Theory and Simulations of Ionic Liquids in Nanoconfinement.

Room-temperature ionic liquids (RTILs) have exciting properties such as nonvolatility, large electrochemical windows, and remarkable variety, drawing much interest in energy storage, gating, electrocatalysis, tunable lubrication, and other applications. Confined RTILs appear in various situations, for instance, in pores of nanostructured electrodes of supercapacitors and batteries, as such electrodes increase the contact area with RTILs and enhance the total capacitance and stored energy, between crossed cylinders in surface force balance experiments, between a tip and a sample in atomic force microscopy, and between sliding surfaces in tribology experiments, where RTILs act as lubricants. The properties and functioning of RTILs in confinement, especially nanoconfinement, result in fascinating structural and dynamic phenomena, including layering, overscreening and crowding, nanoscale capillary freezing, quantized and electrotunable friction, and superionic state. This review offers a comprehensive analysis of the fundamental physical phenomena controlling the properties of such systems and the current state-of-the-art theoretical and simulation approaches developed for their description. We discuss these approaches sequentially by increasing atomistic complexity, paying particular attention to new physical phenomena emerging in nanoscale confinement. This review covers theoretical models, most of which are based on mapping the problems on pertinent statistical mechanics models with exact analytical solutions, allowing systematic analysis and new physical insights to develop more easily. We also describe a classical density functional theory, which offers a reliable and computationally inexpensive tool to account for some microscopic details and correlations that simplified models often fail to consider. Molecular simulations play a vital role in studying confined ionic liquids, enabling deep microscopic insights otherwise unavailable to researchers. We describe the basics of various simulation approaches and discuss their challenges and applicability to specific problems, focusing on RTIL structure in cylindrical and slit confinement and how it relates to friction and capacitive and dynamic properties of confined ions.

Electrochemically controlled rectification in symmetric single-molecule junctions.

Single-molecule electrochemical science has advanced over the past decades and now extends well beyond molecular imaging, to molecular electronics functions such as rectification and amplification. Rectification is conceptually the simplest but has involved mostly challenging chemical synthesis of asymmetric molecular structures or asymmetric materials and geometry of the two enclosing electrodes. Here we propose an experimental and theoretical strategy for building and tuning in situ (in operando) rectification in two symmetric molecular structures in electrochemical environment. The molecules were designed to conduct electronically via either their lowest unoccupied molecular orbital (LUMO; electron transfer) or highest occupied molecular orbital (HOMO; "hole transfer"). We used a bipotentiostat to control separately the electrochemical potential of the tip and substrate electrodes of an electrochemical scanning tunneling microscope (EC-STM), which leads to independent energy alignment of the STM tip, the molecule, and the STM substrate. By creating an asymmetric energy alignment, we observed single-molecule rectification of each molecule within a voltage range of ±0.5 V. By varying both the dominating charge transporting LUMO or HOMO energy and the electrolyte concentration, we achieved tuning of the polarity as well as the amplitude of the rectification. We have extended an earlier proposed theory that predicts electrolyte-controlled rectification to rationalize all the observed in situ rectification features and found excellent agreement between theory and experiments. Our study thus offers a way toward building controllable single-molecule rectifying devices without involving asymmetric molecular structures.

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.

Electrotunable friction with ionic liquid lubricants.

Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction-that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction.

Ionotronics for reverse actuation.

In the midst of an ongoing energy crisis, the search for new methods of energy harvesting has never been more important. Here we explore, analyse and discuss principles of ionotronic reverse-actuator devices based on the effect of double-layer charging. The designs that we consider in this paper operate based on a common principle - using external mechanical work, which would otherwise be wasted, to produce changes in the contact area of electrode and electrolyte, translated into the time variation of the double-layer capacitance. Periodic variation of capacitance, when connected to a reference voltage source, produces alternating electric current through a load. This concept is not new and in some forms was realised in the early works of Boland, Krupenkin and several papers of our group. The goal of the present paper is to build a comprehensive analytical platform for a description of operation of such devices in terms of materials, generated power as a function of the frequency of variation of applied force, electrical load, and other factors; the understanding of which allows us to optimise these systems and navigate their construction. The first design, discussed in the paper, is based on flat electrodes. It is the simplest one and, as such, helps elucidate some key factors determining power generation. While being easy to realise experimentally, it generates relatively low power, even when optimised. The second design, based on microporous electrodes is more sophisticated and allows a much larger power harvest. The results are also compared to the recently proposed capacitive rotor device. The developed theory is set to capture the key factors that determine the functioning of the considered reverse actuators. The structures under study are matched to fit into the sole of a shoe and produce power from walking and running. However, they can also be scaled-up to larger operating systems and various external loads.

