Double layer in ionic liquids: The nature of the camel shape of capacitance

The tail effect on the shape of an electrical double layer differential capacitance curve

The differential capacitance curve for the double layer formed by an electrolyte dissolved in a solvent is commonly believed to be parabolic-like with a minimum at low electrolyte charge concentration and low electrode surface charge density, and independent of electrolyte concentration at high electrolyte concentrations and high electrode charge and would be, in the absence of solvent effects, featureless at these latter conditions. This is the prediction of the popular Gouy-Chapman-Stern theory. In contrast, for an ionic liquid this curve can have a single or a double hump (or a bell or camel shape). Fedorov et al. [Electrochem. Commun. 12, 296 (2010)]10.1016/j.elecom.2009.12.019 have related these humps, particularly the double hump, to the neutral tails of ions in many ionic liquids. Evidence presented here shows, however, that such humps are general features of the differential capacitance of a double layer, whether it be formed by ions with or without a neutral tail. The presence of a double or single hump results from the magnitude of the electrolyte charge concentration. For both spherical ions or non-spherical ions consisting of charged heads and neutral tails, the shape of the differential capacitance transforms continuously from a double hump to a single hump as the electrolyte concentration is increased.

Restructuring of the electrical double layer in ionic liquids upon charging

We have investigated the electrical double layer (EDL) structure at an interface between ionic liquid (IL) and charged surface using molecular dynamics simulations. We show that for three different models of ILs the EDL restructuring, driven by surface charging, can be rationalized by the use of two parameters--renormalized surface charge (κ) and charge excess in the interfacial layers (λ). Analysis of the relationship between the λ and κ parameters provides new insights into mechanisms of over-screening and charge-driven structural transitions in the EDL in ionic liquids. We show that the restructuring of the EDL upon charging in all three studied systems has two characteristic regimes: (1) transition from the bulk-like (κ(Ion) = 0) to the multilayer structure (κ(Ion) ≈ 0.5) through the formation of an ionic bilayer of counter- and co-ions; and (2) transition from the multilayer (κ(Ion) ≈ 0.5) to the crowded (κ(Ion) > 1) structure through the formation of a monolayer of counter-ions at κ(Ion) = 1.

Electrocapillarity and zero-frequency differential capacitance at the interface between mercury and ionic liquids measured using the pendant drop method

Zero-frequency differential capacitance measurements at the ionic liquid|mercury interface using the pendant drop method reveal predicted and unpredicted features of the potential dependence of the capacitance.

Ion structure controls ionic liquid near-surface and interfacial nanostructure

A unique, but unifying, feature of ionic liquids (ILs) is that they are nanostructured on the length scale of the ions; in many ILs well-defined polar and apolar domains exist and may percolate through the liquid. Near a surface the isotropic symmetry of the bulk structure is broken, resulting in different nanostructures which, until now, have only been studied indirectly. In this paper, in situ amplitude modulated atomic force microscopy (AM-AFM) has been used to resolve the 3-dimensional nanostructure of five protic ILs at and near the surface of mica. The surface and near surface structures are distinct and remarkably well-defined, but are very different from previously accepted descriptions. Interfacial nanostructure is strongly influenced by the registry between cations and the mica surface charge sites, whereas near surface nanostructure is sensitive to both cation and anion structure. Together these ILs reveal how interfacial nanostructure can be tuned through ion structure, informing "bottom-up" design and optimisation of ILs for diverse technologies including heterogeneous catalysis, lubrication, electrochemical processes, and nanofluids.

Effects of repulsive interaction on the electric double layer of an imidazolium-based ionic liquid by molecular dynamics simulation

Effects of repulsion between BMIM⁺/PF₆⁻ and the graphite electrode on electric double layer was studied by molecular dynamics simulation.

Solvent effects of 1-ethyl-3-methylimidazolium acetate: solvation and dynamic behavior of polar and apolar solutes

We study the solvation properties of the ionic liquid 1-ethyl-3-methylimidazolium acetate ([eMIM]⁺[ACE]⁻) and the resulting dynamic behavior for differently charged model solutes at room temperature via atomistic molecular dynamics (MD) simulations of 500 ns length.

Switchable silver mirrors with long memory effects

An electrochemically stable and bistable switchable mirror was achieved for the first time by introducing (1) a thiol-modified indium tin oxide (ITO) electrode for the stabilization of the metallic film and (2) ionic liquids as an anion-blocking layer, to achieve a long memory effect. The growth of the metallic film was denser and faster at the thiol-modified ITO electrode than at a bare ITO electrode. The electrochemical stability of the metallic film on the thiol-modified ITO was enhanced, maintaining the metallic state without rupture. In the voltage-off state, the metal film maintained bistability for a long period (>2 h) when ionic liquids were introduced as electrolytes for the switchable mirror. The electrical double layer in the highly viscous ionic liquid electrolyte seemed to effectively form a barrier to the bromide ions, to protect the metal thin film from them when in the voltage-off state.

