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.

In Situ Raman Monitoring of Potential-Dependent Adlayer Structures on the Au(111)/Ionic Liquid Interface

Probing the adlayer structures on an electrode/electrolyte interface is one of the most important tasks in modern electrochemistry for clarifying the electrochemical processes. Herein, we have combined cyclic voltammetry and electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy techniques to explore the potential-dependent adlayer structures on Au(111) in a room-temperature ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) without or with pyridine (Py). It is clearly found that the BMI⁺ cations strongly adsorb on the negatively charged surface with a flat-lying orientation, leaving a little space for Py adsorption. Upon increasing the potentials of the electrode, the variations of Raman band intensities and frequencies reveal that the interaction between the BMI⁺ cations and the Au surface becomes weak; meanwhile, the Py adsorption becomes strong, and its geometry turns from flat, tilted to vertical. Finally, BMI⁺ cations desorb and leave plenty of surface sites for Py adsorption in bulk solution, and a N-bonded compact Py adlayer is formed on the very positively charged surface. This causes obvious anodic peaks in cyclic voltammograms, and the peak currents increase with the square root of the scanning rate. The present work provides a fair molecular-level understanding of electrochemical interfaces and molecular adsorption of Py in ionic liquids.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

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.

Visualizing an Electrochemically Induced Radical Cation of Bipyridine at Au(111)/Ionic Liquid Interfaces toward a Single-Molecule Switch

Room-temperature ionic liquids (RTILs) emerged as ideal solvents, and bipyridine as one of the most used ligands have been widely employed in surface science, catalysis, and molecular electronics. Herein, in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and STM break junction (STM-BJ) technique has been employed to probe the electrochemical process of bipyridine at Au(111)/IL interfaces. It is interestingly found that these molecules undertake a redox process with a pair of well-defined reversible peaks in cyclic voltammograms (CVs). The spectroscopic evidence shows a radical cation generated with rising new Raman peaks related to parallel CC stretching of a positively charged pyridyl ring. Furthermore, these electrochemically charged bipyridine is also confirmed by electrochemical STM-BJ at the single-molecule level, which displays a binary conductance switch ratio of about 400% at the redox potentials. This present work offers a molecular-level insight into the pyridine-mediated reaction process and electron transport in RTILs.

Computational approach to (ZnS)_{i} nanoclusters in ionic liquids

Unique and attractive properties have been predicted for II-VI-type semiconductor nanoclusters within the field of nanotechnology. However, the low reaction kinetics within the usual solvents gives only thermodynamic control during their production process, making the obtention of different metastable polymorphs extremely difficult. The use of ionic liquids as solvents has been proposed to overcome this problem. Identifying how these nanoclusters are solvated within ionic liquids is fundamental if this strategy is to be pursued. While computational chemistry tools are best suited for this task, the complexity and size of the system requires a careful design of the simulation protocol, which is put forward in this work. Taking as reference the (ZnS)_{12} nanocluster and the [EMIM][EtSO_{4}] ionic liquid, we characterize the interactions between the nanoparticle and first solvation shell by density functional theory calculations, considering most of the solvent implicitly. The DFT results are consistent through different theory levels showing a strong interaction between the Zn atoms of the nanocluster and the [EtSO_{4}^{-}] anion of the ionic liquid. A more realistic representation of the system is obtained by classical MD calculations, for which various classical force fields were considered and several atomic interactions parameterized. This new set of parameters correctly describes the interaction of different (ZnS) nanoclusters, supporting its transferability. The resulting MD simulation shows the formation of a structured ionic liquid solvation shell around the nanocluster with no exchange of ions for at least 5 ns, in agreement with the strong interactions observed in the density functional theory calculations.

Probing Interfacial Electronic Effects on Single-Molecule Adsorption Geometry and Electron Transport at Atomically Flat Surfaces

Clarifying interfacial electronic effects on molecular adsorption is significant in many chemical and biochemical processes. Here, we used STM breaking junction and shell-isolated nanoparticle-enhanced Raman spectroscopy to probe electron transport and adsorption geometries of 4,4'-bipyridine (4,4'-BPY) at Au(111). Modifying the surface with 1-butyl-3-methylimidazolium cation-containing ionic liquids (ILs) decreases surface electron density and stabilizes a vertical orientation of pyridine through nitrogen atom σ-bond interactions, resulting in uniform adsorption configurations for forming molecular junctions. Modulation from vertical, tilted, to flat, is achieved on adding water to ILs, leading to a new peak ascribed to CC stretching of adsorbed pyridyl ring and 316 % modulation of single-molecule conductance. The dihedral angle between adsorbed pyridyl ring and surface decreases with increasing surface electronic density, enhancing electron donation from surface to pyridyl ring.

The metal–ionic liquid interface as characterized by impedance spectroscopy and in situ scanning tunneling microscopy

Electrochemical measurements including impedance spectroscopy and in situ scanning tunneling microscopy were performed to study the interface between solid electrodes and ionic liquids. We could reveal that the double layer rearrangement processes are not instantaneous, but that the ions can form ordered clusters at the interface.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

Core-shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core-shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core-shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core-shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core-shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO₂ are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Understanding degradation of platinum catalysts during oxygen reduction is vital for improving proton-exchange membrane fuel cells. Here, the authors identify intermediate stages in the oxidation of Pt(111) and Pt(100) in perchloric acid using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy.