Screening of Ion–Graphene Electrode Interactions by Ionic Liquids

Article pubs.acs.org/JPCC

Screening of Ion−Graphene Electrode Interactions by Ionic Liquids: The Effects of Liquid Structure V. Ivaništšev,† M. V. Fedorov,*,† and R. M. Lynden-Bell*,‡ †

Department of Physics, Scottish Universities Physics Alliance (SUPA), Strathclyde University, John Anderson Building, 107 Rottenrow East, Glasgow G4 0NG, United Kingdom ‡ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom ABSTRACT: We have investigated the screening of solute ion−electrode interactions in two ionic liquids (1-butyl 3-methylimidazolium tetrafluoroborate [BMIm][BF4] and 1,3dimethylimidazolium chloride [MMIm]Cl) by constructing free energy profiles for dissolved charged probes as a function of distance from a charged surface (graphene). The free energy profiles for three types of mutual interactions (surface and solute with opposite charges, solute and uncharged surface, and surface and solute with the same charges) differ from each other, but are remarkably similar in the two ionic liquids. They all show oscillations rather than the monotonic behavior predicted by Debye-type screening models. In both ionic liquids, there are high barriers impeding the motion of charged probes to the oppositely charged surface. We examined the local liquid structure around the probes and found that the free energy minima correspond to positions in which the solvation layers induced by the surface charge and the solvation shells around the probes enhance each other while barriers occur where they perturb each other.

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INTRODUCTION In this paper, we present a molecular dynamics-based approach for investigating the screening of solute ion−electrode interactions in ionic liquids. We focus on the molecular scale effects of the local liquid structure at the interface between a charged electrode surface and an ionic liquid. Interfacial phenomena in ionic liquids are a subject of intensive ongoing research in many areas: from electrochemistry1−6 to nanotribology7−10 and from synthetic chemistry11−13 to biomedical sciences14−18 as well as in theoretical chemical physics.19−21 The emerging interest in this problem is due to several reasons: on one hand, ionic liquids at charged interfaces are a key element of a number of (potential) applications, such as supercapacitors,22 batteries,23−25 solar panels,26 electrolytegated electronics,27 and electrodeposition;5,28,29 on the other hand, ionic liquids at charged interfaces reveal a number of (new) interesting effects, such as overscreening, lattice saturation, and electrostriction.19,21,30−33 At least in combination(s), these effects seem to be specific to the highly concentrated ionic liquid electrolytes. Consequently, these effects are not well-described by classical theories of the double layer (such as the Gouy−Chapman theory) that were developed for low-concentration electrolytes.1 We note that currently, there is a certain lag between theoretical and computational studies of ionic liquids at charged interfaces and experimental works in this area. Without going into detail, we would like to mention just one observation: judging by the ISE Web of Knowledge, there is approximately 1 theoretical/computational publication per 50 experimental papers on this subject. Although there are already a significant number of reported theoretical and computational studies on © 2014 American Chemical Society

neat ionic liquids or their mixtures with polar solvents at charged interfaces (for a recent review, see ref 34), until now, only a few modeling studies have been published on molecular scale mechanisms of interactions between dissolved charged solutes in ionic liquids35,36 and their interactions with charged interfaces.37−39 This subject (solute−electrode interactions in ionic liquids), however, is of large importance for electrodeposition, electrocatalysis, and surface-driven self-assembly in ionic liquids as well as for many other applications.40,41 Because the subject is relatively new, there is no general agreement in the literature, even on basic aspects of the electric double layer formation in ionic liquids and molecular-scale mechanisms of electrostatic screening in these systems. As a result, there are ongoing debates on the general structure of the electric double layer in ionic liquids (multilayer vs monolayer,33,42−45 long-range vs short-range,46−50 etc.) as well as on the role of impurities51 and interfacial dynamics in ionic liquids at charged interfaces.40,52−55 General discussion of these topics is given in ref 1. In our opinion, the apparent controversy of the subject is due to the following reasons: (i) There are a large number of various ionic liquids with different chemical natures of their ions and, consequently, different physical−chemical properties. In fact, the “chemical space” of ionic liquids is vast and quite heterogeneous.12,56−59 That often leads to terminological confusion,59,60 and, overall, makes the task of establishing general trends in ionic liquids behavior at charged interfaces a challenging one. (ii) As discussed above, ionic liquids Received: December 10, 2013 Revised: February 14, 2014 Published: February 16, 2014 5841