Molecular insights into the electric double layers of ionic liquids on Au(100) electrodes

The electric double layer structure and differential capacitance of single crystalline Au(100) electrodes in the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate are investigated using molecular dynamics simulations. Results show strong adsorption on the electrode surface. The potential of zero charge (pzc) and maxima of differential capacitance are strongly dependent on the adsorption layer structure. At potentials near the pzc, cations and anions adjacent to the electrode surface are coadsorbed on the same screening layer. This strong adsorption layer results in overscreening effects on the compact layer and induces both a bell-shaped differential capacitance curve and a positive pzc. Moreover, the potential required for transition from overscreening to overcrowding is about 4.0 V. This transition potential may be attributed to the higher interaction energy between the Au(100) electrode and ions compared with the binding energy in our cation-anion system.

Influence of alkyl chain length and anion species on ionic liquid structure at the graphite interface as a function of applied potential

Atomic force microscopy (AFM) force measurements elucidate the effect of cation alkyl chain length and the anion species on ionic liquid (IL) interfacial structure at highly ordered pyrolytic graphite (HOPG) surfaces as a function of potential. Three ILs are examined: 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM] FAP), 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] FAP), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] TFSA). The step-wise force-distance profiles indicate the ILs adopt a multilayered morphology near the surface. When the surface is biased positively or negatively versus Pt quasireference electrode, both the number of steps, and the force required to rupture each step increase, indicating stronger interfacial structure. At all potentials, push-through forces for [HMIM] FAP are the highest, because the long alkyl chain results in strong cohesive interactions between cations, leading to well-formed layers that resist the AFM tip. The most layers are observed for [EMIM] FAP, because the C2 chains are relatively rigid and the dimensions of the cation and anion are similar, facilitating neat packing. [EMIM] TFSA has the smallest push-through forces and fewest layers, and thus the weakest interfacial structure. Surface-tip attractive forces are measured for all ILs. At the same potential, the attractions are the strongest for [EMIM] TFSA and the weakest for [HMIM] FAP because the interfacial layers are better formed for the longer alkyl chain cation. This means interfacial forces are stronger, which masks the weak attractive forces.

A mean-field theory on the differential capacitance of asymmetric ionic liquid electrolytes

The size of ions significantly influences the electric double layer structure of room temperature ionic liquid (IL) electrolytes and their differential capacitance (Cd). In this study, we extended the mean-field theory (MFT) developed independently by Kornyshev (2007J. Phys. Chem. B 111 5545-57) and Kilic, Bazant, and Ajdari (2007 Phys. Rev. E 75 021502) (the KKBA MFT) to take into account the asymmetric 1:1 IL electrolytes by introducing an additional parameter ξ for the anion/cation volume ratio, besides the ionic compressibility γ in the KKBA MFT. The MFT of asymmetric ions becomes KKBA MFT upon ξ = 1, and further reduces to Gouy-Chapman theory in the γ → 0 limit. The result of the extended MFT demonstrates that the asymmetric ILs give rise to an asymmetric Cd, with the higher peak in Cd occurring at positive polarization for the smaller anionic size. At high potential, Cd decays asymptotically toward KKBA MFT characterized by γ for the negative polarization, and characterized by ξγ for the positive polarization, with inverse-square-root behavior. At low potential, around the potential of zero charge, the asymmetric ions cause a higher Cd, which exceeds that of Gouy-Chapman theory.

Effect of dissolved LiCl on the ionic liquid-Au(111) interface: an in situ STM study

The structure of the electrolyte/electrode interface plays a significant role in electrochemical processes. To date, most studies are focusing on understanding the interfacial structure in pure ionic liquids. In this paper in situ scanning tunnelling microscopy (STM) has been employed to elucidate the structure of the charged Au(111)-ionic liquid (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, [Py1,4]FAP) interface in the presence of 0.1 M LiCl. The addition of the Li salt to the ionic liquid has a strong influence on the interfacial structure. In the first STM scan in situ measurements reveal that Au(111) undergoes the (22 x √3) 'herringbone' reconstruction in a certain potential regime, and there is strong evidence that the gold surface dissolves at negative electrode potentials in [Py1,4]FAP containing LiCl. Bulk deposition of Li is obtained at -2.9 V in the second STM scan.

Coarse-grained simulations of an ionic liquid-based capacitor: II. Asymmetry in ion shape and charge localization

In this work, which is a continuation of part I, we introduce a primitive model for an ionic liquid (IL) that can account for the planar shape of cations typical for ILs like imidazolium. The model consists of a spherical anion and a triangular cation consisting of three spheres, where one or all three vertices of the triangle can carry electric charge. We use molecular dynamics simulations to study the differential capacitance Cd of an ionic liquid confined between two planar electrodes. Our goal is to elucidate the complex dependence of Cd on the electrode potential U in terms of simple entities such as the shape and charge distribution of the ions. For this purpose, we compare the results from the current model to the results based on the models with spherical cations that possess asymmetry in ion valence and shape that were analyzed in detail in part I of this work. We show that the various possible stackings of the triangles near the cathode lead to noticeable new features in Cd(U) as compared to the spherical models. Different distributions of charges on the triangle lead to different preferred orientations of the cations near the cathode that are moreover potential dependent.

