Double Layer In Ionic Liquids Research Articles

Water in the Electrical Double Layer of Ionic Liquids on Graphene

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (CQ) to the total interfacial capacitance (Cint) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (CIL ∼ 80 μF cm-2), so that CQ governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL ∼ 6-9 μF cm-2), which governs Cint already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Cracking Ion Pairs in the Electrical Double Layer of Ionic Liquids

Here we investigate a limiting case of the theory for aggregation and gelation in the electrical double layer (EDL) of ionic liquids (ILs). The limiting case investigated only accounts for ion pairs, ignoring the possibility of larger clusters and a percolating ionic network. This simplification, permits analytical solutions for the properties of the EDL. The resulting equations demonstrate the competition between the free energy of an association and the electrostatic potential in the EDL. For small electrostatic potentials and large negative free energies of associations, ion pairs dominate in the EDL. Whereas, for electrostatic potential energies larger than the free energy of an association, electric-field-induced cracking of ion pairs occurs. The differential capacitance for this consistent ion pairing theory has a propensity to have a “double hump camel” shape. We compare this theory against previous free ion approaches, which do not consistently treat the reversible associations in the EDL.

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.

Double layer in ionic liquids: Temperature effect and bilayer model

This work describes the effect of potential and temperature on the graphene–ionic liquid (EMImBF4) interfacial structure and properties with the focus on a novel phenomenon of ionic saturation. We apply classical molecular dynamics simulations to reproduce well-known phenomena of overscreening, monolayer formation, and temperature-induced smearing of the interfacial structure. Using quantum density functional theory calculations, we show how quantum capacitance dampens the influence of temperature and improves the agreement with the experimental data. Using a bilayer model, we study characteristic features of capacitance–potential dependence and relate them to the changes in interfacial structure. These insights are of fundamental and practical importance for the application of similar interfaces in electrochemical energy storage and transformation devices such as capacitors and actuators.

Cracking Ion Pairs in the Electrical Double Layer of Ionic Liquids

Cracking Ion Pairs in the Electrical Double Layer of Ionic Liquids

CD Stretching Modes are Sensitive to the Microenvironment in Ionic Liquids.

Knowledge of the structure of the electrical double layer in ionic liquids (IL) is crucial for their applications in electrochemical technologies. We report the synthesis and applicability of an imidazolium‐based amphiphilic ionic liquid with a perdeuterated alkyl chain for studies of electric potential‐dependent rearrangements, and changes in the microenvironment in a monolayer on a Au(111) surface. Electrochemical measurements show two states of the organization of ions on the electrode surface. In situ IR spectroscopy shows that the alkyl chains in imidazolium cations change their orientation depending on the adsorption state. The methylene‐d 2 stretching modes in the perdeuterated IL display a reversible, potential‐dependent appearance of a new band. The presence of this mode also depends on the anion in the IL. Supported by quantum chemical calculations, this new mode is assigned to a second ν as(CD2) band in alkyl‐chain fragments embedded in a polar environment of the anions/solvent present in the vicinity of the imidazolium cation and electrode. It is a measure of the potential‐dependent segregation between polar and nonpolar environments in the layers of an IL closest to the electrode.

Theory of Ionic Liquids with Polarizable Ions on a Charged Electrode

We formulate a general mean-field theory of a flat electric double layer in ionic liquids and electrolyte solutions with ions possessing static polarizability and a permanent dipole moment on a charged electrode. We establish a new analytical expression for electric double layer differential capacitance determining it as an absolute value of the ratio of the local ionic charge density to the local electric field on an electrode surface. We demonstrate that this expression generalizes the analytical expressions previously reported by Kornyshev and Maggs and Podgornik. Using the obtained analytical expression, we explore new features of the differential capacitance behavior with an increase in the static polarizability and permanent dipole moment of cations. We relate these features to the behavior of ionic concentrations on the electrode. In particular, we elucidate the role of the competition between the dielectrophoretic attraction and Coulomb repulsion forces acting on polarizable or polar cations in the electric double layer in the behavior of the differential capacitance. The developed theoretical model and obtained theoretical findings could be relevant for different electrochemical applications, e.g. batteries, supercapacitors, catalysis, electrodeposition, etc.