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two rigid graphene slabs, each with an area 3.408 nm ×3.4433 nm and separated by 14.4 nm for [BMIm][BF4] or 14.35 nm for [MMIm]Cl. A total of 364 [BMIm][BF4] (3.6 mol dm−3) or 576 [MMIm]Cl (5.7 mol dm−3) ion pairs were introduced between the surfaces and equilibrated. A single probe was introduced. In simulations with charged probes, the number of anions was increased or decreased to achieve overall charge neutrality. The OPLS-AA force field was used together with partial site charges taken from ref 75 for both ionic liquids. For the original [MMIm]Cl simulations, the charges were not screened, but for [BMIm][BF4], the charges were screened by a factor of 0.79, corresponding to a dielectric constant of 1.6. Apart from this charge scaling, which accounts for the high-frequency electronic polarizability, the interactions were unpolarized. The probe was a Lennard-Jones sphere with εLJ = 0.6192 kJ/mol and σLJ = 0.5 nm. For most of our simulations, we used the Gromacs 4.6 Molecular Dynamics software.76 For a few simulations, a modified version of DL_POLY77 was used with a modified force field in which the [MMIm]+ cation was treated as a rigid body with flexible methyl groups and the flat graphene sheets were replaced by 2-dimensional Lennard-Jones surfaces with the same average interaction. Our previous tests showed that the two sets of simulations give practically identical results. Because of the slow diffusion of ionic liquid ions, long runs were needed for ensemble averaging of the forces on the probe. A single spherical probe (solute particle) was introduced into the ionic liquid and moved to a defined distance from the surface (z), ranging between 0.25 and 1.8 nm in steps of 0.05 nm (32 distances in all). For each distance, eight independent configurations were prepared from a simulation at 1000 K. After equilibration, data was collected from runs of 1.8 ns, starting from each of these configurations. Error bars were calculated from the variance between the runs. The charges on the carbon sites in the surface were chosen to give surface charge densities of ±1e/nm2, ±0.5e/nm2, and 0. The left-hand and right-hand surfaces carried equal and opposite charges. Note that 1e/nm2 = 0.16 C/m2 = 16 μC/cm2. Different probes were distinguished by their charges, +1e and −1e. The probe ion was fixed at a certain distance from one of the surfaces, and the other ions in the system were left free to move under the force field. In the GROMACS simulations, the long-range electrostatic interactions were treated by the particle mesh Ewald method with a correction to give a pseudo-2d condition.78 In the DL_POLY simulations, the long-range electrostatic interactions were treated by an Ewald summation with the same correction.

