Ion Interactions Across Graphene in Electrolyte Aqueous Solutions

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ion Interactions Across Graphene in Electrolyte Aqueous Solutions Martin Pykal, Michal Langer, Barbora Blahova Prudilova, Pavel Banáš, and Michal Otyepka J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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The Journal of Physical Chemistry

Ion Interactions Across Graphene in Electrolyte Aqueous Solutions Martin Pykal, Michal Langer, Barbora Blahová Prudilová, Pavel Banáš, Michal Otyepka* Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University Olomouc, tř. 17. listopadu 12, 771 46 Olomouc, Czech Republic *E-mail: [email protected].

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ABSTRACT The interfacial behavior of graphene is involved in a number of technological processes and applications, ranging from energy storage to sensing and nanofluidics. The organization of ions and structuring of water molecules close to a graphene interface, which represents an atomically thin surface, substantially affects the interfacial physicochemical properties in electrolytes as well as the specific capacitance of supercapacitors. Moreover, adsorption of ions on one side of the ultimately thin material may largely impact the adsorption of additional charge carriers on the opposite side and thus influence the overall supercapacitor performance. However, these phenomena are so far not fully understood. In this study, all-atomic classical molecular dynamics (MD) simulations were conducted with explicitly included polarization, which is essential for accurate description of electrolytes at interfaces in systems containing carbon allotropes. We employed isotropic polarization model using classical Drude oscillators and adjusted Thole parameters for graphene. This approach improved the classical description of graphene-electrolyte interaction, although did not fully cover the inherent anisotropy of graphene polarization because the field components parallel with the graphene sheet were largely reduced but not completely screened as in semimetal. The MD simulations were applied to examine the interface between graphene and potassium halide solutions. The results showed that water molecules formed a well-organized single layer on both sides of graphene, which primarily acted as a hydrophobic structuring agent. This arrangement significantly contributed to effective shielding of ion-ion interactions acting


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through the graphene sheet. Thus, the ion-specific structuring of adjacent electrolytes on opposing sides of graphene was generally independent. The findings help to understand structuring of electrolyte on graphene-based electrode materials of supercapacitors.

INTRODUCTION The successful isolation of graphene in 20041 opened up a new branch of novel twodimensional (2D) materials, which are currently gaining increasing interest in nanosciences. The extraordinary high surface area to volume ratio of 2D materials makes them ideal for use in diverse applications, including sensing,2–5 energy storage,6,7 desalination processes8,9 and nanofluidics.10,11 It has been shown that interaction of electrolytes with graphene and carbonaceous materials significantly affects the performance of supercapacitors.12–14 However, many details of the mechanism of energy storage by supercapacitors remain unknown.14 The interaction of ions with graphenebased materials also determines the final function in desalination technologies.15 Without doubt, further progress in these disciplines relies on a thorough understanding of the nature of interactions of participating molecules, ions and solvents with graphene and related materials. Liquid/solid and liquid/gas phase interfaces induce structuring of solvent and ions at the phase boundary.16,17 At an air/water interface, studies revealed that large halide anions (i.e., Cl–, Br–, I–) preferentially adsorb at the interfacial region.18,19 Later, molecular dynamics (MD) simulations showed that small and non-polarizable ions tend to accumulate mainly


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in the bulk phase, and thus remain excluded from the interface, whereas large and polarizable ions tend to accumulate at the air/electrolyte interface.17 Similar trends were observed in the vicinity of a rigid hydrophobic wall (model of a hydrophobic solid surface), where large halide ions20,21 as well as hydroxide and hydronium ions22–24 exhibited high interfacial affinity. It was suggested, that the ion adsorption on the graphene/water interface was predominantly driven by enthalpic change, unlike the air/water interface, where the entropic component became important.25 Moreover, the hydrophobic wall was shown to induce strong structuring of interfacial water molecules in the proximity of the surface,16,26 influencing physisorption of ions at the interface either by surface-modified ion hydration or, in the case of OH−, by generating an electrical potential gradient that interacted with the dipole moment of the adsorbing species.20,22 At a double layer graphene channel/pore, it was shown that this water structure was only slightly affected by the presence of electrolyte.21,27 Further, Vácha et al.23 showed that the hydroxide ion adsorption and water structuring were considerably suppressed if the thermal motion of the hydrophobic boundary was taken into account. However, 2D materials such as graphene represent a unique case because they are ultimately atomistically thin (i.e., further irreducible) surfaces. There is currently much debate about how a single graphene layer on top of a solid surface may affect its surface properties and its wetting transparency may alter the physicochemical properties.28 It has been reported that graphene coating does not significantly affect the wetting behavior of underlying surfaces