Insight into the Electrical Double Layer of Ionic Liquids Revealed through Its Temporal Evolution

AbstractIonic liquids (ILs) are proposed as potentially ideal electrolytes for use in electrical double layer capacitors. However, recent discoveries of long‐range electrostatic screening in ILs have revealed that this understanding of the electrical double layer in highly concentrated solutions is still incomplete. Through precise time‐dependent measurements of wide‐angle X‐ray scattering and surface forces, novel molecular insight into their electrical double layer is provided. An ultraslow evolution of the nanostructure of three imidazolium ILs is observed, which reflects the reorganization of the ions in confined and unconfined (bulk) states. The observed phase transformation in the bulk consists of the ILs ordering over at least 20 h, reflected in an expansion or contraction of the spacing between the ions organized in domains of ≈10 nm. This transformation justifies the evolution of the electrical double layer measured in force measurements. Subtle differences between the ILs arise from the intricate balance between electrostatic and non‐electrostatic interactions. This work reveals a new time scale of the evolution of the IL structure and offers a new perspective for understanding the electrical double layer in ILs, with implications on diverse areas of inquiry, such as energy storage, lubrication, as well as micro‐ and nanoelectronics devices.

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.

Probing the electrical double layer by operando X‐ray photoelectron spectroscopy through a graphene‐carbon nanotube composite window

AbstractThe electrical double layer is known to spontaneously form at the electrode‐electrolyte interface, impacting many important chemical and physical processes as well as applications including electrocatalysis, electroorganic synthesis, nanomaterial preparation, energy storage, and even emulsion stabilization. However, it has been challenging to study this fundamental phenomenon at the molecular level because the electrical double layer is deeply “buried” by the bulk electrolyte solution. Here, we report a quantitative probing of the electrical double layer of ionic liquids from the solid side of a photoelectron‐transparent graphene‐carbon nanotube hybrid membrane electrode using X‐ray photoelectron spectroscopy. The membrane window is ultrathin (1‐1.5 nm), large (~1 cm2), and robust, enabling a tight seal of the electrolyte and quantitative measurement with excellent photoelectron signals. By operando monitoring the population changes of cations and anions in response to the applied electrical potentials, we experimentally resolve the chemical structure and dynamics of the electrical double layer, which corroborate results from molecular dynamics simulations.image

Water as a catalyst for ion transport across the electrical double layer in ionic liquids

It was previously shown experimentally that the ionic liquid (IL) system based on 1-ethyl-3-methylimidazolium dicyanamide $([{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}])$ outperforms $N$-butyl-$N$-methylpyrrolidinium dicyanamide $([{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}])$ as an electrolyte in a rechargeable zinc battery. Specifically, $[{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}]$ requires a lower overpotential to initiate electrodeposition of zinc, enables a higher zinc deposition peak current density, and supports a greater number of charge-discharge cycles. In the present work, a set of molecular dynamics simulations of two IL systems (i.e., IL + 9 mol % Zn + 3 wt % water) reveals that these differences could be related to the overscreening and crowding phenomena that exist in these IL systems. Furthermore, the work reveals the most likely mechanism by which water acts as an electrochemical catalyst in transferring metal cations to a negatively charged electrode. This transfer is a necessary step for subsequent metal deposition, as required for a rechargeable battery system, for example. In particular, the results of the simulations reveal three primary findings. First, it is shown that water activates the transfer of zinc onto the negative electrodes by temporarily disrupting zinc's [dca] coordination shell while zinc is located within the interfacial region; second, it is shown that the transfer of zinc is obviously greater in the $[{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}]$ system than in the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system, which is consistent with experimental measurements; and third, when the effects of crowding are largely eliminated in the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system (i.e., when the layering instability and the anion layer collapse in the anodic interfacial region are avoided by the use of a smaller electrode charge), the zinc transfer dynamics are completely changed and the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system starts providing a more favorable environment for zinc transfers. These findings present a significant advance in the understanding of the electrode-electrolyte interface in ILs.

Borophene vs. graphene interfaces: Tuning the electric double layer in ionic liquids

In this work we perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), with lithium tetrafluoroborate (LiBF4), confined between two borophene walls of three different surface charges, −1, 0 and +1 e/nm2, where e is the elementary charge. The properties of the system are analyzed by means of ionic density profiles, angular orientations of [BMIM]+ cations close to the wall and vibrational densities of states for the salt cations close to the walls. The lateral structure of the first layer close to the surface is also studied on one hand, calculating Minkowski parameters and the Shannon entropy of the patterns of the 2D density maps of the anions placed there and, on the other hand, computing the 2D-Fourier transform of the positions of these anions. Our results are compared with those obtained previously for the same mixtures confined between two graphene walls. Although similarities exist between both cases, interesting differences are observed in the lateral structure that the ionic liquid adopts near borophene interfaces due to their strong anisotropy. In particular, we have observed that borophene induces more markedly ordered 2D patterns in the innermost layer of the ionic liquid electric double layer, specially when they are charged. It is this feature that makes borophene a potential candidate for battery electrode applications with possibilities beyond those of graphene.