represent a new class of highly concentrated electrolytes that are not well described by standard double layer theories.1 Only recently, reports on theoretical approaches that can be applied to the electrode−ionic liquid interface have started to appear in the literature.21,32,61,62 (iii) Some properties of ionic liquids (high viscosity of most ionic liquids, impurities, hygroscopicity of (some) ionic liquids, etc.) complicate experimental as well as computational studies of these systems.34,51 In this article, we use molecular dynamics simulations to examine the influence of two ionic liquid solvents on the interaction of dissolved ions with a graphene electrode surface. Graphene was chosen as the material for our model electrodes because (i) graphene is a good example of an atomically flat surface and its interactions with ionic liquids can be also studied experimentally63,64 and (ii) graphene−ionic liquids systems show high potential for future applications in energy storage,65−68 sensing,69 and electrochemical gating devices.63,64 For the purpose of qualitative and quantitative comparisons of observed trends, we have chosen 1,3-dimethylimidazolium chloride [MMIm]Cl and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF4]. The [BMIm][BF4] ionic liquid differs from [MMIm]Cl by (i) a longer alkyl chain and (ii) a larger size anion, which allows us to perform comparative analysis of the effects of the length of the alkyl chain and the size of anion on the phenomena under study. [MMIm]Cl was used in our pilot MD simulation study of ion interactions with neutral and charged graphene surfaces.39 [BMIm][BF4] and the similar [BMIm][PF6] are widely used for experimental studies and are common objects of computational studies of the electric double layer structure.70−74 By performing detailed examination of the solute ion−electrode free energy profiles and perturbations of solvent structure at the electrode and near the solute ion, we address the following questions: 1. Are there similarities in the solvent-mediated screening between solute ions and the electrode surface in different ionic liquids? 2. What is the role of the detailed molecular structure of the ions of the ionic liquids on the screening of the solute− electrode interactions? 3. How does the magnitude of the charge density on the electrode affect the energetics (as measured by free energy) of the screened solute−electrode interactions? Are there any qualitative differences between lowly and highly charged surfaces? 4. How extensive are the mutual perturbations of the solvation layers near the electrode and the solvation shells of the solute ion as the solute approaches the electrode? The article is organized as follows: First, we describe the simulation setup and methodology used for our analysis of the solvent-mediated solute ion interactions with the charged surface in the modeled ionic liquid−graphene systems. The following section contains the results of the free energy profiles analysis and details of the interfacial structure characterization. The observations and their interpretations are summarized in the Conclusion section.

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ANALYSIS The free energy profiles were determined from the mean forces on the probes at different distances. These were averaged over all eight runs for a given surface charge at each distance, z. The free energy profiles were then determined by integrating the average force with respect to distance using Simpson’s rule. The measurement of average total densities and number densities of cations and anions as a function of distance from the surface is straightforward. Numbers of ions, total masses, and orientations were averaged in bins lying between z − δz/2 and z + δz/2, with δz = 0.02 nm. Spatial distribution functions about the probe were calculated using Gromacs tool, g_spatial. Finally, contour plots for the cylindrically averaged charge

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TECHNICAL DETAILS Simulations were carried out using molecular dynamics in an NVT ensemble with a temperature of 350 K for [BMIm][BF4] and 500 K for [MMIm]Cl. The temperature was maintained with a velocity-rescaling thermostat. The systems consisted of 5842

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Figure 1. (A) [MMIm]Cl counterion densities with a charge opposite that of the surface charge (±1e nm−2). (B) Free energy profiles for charged probes approaching the surfaces in [MMIm]Cl. The upper two free energy profiles correspond to the two attractive situations (positive probe with negatively charged surface and vice versa) and are offset by 20 kJ/mol, the middle pair of profiles correspond to an uncharged surface, and the lower pair (offset by −20 kJ/mol) correspond to the two repulsive situations. (C) [MMIm]Cl co-ion densities with the same charge as the surface charge (±1e nm−2). Red solid mark: ring center or a probe with charge +1e; blue open mark: Cl− ion or a probe with charge −1e. Square, positively charged surface (+1 e nm−2); triangle, neutral surface; circle, negatively charged surface (−1e nm−2). The highlights show correlations of free energy minima with ion densities (see text). Note the oscillations in free energy and the barriers that must be overcome for a dissolved ion to reach an oppositely charged surface.

Figure 2. (A) [BMIm][BF4] counterion densities with a charge opposite that of the surface charge (±1e nm−2). (B) Free energy profiles for charged probes approaching the surface in [BMIm][BF4]. The upper two free energy profiles correspond to the two attractive situations (positive probe with negatively charged surface and vice versa) and are offset by 20 kJ/mol, the middle pair of profiles correspond to an uncharged surface, and the lower pair (offset by −20 kJ/mol) correspond to the two repulsive situations. (C) [BMIm][BF4] co-ion densities with the same charge as the surface charge (±1e nm−2). Red solid mark, [BMIm]+ ion or a probe with charge +1e; blue open mark, [BF4]− ion or a probe with charge −1e; square, positively charged surface (+1e nm−2); triangle, neutral surface; circle, negatively charged surface (−1e nm−2). The highlights show correlations of free energy minima with ion densities (see text).