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(except superhydrophobic and superhydrophilic substrates).29 However, the interfacial behavior of graphene immersed in an electrolyte solution, i.e., behavior of one-atom thick layer surrounded on both sides by electrolyte, has not yet been explored. It is worth noting that many related features, e.g., the Coulombic coupling of molecules across the walls and molecular transport phenomena, were recently overviewed for nanotubes (1D carbon allotrope) by Král and Wang.30 The interaction of ions with graphene and carbon-based materials is also particularly important for performance of electric double-layer capacitors, i.e., supercapacitors.31 Supercapacitors store electric energy by accumulating charge on electrode/electrolyte interface.32 Carbon-based materials, graphene and graphene derivatives have been identified as suitable electrode materials due to their large surface area and inherent conductivity.31,33–3536,37 The exact mechanism of charge accumulation is still under intensive debate. It is accepted that the mechanism depends on the electrode material, electrode polarization and electrolyte.14 The same authors also claimed that ion-exchange (desorption of the likely charged ions from the electrode) is very important and always present contribution to the supercapacitor charging mechanism. This implies that interaction of electrolyte ions with carbon-based electrode should contribute to determination of supercapacitor capacitance. Furthermore, it was shown that ions of the same charge confined in ultra-narrow electrified nanopores partially broke the Coulombic ordering due to screening of the electrostatic interactions and increased the potential


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concentration at the electrode.38 However, this effect was largely suppressed when a bilayer structure of the adsorbed ions was formed. Without any doubt understanding of interaction of ions with carbon-based electrodes and shielding of interaction of ions across the electrode materials should contribute to rational design of more efficient supercapacitor. Here, we analyze a graphene/electrolyte (precisely electrolyte/graphene/electrolyte) interface and degree of graphene transparency toward hydrated ions in potassium fluoride and potassium iodide solutions using MD simulations with a polarizable force field. The chosen potassium halides represent common electrolytes for aqueous electrochemistry and, moreover, these two species differ markedly in their anion size that has been shown to largely affect their behavior at the interface.39 MD simulations provide a unique tool for studying physicochemical properties of graphene under well-controlled conditions (e.g., no unwanted airborne contamination or agglomeration tendency). Although it has been suggested that the polarizability of both water and the surface has a negligible impact on the water dynamics and structuring at a graphene interface,40 inclusion of explicit polarization in MD simulations was identified as essential for accurate description of electrolytes at interfaces in systems containing carbon allotropes.21,41–43 Our MD simulations showed that fluoride anions are selectively excluded from the interface, whereas I− ions showed a strong affinity to the interfacial region, consistent with prior observations.17 The movement of halide ions across the hydrophobic boundary was


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studied. The results indicated that the structuring of the ionic interfaces was not affected by the presence of the other interface behind the one-atom-thick wall and that both interfaces behaved independently. In other words, the interaction of ions across a graphene sheet was well shielded and a compact and dense layer of structured water molecules on the surface significantly contributed to this shielding. METHODS Computational Methods. The electrolyte/graphene interface was investigated for solutions of two inorganic salts, i.e., potassium fluoride and potassium iodide, which have been shown to exhibit contrasting behavior at a simple electrolyte/air (EA) interface.17 The computer simulations included explicit polarization based on the Drude polarizable model calculated using the NAMD 2.11 program package.44 All simulations used the four-site polarizable SWM4-NDP45 water model together with polarizable monovalent ions that were parametrized in conjunction with the water model.46 However, the original parameters led to excessive ion pairing in solution. Formation of relatively large clusters of ions resulted in their reduced mobility and thus in reduced osmotic pressure. Thus, the Thole parameters were refined to correctly reproduce the experimentally measured osmotic pressure47, as suggested by Luo et al.48 This experimental observable is generally suitable for validation and refinement of force field parameters of ionic solutions. The comparison of original parameters and refined


