Molecular Resolution Nanostructure and Dynamics of the Deep Eutectic Solvent—Graphite Interface as a Function of Potential

Abstract

Interest in deep eutectic solvents (DESs), particularly for electrochemical applications, has boomed in the past decade because they are more versatile than conventional electrolyte solutions and are low cost, renewable, and non‐toxic. The molecular scale lateral nanostructures as a function of potential at the solid–liquid interface—critical design parameters for the use of DESs as electrochemical solvents—are yet to be revealed. In this work, in situ amplitude modulated atomic force microscopy complemented by molecular dynamics simulations is used to probe the Stern and near‐surface layers of the archetypal and by far most studied DES, 1:2 choline chloride:urea (reline), at the highly orientated pyrolytic graphite surface as a function of potential, to reveal highly ordered lateral nanostructures with unprecedented molecular resolution. This detail allows identification of choline, chloride, and urea in the Stern layer on graphite, and in some cases their orientations. Images obtained after the potential is switched from negative to positive show the dynamics of the Stern layer response, revealing that several minutes are required to reach equilibrium. These results provide valuable insight into the nanostructure and dynamics of DESs at the solid–liquid interface, with implications for the rational design of DESs for interfacial applications.

Effect of Potential on the Nanostructure Dynamics of Ethylammonium Nitrate at a Graphite Electrode

“…The minor influence of applied surface potential on the diameters and heights of the EAN disks indicates that the near‐surface nanostructure do not change much with the applied potential, even though the Stern layer changes substantially. [ 25a,26 ] This is likely because the potential is largely neutralized by the Stern layer, or the changes in the Stern layer structure with potential have little influence on the near‐surface structures for graphite.…”

“…[ 3a,17b,24 ] A few AFM studies experimentally probed the lateral structure of Stern layers of ILs, [ 25 ] and other concentrated electrolytes. [ 26 ] These studies showed that the Stern layer composition responds as expected when potential is changed, but that lateral structures can be unexpectedly complex. [ 25a,26,27 ] The lateral IL nanostructure near an electrode surface has not been studied before, and the effect of an applied potential is unknown.…”

“…[ 26 ] These studies showed that the Stern layer composition responds as expected when potential is changed, but that lateral structures can be unexpectedly complex. [ 25a,26,27 ] The lateral IL nanostructure near an electrode surface has not been studied before, and the effect of an applied potential is unknown. This means numerous dynamic interfacial phenomena remain poorly understood, such as the charging/discharging dynamics at the electrode and the relaxation of overscreened electrodes.…”