Electrochemical impedance spectroscopy and Raman spectroscopy studies on electrochemical interface between Au(111) electrode and ethaline deep eutectic solvent

Understanding the Electrode-Electrolyte Interfaces of Ionic Liquids and Deep Eutectic Solvents

Developing unconventional electrolytes such as ionic liquids (ILs) and deep eutectic solvents (DESs) has led to remarkable advances in electrochemical energy storage and conversion devices. However, the understanding of the electrode-electrolyte interfaces of these electrolytes, specifically the liquid structure and the charge/electron transfer mechanism and rates, is lacking due to the complexity of molecular interactions, the difficulty in studying the buried interfaces with nanometer-scale resolution, and the distribution of the time scales for the various interfacial events. This Feature Article outlines the standing questions in the field, summarizes some of the exciting approaches and results, and discusses our contributions to probing the electrified interfaces by electrochemical impedance spectroscopy (EIS), surface-enhanced Raman spectroscopy (SERS), and neutron reflectivity (NR). The related findings are analyzed within electrical double-layer models to provide a framework for studying ILs, DESs, and, more broadly, the concentrated hydrogen-bonded electrolytes.

Unconventional Electrochemical Behaviors of Cu Underpotential Deposition in a Chloride-Based Deep Eutectic Solvent: High Underpotential Shift and Low Coverage

The (5 × 5) Moiré pattern resulting from coadsorption of Cu atoms and chloride ions on the Au(111) electrode is one of the most classical structures for underpotential deposition (UPD) in electrochemical surface science. Although two models have been proposed to describe the pattern, the details of the structure remain ambiguous and controversial, leading to a question that remains to be answered. In this work, we investigate the UPD behaviors of Cu on the Au(111) electrode in a chloride-based deep eutectic solvent ethaline by in situ scanning tunneling microscopy (STM). Benefiting from the properties of the ultraconcentrated electrolyte, we directly image not only Cu but also Cl adlayers by finely tuning tunneling conditions. The structure is unambiguously determined for both Cu and Cl adlayers, where an incommensurate Cu layer is adsorbed on the Au(111) surface with a Cu coverage of 0.64, while the Cl coverage is 0.32 (only half of the expected value); i.e., the atomic arrangement of the observed (5 × 5) Moiré pattern in ethaline matches neither of the models proposed in the literature. Meanwhile, STM results confirm the origin of the cathodic peak in the cyclic voltammogram, which indicates that the underpotential shift of Cu UPD in ethaline indeed increases by ca. 0.40 V compared to its counterpart in a sulfuric acid solution, resulting in a significant deviation from the linear relation between the underpotential shift and the difference in work functions proposed in the literature. The unconventional electrochemical behaviors of Cu UPD reveal the specialty of both the bulk and the interface in the chloride-based deep eutectic solvent.

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

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.

The Role of Water Content of Deep Eutectic Solvent Ethaline in the Anodic Process of Gold Electrode

Traditional coupling of ligands for gold wet etching makes large-scale applications problematic. Deep eutectic solvents (DESs) are a new class of environment-friendly solvents, which could possibly overcome the shortcomings. In this work, the effect of water content on the Au anodic process in DES ethaline was investigated by combining linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Meanwhile, we employed atomic force microscopy (AFM) to image the evolution of the surface morphology of the Au electrode during its dissolution and passivation process. The obtained AFM data help to explain the observations about the effect of water content on the Au anodic process from the microscopic perspective. High water contents make the occurrence of anodic dissolution of gold at higher potential, but enhances the rate of the electron transfer and gold dissolution. AFM results reveal the occurrence of massive exfoliation, which confirms that the gold dissolution reaction is more violent in ethaline with higher water contents. In addition, AFM results illustrate that the passive film and its average roughness could be tailored by changing the water content of ethaline.