Although the contact position at 0.3−0.4 nm from the surface does correspond to a minimum of free energy, this is not the only minimum or necessarily the position of lowest free energy. The screening due to the ionic liquid is more complex than the picture provided by the Debye screening model. We note that it was shown previously that the charge screening in diluted electrolyte solutions at the molecular scale is also more complex than the simple Debye screening (see, e.g., refs 79−84). A distinct qualitative difference between the low and high concentrated electrolytes is the number of barriers on the free energy profiles. In ionic liquids we clearly observe more than one barrier for the probe motion toward the surface. The profiles in the middle (neutral surface case, Figures 1B and 2B) show that even with an uncharged surface, there are multiple oscillations in the free energy profiles. In the lowest profiles (repulsive case), one sees that the ionic liquid gives a broad free energy minimum when the probe is separated from the like-charged surface by a few tenths of a nm. Overall, a clear similarity between [MMIm]Cl and [BMIm][BF4] ionic liquids is seen in the height of the barriers that need to be overcome for the probe to reach an oppositely charged surface. In every case, the minima are correlated with the positions of the solvation layers formed by ionic liquid ions with the same

density distributions were made in the (R,z) plane, where R2 = x 2 + y2 .

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RESULTS AND DISCUSSION Free Energy Profiles. Free energy profiles in two ionic liquids are shown in of Figure 1B for [MMIm]Cl and Figure 2B for [BMIm][BF4] (central portions labeled B). Six profiles are shown for each liquid. The upper pair (offset by 20 kJ/mol) shows the attractive situation, where the surface and the probe have opposite charges. The central pair of the profiles refers to an uncharged surface, and the bottom pair (offset by −20 kJ/ mol) shows the repulsive situation, where the surface and the probe carry the same charge. The first point to notice is the strong similarity of behavior in the two ionic liquids with several free energy minima and high barriers impeding the approach of a charged probe to an oppositely charged surface. If the solvent provided Debye-type screening, in the attractive case when the surface and the probe attract each other, one could expect a monotonic increase of free energy from the minimum point where the probe is in contact with the surface, that is, from the “contact” minimum on the free energy profile. 5843

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charge as the probe. The results of our and previous simulations70−74 show that the interfacial structure can be represented by alternating ion layers characterized by different packing and charge densities. This is illustrated in Figures 1A and 2A for the attractive case and Figures 1C and 2C for the repulsive case. The correlations of ionic liquid ion density and free energy minima are highlighted for the two situations. In our earlier works, we interpreted this correlation as showing that the probe tends to replace an ionic liquid ion of the same charge in the layered structure of [MMIm]Cl induced by the surface charge.39,85 This implies that the positions of free energy minima are determined by the relative size of ions, which truly can be seen in Figures 1B and 2B. In this work, we deepen the interpretation by examining the local interfacial structure in more detail and show that it applies to both [MMIm]Cl and [BMIm][BF4] ionic liquids. Local Structure. To obtain more detailed insights into the shape of the free energy profiles, we examined the local solvent structure around the probe at different probe distances from the electrode. Because, on average, the local fluctuations of the molecular environment should be cylindrically symmetrical around the normal to the electrode which passes through the probe,36 one can decrease the noise and show more information by making cylindrical averages of the three-dimensional spatial distribution function about this normal. The process is illustrated in Figure 3.

Figure 4. Contour plots of the cylindrically averaged charge density around probes (solid circle) at different distances from the charged surfaces in [MMIm]Cl. (A) Positive probe near a negatively charged surface. (B) Positive probe near a positively charged surface.

Figure 5. Free energy profiles for charged probes approaching low(±0.5e nm−2) and highly (±1e nm−2) charged surfaces in [MMIm]Cl. The upper free energy profile corresponds to the attractive situation (positive probe with negatively charged surface) and are offset by 20 kJ/mol; the lower, to the repulsive situation (positive probe with positively charged surface). Square, positively charged surface (+1e nm−2); triangle up, positively charged surface (+0.5e nm−2); triangle down, negatively charged surface (−0.5e nm−2); circle, negatively charged surface (−1e nm−2). Open marks indicate positions selected for the analysis shown in Figure 4.