Polar liquids at charged interfaces: A dipolar shell theory.

The structure of polar liquids and electrolytic solutions, such as water and aqueous electrolytes, at interfaces underlies numerous phenomena in physics, chemistry, biology, and engineering. In this work, we develop a continuum theory that captures the essential features of dielectric screening by polar liquids at charged interfaces, including decaying spatial oscillations in charge and mass, starting from the molecular properties of the solvent. The theory predicts an anisotropic dielectric tensor of interfacial polar liquids previously studied in molecular dynamics simulations. We explore the effect of the interfacial polar liquid properties on the capacitance of the electrode/electrolyte interface and on hydration forces between two plane-parallel polarized surfaces. In the linear response approximation, we obtain simple formulas for the characteristic decay lengths of molecular and ionic profiles at the interface.

Gelation, clustering, and crowding in the electrical double layer of ionic liquids.

Understanding the bulk and interfacial properties of super-concentrated electrolytes, such as ionic liquids (ILs), has attracted significant attention lately for their promising applications in supercapacitors and batteries. Recently, McEldrew et al. [J. Phys. Chem. B 125, 2677 (2021)] developed a theory for reversible ion associations in bulk ILs, which accounted for the formation of all possible (Cayley tree) clusters and a percolating ionic network (gel). Here, we adopt and develop this approach to understand the associations of ILs in the electrical double layer at electrified interfaces. With increasing charge of the electrode, the theory predicts a transition from a regime dominated by a gelled or clustered state to a crowding regime dominated by free ions. This transition from gelation to crowding is conceptually similar to the overscreening to crowding transition.

Molecular understanding of charge storage and charging dynamics in supercapacitors with MOF electrodes and ionic liquid electrolytes.

We performed constant-potential molecular dynamics simulations to analyse the double-layer structure and capacitive performance of supercapacitors composed of conductive metal-organic framework (MOF) electrodes and ionic liquids. The molecular modelling clarifies how ions transport and reside inside polarized porous MOFs, and then predicts the corresponding potential-dependent capacitance in characteristic shapes. The transmission line model was adopted to characterize the charging dynamics, which further allowed evaluation of the capacitive performance of this class of supercapacitors at the macroscale from the simulation-obtained data at the nanoscale. These 'computational microscopy' results were supported by macroscopic electrochemical measurements. Such a combined nanoscale-to-macroscale investigation demonstrates the potential of MOF supercapacitors for achieving unprecedentedly high volumetric energy and power densities. It gives molecular insights into preferred structures of MOFs for accomplishing consistent performance with optimal energy-power balance, providing a blueprint for future characterization and design of these new supercapacitor systems.

Structural effects in nanotribology of nanoscale films of ionic liquids confined between metallic surfaces.

Room Temperature Ionic Liquids (RTILs) attract significant interest in nanotribology. However, their microscopic lubrication mechanism is still under debate. Here, using non-equilibrium molecular dynamics simulations, we investigate the lubrication performance of ultra-thin (<2 nm) films of [C2MIM]+ [NTf2]- confined between plane-parallel neutral surfaces of Au(111) or Au(100). We find that films consisting of tri-layers or bilayers, form ordered structures with a flat orientation of the imidazolium rings with respect to the gold surface plane. Tri-layers are unstable against loads >0.5 GPa, while bi-layers sustain pressures in the 1-2 GPa range. The compression of these films results in monolayers that can sustain loads of several GPa without significant loss in their lubrication performance. Surprisingly, in such ultra-thin films the imidazolium rings show higher orientational in-plane disorder, with and the rings adopting a tilted orientation with respect to the gold surface. The friction force and friction coefficient of the monolayers depends strongly on the structure of the gold plates, with the friction coefficient being four times higher for monolayers confined between Au(100) surfaces than for more compact Au(111) surfaces. We show that the general behaviour described here is independent of whether the metallic surfaces are modelled as polarizable or non-polarizable surfaces and speculate on the nature of this unexpected conclusion.

