Toward an Atomistic Understanding of Deep Eutectics Solvents Electrochemical Interfacial Structure

Green, stable, and wide electrochemical window deep eutectics solvents (DESs) are ideal candidates for electrochemical systems, such as batteries, supercapacitors, catalysis, electrodeposition, and many more. However, since these mixtures are highly dense and composed of only large asymmetric ionic and molecular components, short-range ion-ion, ion-molecule interactions play a dominant role both in the bulk as well as at the interface properties. This results in breakdown of the standard mean-field approximation, which is the basis of dilute solution theory. In addition, due to the hygroscopic nature of DESs, the presence of latent water is unavoidable. Therefore, understanding the DESs-electrode interface (electrical double layer structure, EDLS) together with the role of water at a molecular scale is of great importance for the widespread use of these solvents in electrochemical systems [1-3]. In this presentation, we will first review the benchmark study on conventional dilute solution electrical double layer structure and secondly present our recent findings and theoretical developments on the electrochemical interfacial structure of deep eutectic solvents using a combined experimental and a novel atomistic molecular dynamics approach. In the presentation, we focused on the computational approach. The use of such an atomistic-molecular approach allows investigation of all the possible bulk and interfacial interactions and, consequently, helps to shed light on the nanoscale electrochemical interfacial structure of DESs. Unlike the interfacial structures observed and proposed for other electrolyte-electrode interfaces (compact- diffuse double layer for dilute solution [4]; overscreening or crowding for concentrated solutions [5]), the simulations of DESs-electrode interfacial structure show an unexpected and previously unrecognized phenomenon: the electrochemical interface is composed of a mixed layer structure followed by a mixed charged clustered layer regardless of the surface polarization [1,2]. Some of the findings will be discussed in this presentation. References [1] M. H. Mamme, S. Moors, H. Terryn, J. Deconinck, J. Ustarroz, and F. De Proft, “Atomistic Insight into the Electrical Double Layer of Choline Chloride-Urea Deep Eutectic Solvents: Clustered Interfacial Structuring”, JPCL,9:6296,2018. [2] El A. M. Cherigui, K. Sentosun, M. H. Mamme, M. Lukaczynska, S. Bals, H. Terryn, and J. Ustarroz, “On the Control and Effect of Water Content During the Electrodposition of Ni Nanostructure from Deep Eutectic Solvents”, JPCC, 122: 23129, 2018. [3] M. H. Mamme, S. Moors, El A. M. Cherigui, H. Terryn, J. Deconinck, J. Ustarroz, and F. De Proft, “Water Distribution at the Electrified interface of Deep Eutectics solvent", under review, 2019. [4] G. Jiang, C. Cheng, D. Li, J.Z. Liu, “Molecular dynamics simulations of the electric double layer capacitance of graphene electrodes in mono-valent aqueous electrolytes.” Nano Res. 9, 174−186,2016. [5] M.V. Fedorov, A. A. Kornyshev, “Ionic liquids at electrified interfaces.” Chem. Rev. 114, 2978−3036, 2014.