Fall 2020 Virtual Meeting

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