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parameters for KF and KI is shown in Figure 1. Parameters used in this work are summarized in Table 1.

Figure 1. Osmotic pressure dependence on salt concentration of A) KF and B) KI solution showing experimental values (black squares), MD simulation with original parameters (red diamonds) and simulation with refined Thole parameters (blue circles). C) Side and top view of a graphene interface showing the excessive KI pairing of original force field parameters. Table 1. Thole parameters used for MD simulations. Atom types KF (POTD-FAD) KCl (POTD-CLAD) KBr (POTD-BRAD) KI (POTD-IAD)

Used Thole parameter 1.24467 2.91744 2.58776 2.75567

The carbons in graphene were treated as Lennard-Jones (LJ) spheres by using the forcefield parameters proposed by Cheng and Steele.49 The lateral dimensions of graphene were set to approximately 30 × 30 Å, whereas the z-dimension was set sufficiently high


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to be unaffected by the periodic image.50 To include the effect of graphene polarization, we used the approach published by Ho et al.40 A fixed charge based on the predicted graphene polarizability of α = 0.878 Å3 was assigned to each carbon atom. The auxiliary Drude particle carried a charge opposite in sign to that at the center. Although the inplane polarizability in C atoms of graphite-like structures should be ~3.5 times larger,51 it was shown that calculations using isotropic polarizability provide about the same geometries and binding energies as those obtained in calculations with an anisotropic polarizability.52 This might be rationalized by the fact that the in-plane component of the electric field intensity vector of a given ion drops down in lateral direction slower than the corresponding out-of-plane component. As a consequence, more atoms (Drude oscillators) contribute to the in-plane polarizability response than to the out-of-plane one, thus the overall response of the graphene acts effectively in anisotropic manner. However, the abovementioned value was suggested for the out-of-plane polarizability of graphene which can result in almost a 20% difference in polarization interaction energy at a typical graphene contact distance (3.5 Å) (see Figure 2). Unfortunately, the used simulation package was not able to address this phenomenon due to inability to exclude the C-C interactions in the graphene plane (for this specific purpose the LAMMPS package53 was used; we did not use this package in the main study for performance reasons). Thus, the isotropic polarizability was increased accordingly (together with adjustment of the Thole parameter) to provide the same polarization interaction energies at the contact distances


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as the out-of-plane polarizability. The polarization interaction energy profile was obtained by scanning the distance (in the interval of 1.5-99.5 Å; bin size 2 Å) between a periodic graphene sheet (30 × 30 Å) and a non-polarizable atom with a charge of +1e. System was neutralized by an additional negative charge that was placed in a constant distance from the positively charged atom. The undesired periodic interactions in z-axis between replicas were treated by the correction from Yeh and Berkowitz.54 The simulation options were kept same as far as possible with the main work. In each window, the system was constrained except the Drude atoms which were let to move freely to adapt their lowest energy positions. We found that the rescaling of α by 1.09 and Thole parameter by 0.29 provided the best consistency with the polarization interaction energy obtained by outof-plane polarizability (see Figure 2). The used setup can be applied to the graphene/electrolyte interface safely, however, its applications for other phenomena should be considered with care due to the inherent limitations of the setup. It should be stressed that the graphene was considered to be polarizable but not behaving as a semimetal, i.e., the field components parallel with the graphene plane were reduced but not screened as in semimetal. The setup may fail for instance in material drag phenomena, where the lateral polarizability plays a vital role. Thus, the results should be interpreted with care, keeping in mind limitations of the used approach.