Author Correction: Electrotunable nanoplasmonic liquid mirror.

In the version of this Article originally published, the last sentence of the acknowledgements incorrectly read 'L.V. acknowledges the support of a Marie Skodowska-Curie fellowship (N-SHEAD)'; it should have read 'L.V. and D.S. acknowledge the support of Marie Sklodowska-Curie fellowships, N-SHEAD and S-OMMs, respectively'.

Interfacial Layering in the Electric Double Layer of Ionic Liquids.

Ions in ionic liquids and concentrated electrolytes reside in a crowded, strongly interacting environment, leading to the formation of discrete layers of charges at interfaces and spin-glass structure in the bulk. Here, we propose a simple theory that captures the coupling between steric and electrostatic forces in ionic liquids. The theory predicts the formation of discrete layers of charge at charged interfaces. Further from the interface, or at low polarization of the electrode, the model outputs slowly decaying oscillations in the charge density with a wavelength of a single ion diameter, as shown by analysis of the gradient expansion. The gradient expansion suggests a new structure for partial differential equations describing the electrostatic potential at charged interfaces. We find quantitative agreement between the theory and molecular simulations in the differential capacitance and concentration profiles.

Theory of ion aggregation and gelation in super-concentrated electrolytes.

In concentrated electrolytes with asymmetric or irregular ions, such as ionic liquids and solvent-in-salt electrolytes, ion association is more complicated than simple ion-pairing. Large branched aggregates can form at significant concentrations at even moderate salt concentrations. When the extent of ion association reaches a certain threshold, a percolating ionic gel network can form spontaneously. Gelation is a phenomenon that is well known in polymer physics, but it is practically unstudied in concentrated electrolytes. However, despite this fact, the ion-pairing description is often applied to these systems for the sake of simplicity. In this work, drawing strongly from established theories in polymer physics, we develop a simple thermodynamic model of reversible ionic aggregation and gelation in concentrated electrolytes accounting for the competition between ion solvation and ion association. Our model describes, with the use of several phenomenological parameters, the populations of ionic clusters of different sizes as a function of salt concentration; it captures the onset of ionic gelation and also the post-gel partitioning of ions into the gel. We discuss the applicability of our model, as well as the implications of its predictions on thermodynamic, transport, and rheological properties.

Optical response of electro-tuneable 3D superstructures of plasmonic nanoparticles self-assembling on transparent columnar electrodes.

Electrically tuneable, guided self-assembly of plasmonic nanoparticles (NPs) at polarized, patterned solid-liquid interfaces could enable numerous platforms for designing nanoplasmonic optical devices with new tuneable functionalities. Here, we propose a unique design of voltage-controlled guided 3D self-assembly of plasmonic NPs on transparent electrodes, patterned as columnar structures-arrays of vertical nanorods. NP assembly on the electrified surfaces of those columnar structures allows formation of a 3D superstructure of NPs, comprising stacking up of NPs in the voids between the columns, forming multiple NP-layers. A comprehensive theoretical model, based on quasi-static effective medium theory and multilayer Fresnel reflection scheme, is developed and verified against full-wave simulations for obtaining optical responses-reflectance, transmittance, and absorbance-from such systems of 3D self-assembled NPs. With a specific example of small gold nanospheres self-assembling on polarized zinc oxide columns, we show that the reflectance spectrum can be controlled by the number of stacked NP-layers. Numerical simulations show that peak reflectance can be enhanced up to ∼1.7 times, along with spectral broadening by a factor of ∼2-allowing wide-range tuning of optical reflectivity. Smaller NPs with superior mobility would be preferable over large NPs for realizing such devices for novel photonic and sensing applications.