Intermolecular Interactions of Edaravone in Aqueous Solutions of Ethaline and Glyceline Inferred from Experiments and Quantum Chemistry Computations

Edaravone, acting as a cerebral protective agent, is administered to treat acute brain infarction. Its poor solubility is addressed here by means of optimizing the composition of the aqueous choline chloride (ChCl)-based eutectic solvents prepared with ethylene glycol (EG) or glycerol (GL) in the three different designed solvents compositions. The slurry method was used for spectroscopic solubility determination in temperatures between 298.15 K and 313.15 K. Measurements confirmed that ethaline (ETA = ChCl:EG = 1:2) and glyceline (GLE = ChCl:GL = 1:2) are very effective solvents for edaravone. The solubility at 298.15 K in the optimal compositions was found to be equal x_E = 0.158 (c_E = 302.96 mg/mL) and x_E = 0.105 (c_E = 191.06 mg/mL) for glyceline and ethaline, respectively. In addition, it was documented that wetting of neat eutectic mixtures increases edaravone solubility which is a fortunate circumstance not only from the perspective of a solubility advantage but also addresses high hygroscopicity of eutectic mixtures. The aqueous mixture with 0.6 mole fraction of the optimal composition yielded solubility values at 298.15 K equal to x_E = 0.193 (c_E = 459.69 mg/mL) and x_E = 0.145 (c_E = 344.22 mg/mL) for glyceline and ethaline, respectively. Since GLE is a pharmaceutically acceptable solvent, it is possible to consider this as a potential new liquid form of this drug with a tunable dosage. In fact, the recommended amount of edaravone administered to patients can be easily achieved using the studied systems. The observed high solubility is interpreted in terms of intermolecular interactions computed using the Conductor-like Screening Model for Real Solvents (COSMO-RS) approach and corrected for accounting of electron correlation, zero-point vibrational energy and basis set superposition errors. Extensive conformational search allowed for identifying the most probable contacts, the thermodynamic and geometric features of which were collected and discussed. It was documented that edaravone can form stable dimers stabilized via stacking interactions between five-membered heterocyclic rings. In addition, edaravone can act as a hydrogen bond acceptor with all components of the studied systems with the highest affinities to ion pairs of ETA and GLE. Finally, the linear regression model was formulated, which can accurately estimate edaravone solubility utilizing molecular descriptors obtained from COSMO-RS computations. This enables the screening of new eutectic solvents for finding greener replacers of designed solvents. The theoretical analysis of tautomeric equilibria confirmed that keto-isomer edaravone is predominant in the bulk liquid phase of all considered deep eutectic solvents (DES).

Spectroscopic Investigation of the Structure of a Pyrrolidinium-Based Ionic Liquid at Electrified Interfaces

The molecular structure of electric double layers (EDLs) at electrode-electrolyte interfaces is crucial for all types of electrochemical processes. Here we probe the EDL structure of an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPy-TFSI), using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS). We extract the position and intensity of individual peaks corresponding to either intra- or inter-molecular vibrational modes, and examine their dependence on the electrode potential. The observed trends suggest that the molecular reconfiguration mechanism is distinct between cations and anions. BMPy+ is found to always adsorb on the Au electrode surface via the pyrrolidinium ring while the alkyl chains strongly change their orientation at different potentials. In contrast, TFSI- is observed to have pronounced position shifts but negligible orientation changes as we sweep the electrode potential. Despite their distinct reconfiguration mechanisms, BMPy+ and TFSI- in the EDL are likely paired together through strong intermolecular interaction.

Effect of hydrogen bond donor molecules ethylene glycerol and lactic acid on electrochemical interfaces in choline chloride based-deep eutectic solvents

Choline chloride (ChCl)-based-deep eutectic solvents (DESs) are widely used in electrochemical fields. In this work, the effect of two types of hydrogen bond donor (HBD) molecules, ethylene glycerol and lactic acid (LA), on electrochemical interfaces between the Au electrode and DESs has been investigated by employing voltammetry and electrochemical impedance spectroscopy. The anodic dissolution and passivation behaviors of the Au electrode are revealed in both ethaline and ChCl:LA. In ChCl:LA, the anodic dissolution of Au is slowed down, and the passivation film is relatively dense and stable due to the existence of the carboxyl group in HBD molecule LA. In the double layer region, the lifting and formation of Au(111) surface reconstruction and a disorder-order phase transition of the chloride ion adlayer were observed in the two DESs. Moreover, compared with ethaline, an extra pair of current peaks appears in ChCl-LA possibly due to the adsorption and desorption of LA on the Au(111) surface, which might imply the stronger interaction of LA with the Au electrode in ChCl:LA. HBD LA could even have marked an impact on the disorder-order phase transition of the chloride ion adlayer. The above results provide new insight into the significant effect of HBD molecules on the anodic dissolution and the passivation of the Au electrode and the electrochemical behaviors in the double layer region.