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Figure 2. Polarization interaction energy of graphene with (black squares) and without (red circles) inclusion of C-C interactions. The blue curve represents the fitted potential with rescaled α and Thole parameters. The simulations were run at 300 K. The equations of motion were integrated with a time step of 0.5 fs. The cutoff distance for non-bonded interactions was set to 1.2 nm. The production run was preceded by a thermalization step (from 10 to 300 K for 0.5 ns). System coordinates were recorded every 10 ps. Each simulation lasted 15 ns. The first 5 ns were considered as an equilibration period, and density profiles and subsequent data and analysis were calculated from the last 10 ns of simulations. RESULTS AND DISSCUSSION


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To study the structure of different interfaces, including graphene, several starting structures were considered (Figure 3): i) periodic rigid graphene placed within a box of water and ions, representing an electrolyte/graphene (EGE) interface; ii) electrolyte solution enclosed between two periodic graphene layers (both rigid and flexible) in pure water, representing an electrolyte/graphene/water (EGW) interface; and iii) a water slab with ions enclosed between two periodic flexible graphene layers in air/vacuum (that are indistinguishable in any given dimension), representing an electrolyte/graphene/air (EGA) interface. The results for the rigid and flexible models were nearly identical, but flexible graphene had a less structured vicinity due to its phonon states (Figure 4). Although two different salt concentrations of the individual electrolytic systems were considered, i.e., 0.6 M and 1.2 M, both gave similar results. Thus, only the results for 1.2 M concentration are discussed below. The low sensitivity of the hydrophobic surface to I– concentration was also proven experimentally by equilibrium contact angle measurements on graphene (see SI for details), which showed no substantial change for salt concentrations below 3.0 M (Figure S1).


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Figure 3. Initial structures with randomly generated ions used in MD simulations to study


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an A) EGE, B) EGW, and C) EGA interface. For clarity, beside each structure is shown a simulation box (bordered by a blue rectangle) replicated periodically three times along

the z-direction, perpendicular to the interface. Coloring scheme: grey - graphene, red spheres - potassium, green spheres - fluorine.


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Figure 4. Axial density profiles for KF and KI solutions at A) a simple EA interface, and B) EGE (rigid graphene), C) EGW (two thermal graphenes) and D) EGA (left: rigid graphene; right: thermal graphene) interfaces (insets show nearest 5 Å region at the EGW/EGA interface). E) Side and top view of ion arrangement in the interfacial region of an EA, EGW and EGA interface in KI (in the latter cases, graphene is omitted for clarity). Coloring scheme: blue - water, black - graphene, red - potassium, green - fluorine, yellow - iodine. First, we examined the position of ions in KF and KI solutions at different interfaces comprising graphene (EGE, EGW and EGA). Figure 4 shows density profiles along the direction perpendicular to the interfaces. To ensure that the parameters selected correctly described previously observed features21,39,55 and were consistent with our premise about the different behavior of these ions on interfaces (see above), we also ran simulations at a simple EA interface. In accord with the literature, in KF solution, fluoride ions were repelled from the surface layer, showing a higher propensity toward the bulk region. In contrast, in KI solution, iodide ions accumulated significantly at the EA interface. In both cases, potassium cations were dragged with the anions and, in the case of KI, formed an apparent ionic double layer near the interface. This behavior is consistent with the condition of electroneutrality and agrees with previously published works.55,56 Formation of two distinguishable water layers on the graphene surface (regardless of rigid/flexible surface or type of dissolved ions) was observed (Figure 4B, blue curve). As suggested before,23 the presence of electrolyte did not significantly influence the


