Water in Ionic Liquids at Electrified Interfaces: The Anatomy of Electrosorption

Complete removal of water from room-temperature ionic liquids is nearly impossible. For the electrochemical applications of ionic liquids, how water is distributed in the electrical double layers when the bulk liquids are not perfectly dry can potentially determine whether key advantages of ionic liquids, such as a wide electrochemical window, can be harnessed in practical systems. In this paper, we study the adsorption of water on electrode surfaces in contact with humid, imidazolium-based ionic liquids using molecular dynamics simulations. The results revealed that water molecules tend to accumulate within sub-nanometer distance from charged electrodes. At low amount of water in the bulk, the distributions of ions and of electrostatic potential in the double layer are affected weakly by the presence of water, but the spatial distribution of water molecules is strongly dependent on both. The preferential positions of water molecules in double layers are determined by the balance of several factors: the tendency to follow the positions of the maximal absolute value of the electrical field, the association with their ionic surroundings, and the propensity to settle at positions where more free space is available. The balance between these factors changes with charging the electrode, but the adsorption of water generally increases with voltage. The ion specificity of water electrosorption is manifested in the stronger presence of water near positive electrodes (where anions are the counterions) than near negative electrodes (where cations are counterions). These predictions await experimental verification

Less Is More: Can Low Quantum Capacitance Boost Capacitive Energy Storage?

We present a theoretical analysis of charge storage in electrochemical capacitors with electrodes based on carbon nanotubes. Using exact analytical solutions supported by Monte Carlo simulations, we show how the limitations of the electron density of states in such low-dimensional electrode materials may help boost the energy stored at increased voltages. While these counterintuitive predictions await experimental verification, they suggest exciting opportunities for enhancing energy storage by rational engineering of the electronic properties of low-dimensional electrodes

Less Is More: Can Low Quantum Capacitance Boost Capacitive Energy Storage?

We present a theoretical analysis of charge storage in electrochemical capacitors with electrodes based on carbon nanotubes. Using exact analytical solutions supported by Monte Carlo simulations, we show how the limitations of the electron density of states in such low-dimensional electrode materials may help boost the energy stored at increased voltages. While these counterintuitive predictions await experimental verification, they suggest exciting opportunities for enhancing energy storage by rational engineering of the electronic properties of low-dimensional electrodes

Mechanisms of Electrotunable Friction in Friction Force Microscopy Experiments with Ionic Liquids

Using molecular dynamics simulations and a coarse-grained model of ionic liquids (ILs), we study mechanisms of electrotunable friction measured in friction force microscopy experiments, where only one layer of IL is present between the tip and the electrode (substrate). We show that the variation of the friction force with the electrode surface charge density is determined by the regime of motion of the confined IL relative to the substrate and tip. The latter depends on the strengths of the ion–substrate and ion–tip interactions and on the commensurability between the characteristic ion dimensions and lattice spacings of the substrate and tip surfaces. Related with those factors, our simulations predict two strictly different scenarios for the variation of the friction force with the electrode surface charge. Revealing mechanisms of frictional energy dissipation in nanoscale IL films offers a way for controlling friction by tuning ion–substrate interactions and electrical polarization of sliding surfaces

Orientational Ordering in Nano-confined Polar Liquids

Water and other polar liquids exhibit nanoscale structuring near charged interfaces. When a polar liquid is confined between two charged surfaces, the interfacial solvent layers begin to overlap, resulting in solvation forces. Here, we perform molecular dynamics simulations of polar liquids with different dielectric constants and molecular shapes and sizes confined between charged surfaces, demonstrating strong orientational ordering in the nanoconfined liquids. To rationalize the observed structures, we apply a coarse-grained continuum theory that captures the orientational ordering and solvation forces of those liquids. Our findings reveal the subtle behavior of different nanoconfined polar liquids and establish a simple law for the decay length of the interfacial orientations of the solvents, which depends on their molecular size and polarity. These insights shed light on the nature of solvation forces, which are important in colloid and membrane science, scanning probe microscopy, and nano-electrochemistry

Dynamic Charge Storage in Ionic Liquids-Filled Nanopores: Insight from a Computational Cyclic Voltammetry Study

Understanding the dynamic charge storage in nanoporous electrodes with room-temperature ionic liquid electrolytes is essential for optimizing them to achieve supercapacitors with high energy and power densities. Herein, we report coarse-grained molecular dynamics simulations of the cyclic voltammetry of supercapacitors featuring subnanometer pores and model ionic liquids. We show that the cyclic charging and discharging of nanopores are governed by the interplay between the external field-driven ion transport and the sloshing dynamics of ions inside of the pore. The ion occupancy along the pore length depends strongly on the scan rate and varies cyclically during charging/discharging. Unlike that at equilibrium conditions or low scan rates, charge storage at high scan rates is dominated by counterions while the contribution by co-ions is marginal or negative. These observations help explain the perm-selective charge storage observed experimentally. We clarify the mechanisms underlying these dynamic phenomena and quantify their effects on the efficiency of the dynamic charge storage in nanopores

Plasmonic Ruler at the Liquid–Liquid Interface

We report on a simple, fast, and inexpensive method to study adsorption and desorption of metallic nanoparticles at a liquid/liquid interface. These interfaces provide an ideal platform for the formation of two-dimensional monolayers of nanoparticles, as they form spontaneously and are defect-correcting, acting as 2D “nanoparticle traps”. Such two-dimensional, self-assembled nanoparticle arrays have a vast range of potential applications in displays, catalysis, plasmonic rulers, optoelectronics, sensors, and detectors. Here, we show that 16 nm diameter gold nanoparticles can be controllably adsorbed to a water/1,2-dichloroethane interface, and that we can control the average interparticle spacing at the interface over the range 6–35 nm. The particle density and average interparticle spacing are experimentally assessed by measuring the optical plasmonic response of the nanoparticles in the bulk and at the interface and by comparing the experimental data with existing theoretical results

Optical Properties of Ordered Self-Assembled Nanoparticle Arrays at Interfaces

Nanoplasmonic metamaterials are rapidly finding uses in optical devices. Self-assembled soft matter optical nanostructures are straightforward to manufacture and are low cost, self-healing, and tunable. The simplest way to self-assemble such structures is to bring nanoparticles to interfaces where they can build lattices. While being simple to manufacture, these systems are difficult to model analytically. Here we develop an analytical model that is suitable for interfacial systems, that takes account of interactions of the nanoplasmonic structures at various interfaces and electrodes. The model is applicable to both thin-film and bulk electrodes, and it compares well with numerical calculations. On the basis of our model we propose designs suitable for simple surface-enhanced Raman scattering and optical mirror devices

Self-Assembly and Applications of Ultraconcentrated Nanoparticle Solutions

We demonstrate a highly efficient method for concentrating, purifying and separating gold nanoparticles. The method relies on localized density gradients that can be formed at an aqueous | organic phase interface. We show that this method is able to concentrate aqueous gold nanoparticles to the point where confinement leads to variable interparticle separations. Furthermore, the physical properties of the resulting solution are drastically altered when compared to water. For example, densities higher than 4.5 g/cm³ could be generated without nanoparticle aggregation. As far as we are aware, this is one of the highest reported densities of an aqueous solution at room temperature. Finally, the compositions of the solutions generated are highly dependent on parameters such as particle size and background analyte making this technique highly advantageous for the separation of multimodal NP populations and chemical purification, with 99.5% and >99.9% efficiency, respectively