Coarse-grained simulations of an ionic liquid-based capacitor: I. Density, ion size, and valency effects

We introduce a hierarchy of generic coarse-grained models of ionic liquids of increasing complexity. We use them in molecular dynamics simulations to study the differential capacitance of a capacitor consisting of an ionic liquid between two planar electrodes. The primary goal is to explain the complex dependence of the differential capacitance Cd on the electrode potential U in simple terms, e.g. in terms of the size and valency of the ions. For this purpose we introduce the symmetric model A, which qualitatively reproduces the Cd(U) dependence predicted by the mean-field theory but also reveals strong quantitative deviations. We further introduce size asymmetry in model A by increasing the cation size. In model B we vary the cation valency, keeping the sizes of both ions constant. We show that simultaneous increases in size and valency may compensate for each other, leading to a Cd(U) very similar to that for the symmetric case. We interpret distinct features in Cd(U) on the basis of the density profiles of the ions and charge density profiles. We focus on the first two ion layers at the electrode, and demonstrate that the polarization of the ionic liquid proceeds through replacement of one ion type by the other, in contrast to the simple increase in ion concentrations typical for dilute systems. The understanding gained for the simple models serves as a reference for interpretation of complex effects of ion size, valency and shape. This is carried through in part II (a separate article) where we show how the planar shape of ions in model C brings new features to the Cd(U) curve and also to the polarization mechanism.

Interfaces of dicationic ionic liquids and graphene: a molecular dynamics simulation study

Molecular dynamics simulations were performed to investigate the interfacial structure and capacitance of electrical double layers (EDLs) in dicationic ionic liquids (DILs) 1-alkyl-3-dimethylimidazolium tetrafluoroborate [Cn(mim)2](BF4)2 (n = 3, 6, 9), with respect to a baseline of a monocationic ionic liquid [C3mim][BF4], near planar carbon electrodes consisting of graphene sheets. The simulation results show that an adsorbed layer with double peaks is exclusively found for [C3(mim)2](BF4)2, while a single peak of the other three cations is observed at the neutral electrode, due to the difference in ion-wall interaction and cation-anion association. As the electrode becomes negatively charged, the second peak of [C3(mim)2](2+) is dramatically reduced, whereas those of [C6(mim)2](2+) and [C9(mim)2](2+) become non-trivial. The capacitance-potential curve of EDLs in DILs manifests a transition from camel shape to bell shape as the cation chain length increases, which is attributed to the enlargement of ion adsorption (per unit charge) on the electrode and the decrease of attractive interaction between ions.

In situ small angle neutron scattering revealing ion sorption in microporous carbon electrical double layer capacitors

Experimental studies showed the impact of the electrolyte solvents on both the ion transport and the specific capacitance of microporous carbons. However, the related structure-property relationships remain largely unclear and the reported results are inconsistent. The details of the interactions of the charged carbon pore walls with electrolyte ions and solvent molecules at a subnanometer scale are still largely unknown. Here for the first time we utilize in situ small angle neutron scattering (SANS) to reveal the electroadsorption of organic electrolyte ions in carbon pores of different sizes. A 1 M solution of tetraethylammonium tetrafluoroborate (TEATFB) salt in deuterated acetonitrile (d-AN) was used in an activated carbon with the pore size distribution similar to that of the carbons used in commercial double layer capacitors. In spite of the incomplete wetting of the smallest carbon pores by the d-AN, we observed enhanced ion sorption in subnanometer pores under the applied potential. Such results suggest the visible impact of electrowetting phenomena counterbalancing the high energy of the carbon/electrolyte interface in small pores. This behavior may explain the characteristic butterfly wing shape of the cyclic voltammetry curve that demonstrates higher specific capacitance at higher applied potentials, when the smallest pores become more accessible to electrolyte. Our study outlines a general methodology for studying various organic salts-solvent-carbon combinations.

Molecular dynamics simulations of the structure of the graphene–ionic liquid/alkali salt mixtures interface

Ionic liquids mixed with lithium or potassium salts are layered near charged and uncharged graphene.

Model-free test of local-density mean-field behavior in electric double layers

We derive a self-similarity criterion that must hold if a planar electric double layer (EDL) can be captured by a local-density approximation (LDA), without specifying any specific LDA. Our procedure generates a similarity coordinate from EDL profiles (measured or computed), and all LDA EDL profiles for a given electrolyte must collapse onto a master curve when plotted against this similarity coordinate. Noncollapsing profiles imply the inability of any LDA theory to capture EDLs in that electrolyte. We demonstrate our approach with molecular simulations, which reveal dilute electrolytes to collapse onto a single curve, and semidilute ions to collapse onto curves specific to each electrolyte, except where size-induced correlations arise.

Effect of dissolved LiCl on the ionic liquid-Au(111) electrical double layer structure

The electrical double layer at ionic liquid (IL)-Au(111) interfaces is composed of alternating ion layers. Interfacial layering is markedly weaker when small amounts of LiCl are dissolved in the IL for all potential between -2.0 V and +2.0 V (vs. Pt). This means that models developed for pure IL electrical double layers may not be valid when solutes are present.