Structural Forces in Mixtures of Ionic Liquids with Organic Solvents.

Using molecular dynamics simulations, we study the impact of electrode charging and addition of solvent (acetonitrile, ACN) on structural forces of the BMIM PF6 ionic liquid (IL) confined by surfaces at nanometer separations. We establish relationships between the structural forces and the microscopic structure of the confined liquid. Depending on the structural arrangements of cations and anions across the nanofilm, the load-induced squeeze-out of liquid layers occurs via one-layer or bilayer steps. The cations confined between charged plates orient with their aliphatic chain perpendicular to the surface planes and link two adjacent IL layers. These structures facilitate the squeeze-out of single layers. For both pure IL and IL-ACN mixtures, we observe a strong dependence of nanofilm structure on the surface charge density, which affects the simulated pressure-displacement curves. Addition of solvent to the IL modifies the layering in the confined film. At high electrode charges and high dilution of IL (below 10% molar fraction), the layered structure of the nanofilm is less well defined. We predict a change in the squeeze-out mechanism under pressure, from a discontinuous squeeze-out (for high IL concentrations) to an almost continuous one (for low IL concentrations). Importantly, our simulations show that charged electrodes are coated with ions even at low IL concentrations. These ion-rich layers adjacent to the charged plate surfaces are not squeezed out even under very high normal pressures of ∼5 GPa. Hence, we demonstrate the high performance of IL-solvent mixtures to protect surfaces from wear and to provide lubrication at high loads.

Superionic liquids in conducting nanoslits: A variety of phase transitions and ensuing charging behavior.

We develop a theory of charge storage in ultranarrow slitlike pores of nanostructured electrodes. Our analysis is based on the Blume-Capel model in an external field, which we solve analytically on a Bethe lattice. The obtained solutions allow us to explore the complete phase diagram of confined ionic liquids in terms of the key parameters characterizing the system, such as pore ionophilicity, interionic interaction energy, and voltage. The phase diagram includes the lines of first- and second-order, direct and re-entrant phase transitions, which are manifested by singularities in the corresponding capacitance-voltage plots. Testing our predictions experimentally requires monodisperse, conducting ultranarrow slit pores, to permit only one layer of ions, and thick pore walls, to prevent interionic interactions across the pore walls. However, some qualitative features, which distinguish the behavior of ionophilic and ionophobic pores and their underlying physics, may emerge in future experimental studies of more complex electrode structures.

Electrotunable nanoplasmonic liquid mirror.

Recently, there has been a drive to design and develop fully tunable metamaterials for applications ranging from new classes of sensors to superlenses among others. Although advances have been made, tuning and modulating the optical properties in real time remains a challenge. We report on the first realization of a reversible electrotunable liquid mirror based on voltage-controlled self-assembly/disassembly of 16 nm plasmonic nanoparticles at the interface between two immiscible electrolyte solutions. We show that optical properties such as reflectivity and spectral position of the absorption band can be varied in situ within ±0.5 V. This observed effect is in excellent agreement with theoretical calculations corresponding to the change in average interparticle spacing. This electrochemical fully tunable nanoplasmonic platform can be switched from a highly reflective 'mirror' to a transmissive 'window' and back again. This study opens a route towards realization of such platforms in future micro/nanoscale electrochemical cells, enabling the creation of tunable plasmonic metamaterials.

Theory of polymer-electrolyte-composite electroactuator sensors with flat or volume-filling electrodes.

In reverse actuation, a voltage/electrical-current signal can be generated from applying a mechanical force to an electroactuator. Such processes are of interest in touch sensing and soft robotics applications. We develop a classical density functional theory of reverse actuation for polymer-electrolyte-composite electroactuators, which treats mobile cations in the same spirit as forward actuation (curving in response to applied voltage). The proposed framework is applied to electroactuators with micro-structured porous electrodes (with cylindrical or slit pores) and flat electrodes, the dynamic response of which has to be modelled differently. Open- and short-circuit operation modes are investigated separately. A detailed analysis of the proposed theory indicates the preferred architectures for sensing, depending on the operation regimes.