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structuring of water at the interface. In other words, the water structure near the hydrophobic wall was insensitive to the presence of ionic species - there were no substantial changes in either the O/H density plots or dipole orientation of water molecules involved in the first water layer induced by the hydrophobic wall (see Figure S2). Only water molecules in close proximity to ions (< 5 Å distance) altered their mutual orientations slightly due to the ion’s electrostatic field. However, the vicinity of graphene at the EGA interface was less ordered than at the EGW interface (see insets in Figure 4). This can be attributed to water-water interactions acting across the graphene layer, i.e., mutual attraction of water molecules on opposite sides of the interface. The coupling of water molecules through graphene was also previously observed during the self-assembly process of graphene nanostructures.57 The maximum density of the first layer of water was located 3.3 ± 0.4 Å above the graphene interface when either water or electrolyte was present on the opposite side of the surface (EGE and EGW systems), and 3.4 ± 0.4 Å when only air was on the other side (EGA). The maximum density of the second layer in all three systems was at 6.3 ± 0.8 Å. Formation of this dense layer of water molecules around the hydrophobic surface was also reflected in lower dynamics (higher residence time) of water molecules at the surface (Figure S3). Fluoride ions on the graphene surface showed similar behavior to that observed at the EA interface, where the anions tended to prefer the bulk solution over the electrolyte/graphene interface (also due to reduced hydration of ions at the graphene interface; see Figure S4). However, the hydrophobic graphene induced a


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small increase in F– density near the graphene wall coinciding with the first water layer. In contrast, I- ions mostly occurred near the boundary, similarly to the EA interface. However, the first layer of water molecules in contact with the hydrophobic surface was very compact and impermeable to iodides, unlike at the EA interface, where the ions were largely exposed to an air environment (Figure 4E). Nevertheless, interfaces including graphene (i.e., EGE, EGW and EGA) exhibited very similar basic characteristics. I– formed a dominant peak (located at 4.1 ± 0.4 Å for EGE and EGW, and 4.2 ± 0.7 Å for EGA) with a broader K+ layer compensating the accumulated charge near the boundary. Concisely, ions were generally attracted to the graphene surface and this tendency was much more apparent for I–. The less ordered water structuring (insets in Figure 4) at the EGA interface was also reflected by a reduced density of iodide ions compared to at the EGE and EGW interfaces. However, the hydration number of the ions was independent of the type of interface (Figure S4). Owing to the organization of water molecules induced by the presence of graphene, I– ions were forced out of the nearest interfacial region and instead accumulated beyond the first water band. This indicates that confined water molecules principally governed the electrolyte structure near the hydrophobic surface partly by increasing the potential minimum separation distance between the ions on opposing sides and partly by shielding the ion-ion interactions, thus influencing the shape of the interfacial ion bands (see insets in Figure 4C, D). To further analyze the interaction of ions across the interface, we examined lateral density maps of iodine ions. We inspected two


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contrasting arrangements where ions were either in contact-distance (EGE) or without the presence of interfering factors/interacting species (EGA, EGW) on the imaginary adjacent faces. The densities were summed over the nearest 5 Å region oriented perpendicularly to the interface/graphene and averaged over the last 10 ns of the simulation. The resulting plots are shown on Figure 5. At the first glance, there was an evident reduction in density in the EGA system (Figure 5C). This was consistent with the less ordered vicinity of graphene with shallower iodide distribution compared to the EGE and EGW interfaces (see insets in Figure 4C, D). Nevertheless, this representation did not provide any unambiguous

information

about

the

interaction

shielding

(movement

correlation/anticorrelation) between ions located on opposite sides of the interface. Thus, to investigate this further, we divided the opposite interfacial region into small domains (small rectangular fractions with a projected square area of ~3 Å2; for a schematic, see Figure S6) and compared the presence of ions on corresponding sides of the interface through the production run. Fluorides, which were generally repelled from the interface and thus not expected to interact, were found in opposing interfacial domains in < 1 % of cases. In contrast, in KI, I– ions were found on both sides of the interface in 11 % of cases regardless of the type of interface (i.e., the value coincided with the case where no mutual interaction could be expected – at the EGW and EGA interfaces; see Figure S6). Image analysis, i.e., colocalization quantification, was performed on corresponding pairs of 2D density plots by calculating Pearson’s correlation coefficient measurement. The