Coarse-Grained Electrostatic Model Including Ion-Pairing Equilibrium That Explains DC and AC X-ray Photoelectron Spectroscopy Measurements on Ionic Liquids

The dynamics of the electrochemical double layer in ionic liquids can be experimentally probed by a number of experimental techniques. Earlier, we reported on the results of an X-ray photoelectron ...

Observation of Structure of Surfaces and Interfaces by Synchrotron X-ray Diffraction: Atomic-Scale Imaging and Time-Resolved Measurements

The recent developments in synchrotron optics, X-ray detectors, and data analysis algorithms have enhanced the capability of the surface X-ray diffraction technique. This technique has been used to clarify the atomic arrangement around surfaces in a non-contact and nondestructive manner. An overview of surface X-ray diffraction, from the historical development to recent topics, is presented. In the early stage of this technique, surface reconstructions of simple semiconductors or metals were studied. Currently, the surface or interface structures of complicated functional materials are examined with sub-Å resolution. As examples, the surface structure determination of organic semiconductors and of a one-dimensional structure on silicon are presented. A new frontier is time-resolved interfacial structure analysis. A recent observation of the structure and dynamics of the electric double layer of ionic liquids, and an investigation of the structural evolution in the wettability transition on a TiO2 surface that utilizes a newly designed time-resolved surface diffractometer, are presented.

On the Impact of Electrostatic Correlations on the Double-Layer Polarization of a Spherical Particle in an Alternating Current Field.

At concentrated electrolytes, the ion-ion electrostatic correlation effect is considered an important factor in electrokinetics. In this paper, we compute, in theory and simulation, the dipole moment for a spherical particle (charged, dielectric) under the action of an alternating electric field using the modified continuum Poisson-Nernst-Planck (PNP) model by Bazant et al. [ Double Layer in Ionic Liquids: Overscreening Versus Crowding . Phys. Rev. Lett. 2011 , 106 , 046102 ] We investigate the dependency of the dipole moment in terms of frequency and its variation with such quantities like ζ-potential, electrostatic correlation length, and double-layer thickness. With thin electric double layers, we develop simple models through performing an asymptotic analysis of the modified PNP model. We also present numerical results for an arbitrary Debye screening length and electrostatic correlation length. From the results, we find a complicated impact of electrostatic correlations on the dipole moment. For instance, with increasing the electrostatic correlation length, the dipole moment decreases and reaches a minimum and then it goes up. This is because of initially decreasing of surface conduction and finally increasing due to the impact of ion-ion electrostatic correlations on ion's convection and migration. Also, we show that in contrast to the standard PNP model, the modified PNP model can qualitatively explain the data from the experimental results in multivalent electrolytes.

Simulations and Experiments of the Kinetics of the Electrochemical Double Layer in Ionic Liquids