Figure 3. Figure showing the relation of the cylindrically averaged charge density to the three-dimensional spatial charge distribution around a positively charged probe. (A) Three-dimensional spatial distribution of [BF4]− anions around a spherical probe. (B) Schematic procedure of cylindrical averaging. (C) Cylindrically averaged charge density distribution. The surface is schematically shown as a rectangle, and the probe, as a solid circle. The dashed line indicates the first solvation shell around the probe.

near the positive probe and the positive solvation layer is enhanced, stabilizing the system. In the repulsive case (Figure 4B), the first minimum is stabilized by a ring of chloride ions above the surface, but below the probe. These ions provide shielding, although they are not directly between the probe and the wall. In the barrier position, the negative solvation layer of the wall distorts the solvation shell of the probe, leaving an unfavorable gap between the shell and the layer. This is somewhat similar to the results reported by Gelb, Patrick, and Lynden-Bell for the forces between a probe and surface in a Lennard-Jones solvent.36,86,87 Finally, in the second minimum (z = 1.1 nm), the solvation shell of the probe is more nearly complete, and the screening is favorable. The general features of the solute ion−electrode interactions screening in [BMIm][BF4] are very similar to those in the [MMIm]Cl ionic liquid (Figures 5 and 7). This is apparent in Figure 6, which shows the cylindrically averaged charge density around a positive probe in [BMIm][BF4] at various distances from a charged surface. In the contact position, there is very little shielding of the probe−surface interaction by the solvent; in the barrier position, the solvation layers are significantly restructured; in the positions of free energy minima, the order of solvation layers is less affected.

Figures 4−5 show how the solvation layering near the surface and the solvation shell around the probe interact and interfere with each other in [MMIm]Cl. In Figure 4A,B there are shown the cylindrically averaged charge density distributions around a positive probe at different distances from a negatively charged surface (the attractive case) and a positively charged surface (the repulsive case). The positions shown are the following: at the first minimum (left); at the barrier (middle), and at the second minimum (right). For reference, they are marked with open marks in Figure 5, presenting the corresponding free energy profiles. In the attractive case (Figure 4A), the first minimum is the contact minimum in which the short-range repulsive force from the wall on the probe balances the direct electrostatic force from the wall on the probe. The ionic liquid has little effect on this minimum. The existence of a barrier and a second minimum is, however, entirely due to the changes in screening due to the ionic liquid. The figure shows how the solvation shell of the probe gradually develops as z gets larger. In the second minimum (z = 0.8 nm), the part of the solvation shell 5844

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Figure 6. Contour plots of the cylindrically averaged charge density around probes (solid circle) at different distances from the charged surfaces in [BMIm][BF4]. (A) Positive probe near a negatively charged surface. (B) Positive probe near a positively charged surface.

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CONCLUSIONS Our simulation results show that the structure of the solvation layers near a graphene electrode in the studied [MMIm]Cl and [BMIm][BF4] is rather complex and can be described as a succession of ionic layers with different packing and charge densities. As a result of this layered structure, the free energy profiles (potentials of mean force) for charged probes to approach the electrode have multiple oscillations. The free energy profiles show high barriers impeding the passage of an ionic probe to an oppositely charged surface. Interestingly these barriers are qualitatively comparable in the two ionic liquids, although the number density of ions is considerably lower in [BMIm][BF4] compared with [MMIm]Cl. The forms of the free energy profiles in the three situations, attractive surface− probe, neutral surface−probe, and repulsive surface−probe, differ from each other but are similar (apart from a length scaling) in the two ionic liquids. Generally, the free energy maxima occur at those positions of the probe wherein the solvation layer there is a high concentration of ionic liquid ions of the same charge as the probe. However, more detailed examination of the ionic liquid structure in the presence of both the surface and the probe shows the importance of the interaction between the solvation layers induced by the surface and the solvation shells around the charged probe. At those positions where these solvation structures enhance each other, the free energy profile is lower, whereas at those positions where they distort each other, the free energy is higher. We note that the probe can significantly perturb the layered structure of the ionic liquid at short distances from the electrode; that is presumably the reason for the high free energy barriers at the interface. By comparing the free energy profiles at different electrode charge densities, we conclude that the structure of solvation layers at the electrodes determines the positions of the minima and maxima on the profiles that qualitatively do not change with the electrode charge. However, the heights of the barriers strongly depend on the ionic liquid composition and the electrode charge: they increase when the magnitude of the electrode charge density increases due to the overall increase of liquid density at the interface. We think that the observed dependencies of positions and heights of the ion−electrode free energy barriers in ionic liquids from the electrode charge and the liquid structure can explain an apparent discrepancy in the reported experimental results of redox reaction rates at charged interfaces in ionic liquids, where