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obtained values were close to zero for all the studied interfaces (0.01, 0.008 and -0.039 for EGE, EGW and EGA, respectively), suggesting there was no correlation/anticorrelation in the ion movements across the boundary. We fixed one iodide ion 4.1 Å above graphene surface and monitored behavior of ions on the opposite side (at the EG and EGW interfaces). Any specific structuring of ions (both positively and negatively charged) was not observed on the opposite side and corroborated our prior observations (Figure S5). Thus, ion-ion interactions were reduced by the environment, i.e., structuring of the individual electrolytes each side of the graphene layer was completely independent (acted as separate electrolytes). To support this observation, we ran an additional simulation similar to the EGW arrangement but with individual adjacent volumes containing only cations or anions on their corresponding side (zero net charge; see Figure S7A for details). In such an arrangement, the ions accumulated mainly at the interface (regardless of the type of ion, i.e., anions or cations; Figure S7B) and interfered due to strong electrostatic interactions between separated ions. Ions in corresponding domains at the interface were found in 28 % and 33 % of cases in KF and KI, respectively. These findings suggest that movement of the solutions in this case was correlated and that the electrolytes did not behave independently, in contrast to the previously discussed cases.


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Figure 5. Lateral density maps of iodide ions summed over a 5 Å region (in the z direction perpendicular to graphene plane; highlighted in the middle by a blue rectangular box


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from the side view) of A) opposite sides of the same graphene layer at the EGE interface, and ion-exposed sides of two distant graphenes at B) EGW and C) EGA interfaces. CONCLUSIONS The behavior of two different potassium halide solutions was examined by molecular dynamics simulations with explicitly included polarization and adjusted Thole parameters. It was shown that F– ions were preferentially present in bulk, whereas I- ions tended to occupy the interfacial area. When graphene was present, the interface arrangement of ions was predominantly governed by the structured layer of water molecules formed around the hydrophobic surface. The presence of water molecules on both sides of the graphene interface resulted in more dense organization of water around graphene compared to when the interface was surrounded by air on one side (water molecules were able to interact through graphene by dispersion). The water layer formed around hydrophobic graphene significantly contributed to shielding of ion-ion interactions acting between ions on opposite sides of the graphene layer. This resulted to an independent structuring of ions on each side of the graphene interface. These findings may be particularly relevant in supercapacitor applications showing that the single layered graphene binds ions on both sides without any potential interference. It implies that thinning of graphite-based materials up to ultimate single atomic graphene layer represents an efficient way to obtaining large surface area electrode materials likely without any loss of their charge storage capacity. We believe that our findings could help to


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shed light on design of more advanced and efficient energy storage materials. However, it is worth noting that our results are relevant for a resting (neutral) state of electrode and behavior of electrolytes during charging/discharging of supercapacitor electrode materials should be carefully explored. Supporting Information. Supporting Information available: additional methodology and experimental setup; refinement of the ion parameters; adjustment of graphene polarizability; hydration number profiles; contact angle measurements on graphene in different ionic concentrations of KF and KI; residence time of water molecules; water dipole orientation at the interface; additional axial density graphs. The material is available free of charge at http://pubs.acs.org/. AUTHOR INFORMATION Notes Corresponding author: Michal Otyepka, e-mail: [email protected]. The authors declare no competing financial interests. ACKNOWLEDGMENT Funding is gratefully acknowledged from the Ministry of Education, Youth and Sports of the Czech Republic (LO1305, CZ.1.05/2.1.00/19.0377), the Operational Programme Research, Development and Education – European Regional Development Fund (project no. CZ.02.1.01/0.0/0.0/16_019/0000754), the European Research Commission (ERC


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683024 from the European Union's Horizon 2020 program) and Palacký University Olomouc (student project IGA_PrF_2018_015). We would like to thank Petr Jurečka for helpful comments and discussions. REFERENCES (1)

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Ion Interactions Across Graphene in Electrolyte Aqueous Solutions

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