Ionic liquids are an interesting class of electrolytes both from a fundamental scientific standpoint and from an applications standpoint due to a number of relevant applications like batteries, supercapacitors and solar cells. Even though the constituents of ionic liquids are only ions, the conductivities are known to be pretty low. The suggested reasons for this low conductivity are low mobility due to high viscosity and the existence of most ions as ion pairs. Even though they have low conductivities, their electrochemical potential window and their low to non-existent vapor pressures make them very attractive as electrolytes. Because ionic liquids have no measurable vapor pressure, in-situ X-Ray Spectroscopy (XPS) can be performed directly on the ionic liquids during electrochemical experiments. This is not possible with more conventional electrolytes since any other electrolytic solution would not be conducive to running experiments inside the ultra-high vacuum (UHV) chamber. We have been doing experiments investigating both Faradaic and non-Faradaic aspects of electrochemical experiments on ionic liquids inside the XPS chamber. On the Faradaic side we have recently shown that the reduction of imidazolium based ionic liquids can yield stable carbene species (Aydogan-Gokturk et.al. New Journal of Chemistry 18(2017), 10299-10304 and Aydogan-Gokturk et.al. Electrochimica Acta 234(2017), 37-42) and that gold nanoparticles can be oxidatively synthesized starting from metal electrodes (Camci et.al. ACS Omega, 2(2017), 478-486). On the non-Faradaic front, investigations of the electrochemical double layer revealed that the applied potential has transient effects that are sensed unexpectedly far away from the electrode surface (Camci et.al. Phys. Chem. Chem. Phys. 18(2016)28434-28440). Specifically, our experiments indicate that the effects of the applied potential are felt up to millimeters away from the electrode surface during the transients. These transients can last hundreds of seconds and dominate the response. The current contribution is going to elaborate more on the unexpected electrochemical double layer effects. Even though the electrochemical double layer in electrolytes is well understood since the days of Gouy-Chapman-Stern model development, the double layer in the ionic liquid media is known to be somewhat different. There is a sizable body of recent modeling literature on the electrochemical double layer in ionic liquids. For example, work by Bazant et.al. (Phys. Rev. Lett. 106(2011) 046102), Lee et.al.(Phys. Rev. Lett. 115(2015)106101) and Gavish et.al.(J. Phys. Chem. Lett. 7(2016), 1121-1126) consider continuum type models of various sorts to model the behavior of the anions and the cations in great detail. The results indicate that there is some layering behavior that are either close to the electrode surface or throughout the region between the electrodes. The models used, though very detailed and thorough neglect ion pairing and thermal fluctuations. The effect of ion pairing is a crucial part of the explanation of the low conductivity of ionic liquids. On the other hand, thermal fluctuations are a necessary part of the conventional explanation of the diffuse double layer. In most cases, the models developed predict a layered steady state structure and also predict that the Debye length would be on the order of 10-20 times the size of a monolayer for most cases. Our experiments indicate that the effect of the applied potential is felt way farther away from the electrode in the transient times. We have developed a simpler modeling framework that model diffusion, convection and ion pairing using simple algebraic expressions on a simple grid that is set up. This framework involves setting up a grid between the two electrodes and starting with a homogeneous ionic liquid. At every time step, the algorithm iterates over the grid and treats diffusion, migration and ion pairing separately using simple constructs. Diffusion is simply handled by a random number generator that determine which way the ions will move, if at all. Then, the potential difference between neighboring points of the grid is used to evaluate the electric fields, which is used to then treat migration. Finally, for every position on the grid, a simple chemical equilibrium calculation is employed to solve for the number of ions that pair up. Using this framework to model the behavior of the ionic liquids and tuning the parameters of the simulation based on the results of transient measurements inside the XPS chamber using Squarewave applied potentials, we will report behavior of the electrochemical double layer in ionic liquids and insights about various parameters of ionic liquids such as ion-pairing constants, mobilities and diffusion coefficients. Figure 1

On the thickness of the double layer in ionic liquids.

In this study, we examined the thickness of the electrical double layer (EDL) in ionic liquids using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. We focused on BF4- anion adsorption from the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) ionic liquid on the Au(111) surface. At both DFT and MD levels, we evaluated the capacitance-potential dependence for the Helmholtz model of the interface. Using MD simulations, we also explored a more realistic, multilayer EDL model accounting for the ion layering. Concurrent analysis of the DFT and MD results provides a ground for thinking whether the electrical double layer in ionic liquids is one- or multi-ionic-layer thick.

The effect of dispersion interactions on the structure and performance of electrical double layer of ionic liquids

A classical density functional theory has been used to study the structure and phase behavior of the electrical double layer of a dense ionic liquid. The model for IL consists of a trimer cation (with a charged head and two neutral segments) and a monomer anion. The effect of dispersion interactions on the density profile and differential capacitance curve has been investigated. Increasing the contribution of dispersion interactions leads to a camel-shape differential capacitance curve. In the case of bell-shape curve, the maximum of the differential capacitance increases with decreasing the dispersion forces. These observations are related to the depletion or accumulation of ions near electrode with zero or low surface charge density.

Molecular-level understanding of electric double layer in ionic liquids

Summary Electrochemical interfaces in ionic liquids (ILs) are structurally anisotropic with multi-scaled and multi-natured interactions between electrode and IL as well as among ions of IL. Combined short-ranged and long-ranged surface techniques with high spatial, time and energy resolutions need to be employed to precisely characterize the lateral structure and vertical arrangement of IL at the interface. We briefly review recent advancements on the understanding of the molecular structure of electric double layer (EDL) at electrode–IL interfaces, with emphasis laid on the necessity of detailed and systematic potential-dependent measurements across and at close to the potential of zero charge (PZC). Future directions of fundamental investigations on EDL are briefly discussed.

Mean-Field Theory of Electrical Double Layer In Ionic Liquids with Account of Short-Range Correlations

We develop the theory of the electrical double layer in ionic liquids as proposed earlier by Kornyshev (2007). In the free energy function we keep the so called ‘short-range correlation terms’ which were omitted there. With some simplifying assumptions, we arrive at a modified expression for differential capacitance, which makes differential capacitance curves less sharply depending on electrode potential and having smaller values at extrema than in the previous theory. This brings the results closer to typical experimental observations, and makes it appealing to use this formalism for treatment of experimental data. Implications on Debye length and the extent of ion paring in ionic liquids are then briefly discussed.