Figure 7. Free energy profiles for charged probes approaching lowly (±0.5e nm−2) and highly (±1e nm−2) charged surfaces in [BMIm][BF4]. The upper free energy profile corresponds to the attractive situation (positive probe near negatively charged surface) and are offset by 20 kJ/mol; the lower, to the repulsive situation (positive probe with positively charged surface). Square, positively charged surface (+1e nm −2); triangle up, positively charged surface (+0.5e nm −2 ); triangle down, negatively charged surface (−0.5e nm−2); circle, negatively charged surface (−1e nm−2). Open marks indicate positions selected for the analysis shown in Figure 6.

Overall, the shapes of the free energy profiles depend on changes in the system energy and the system entropy as the position of the probe changes. The existence of the contact minima is mainly due to the direct probe−surface interactions, but the shapes of the rest of the free energy profiles additionally depend on the probe−solvent and solvent−solvent interactions. Molecular scale interactions between a reactive solute immersed in a solvent are known to affect the potential and the rate constant of redox reactions.88−92 Our results can shed light on the puzzling variation of the rates of different redox reactions in ionic liquids (it was shown that some reactions in ionic liquids are faster than in organic solvents,91,93−97 and some are significantly slower88,98). Indeed, in those situations when there is a high free energy barrier for the reactant to come to the electrode, one can expect a significant slowing down of the redox reaction. On the other hand, in those situations when there is a free energy minimum for the reactant close to the electrode surface, one can expect an acceleration of the corresponding redox reaction. We note that there are other possible explanations for the variations in the reaction rates in ionic liquids (see refs 51, 89, 99), but we think that this hypothesis is worthy of further theoretical and experimental investigation and this is a subject of our future works. 5845

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both slowing down as well as acceleration of reaction rates were observed (see, e.g., refs 89, 91, 97) In line with the recent experimental results by Xiong et al.97 and Li et al.,100,101 the simulation results presented in our work show that the interfacial structure as well as the energetics of the ion− electrode interactions in ionic liquids (and, consequently, the rate of the charge transfer reactions) can be rationally tuned by (i) electrode charge density and (ii) molecular structure of ionic liquids.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the supercomputing support from the von Neumann-Institut für Computing, FZ Jülich (Project ID ESMI11). A large part of the results were obtained using the EPSRC funded ARCHIE-WeSt High Performance Computer (www.archie-west.ac.uk). EPSRC Grants No. EP/K000586/1 and EP/K000195/1. We thank Alexei Kornyshev for useful discussions on theories of the electric double layer in ionic liquids. We thank Frank Endres, Susan Perkin, and Tom Welton for useful discussions concerning experiments on interfacial structure in ionic liquids. We thank Bernhard Roling and Marcel Drüschler for useful discussions on interfacial electrochemistry in ionic liquids. We thank Steve Baldelli for useful comments and discussions on molecular-scale interactions of graphene with ionic liquids. We are thankful to our colleagues, Kathleen Kirchner, Tom Kirchner, and Andrey Frolov, for useful discussions and help with setting up the simulations.

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