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Descriptions of an EDL structure using the ''counter-charge layer in generalized solvents'' (CGS) framework. (a) Distribution of ions (top panel) and the corresponding space charges (bottom panel) inside an EDL near an electrode with a surface charge density of s. An EDL is divided into N layers: the zeroth layer balances the electrode charge, and all other layers consist of a counter-ion sub-layer and a co-ion sub-layer. All EDL layers except the zeroth layer carry zero net charge and are thus considered as ''generalized solvents''. (b) A description of the effective space charge layers inside an EDL focusing on the change of space charge as the surface charge density of the electrode changes from zero to s. Downloaded by Vanderbilt University on 03 August 2011
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Room-temperature ionic liquids (RTILs) have received significant attention as electrolytes due to a number of attractive properties such as their wide electrochemical windows. Since electrical double layers (EDLs) are the cornerstone for the applications of RTILs in electrochemical systems such as supercapacitors, it is important to develop an unde...
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... on the EDL structures shown in Section 2.2 and those observed in prior MD simulations of EDLs in RTILs, 28,29,34,35,39 we propose to describe the structure of these EDLs from a completely different perspective. As shown in Fig. 5a, the EDL is treated as an N-layer entity. The zeroth layer only consists of the counter-ions closest to the electrode surface that exactly balance the charge on the electrode surface. The first layer consists of a counter-ion sub-layer and a co-ion sub-layer with equal but opposite net charges, and the counter-ion sub-layer is located ...
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... as ''green'' solvents, they are almost always viewed as individual ions in prior EDL studies. By emphasizing that counter-ions and co-ions must be considered together and further assuming that counter-ion sub-layers are always closer to the electrode than the co-ion sub-layer in odd-numbered layers and the opposite in even-numbered layers (see Fig. 5a), the CGS framework introduces several important advantages. Specifically, the overscreening of the electrode and the alternating layering of counter-ions and co-ions, two key properties of the EDLs in RTILs, are explicitly incorporated into the EDL model. Furthermore, as will be discussed later, incorporating these features will ...
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... is a generic feature of EDLs in RTILs: it not only occurs under small to moderate electrode charge densities, but also occurs (simultaneously with lattice saturation) under high electrode charge densities as evident from recent MD simulations and continuum modeling. 38 To derive a capacitance model for the EDL with the structure shown in Fig. 5a, we note that the potential drop Df EDL across such an EDL can be obtained by solving the Poisson equation. 39 If the background dielectric constant, although used in a number of force fields to account for the electronic polarizability of RTILs, 35,36 is neglected (i.e., its value is taken to be one as in the current MD simulations) ...
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... issue can be resolved by an alternative description of the EDL, in which the variation of the EDL structure as the electrode surface charge density increases from zero to s is explicitly delineated. Fig. 5b shows such an alternative description of the EDL. For the sake of simplicity, the effects of the molecular solvents are not considered but it is straight- forward to include their contributions. Similar to that in Fig. 5a, the EDL is divided into N layers. Inside each layer, however, the counter-ion and co-ion sub-layers are replaced ...
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... the variation of the EDL structure as the electrode surface charge density increases from zero to s is explicitly delineated. Fig. 5b shows such an alternative description of the EDL. For the sake of simplicity, the effects of the molecular solvents are not considered but it is straight- forward to include their contributions. Similar to that in Fig. 5a, the EDL is divided into N layers. Inside each layer, however, the counter-ion and co-ion sub-layers are replaced by counter-charge and co-charge sub-layers. A counter-charge sub-layer located at a given position denotes the accumulation of counter-charge due to both addition of counter-ions and removal of co-ions at that position as ...
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... model described in Fig. 5a and b and eqn (5) provides a framework for understanding the EDLs in RTILs. Unlike other existing models, 20,26,38 this model incorporates only a few, but critical physics such as overscreening and alternating layering of counter-ions and co-ions, and does not directly provide a quantitative prediction of the EDL structure and ...
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... Furthermore, we consider only two limiting cases, i.e., EDLs in neat RTILs and in hybrid electrolytes with 50% mass fraction of ACN. Since the PZC in both electrolytes are the same and C 0 exhibits the same trend as the capacitance C (see Fig. 4), we will analyze the apparent capacitance C 0 defined in eqn (2) using the EDL model given by Fig. 5a. Such an analysis is far less tedious than analyzing the capacitance C, and is sufficient for understanding the dependence of C on the electrolyte composition. The analysis of capacitance C using the EDL model given by Fig. 5b will be demonstrated in Section ...
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... capacitance C (see Fig. 4), we will analyze the apparent capacitance C 0 defined in eqn (2) using the EDL model given by Fig. 5a. Such an analysis is far less tedious than analyzing the capacitance C, and is sufficient for understanding the dependence of C on the electrolyte composition. The analysis of capacitance C using the EDL model given by Fig. 5b will be demonstrated in Section ...
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... analyze an EDL using the CGS framework in Fig. 5a and the capacitance model in eqn (5), we first divide it into separate layers (as shown in Fig. 5a) and compute g i in layer i. To this end, we integrate the counter/co-ion number density from the electrode surface toward the bulk electrolyte and compute the EIA factor. The results for the EDL in neat [BMIM][BF 4 ] are shown in Fig. 1b ...
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... analyze an EDL using the CGS framework in Fig. 5a and the capacitance model in eqn (5), we first divide it into separate layers (as shown in Fig. 5a) and compute g i in layer i. To this end, we integrate the counter/co-ion number density from the electrode surface toward the bulk electrolyte and compute the EIA factor. The results for the EDL in neat [BMIM][BF 4 ] are shown in Fig. 1b and c. Following the description made in Section 3, the BF 4 À ions within 0.27 nm from the ...
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... EDL capacitance. For this reason, subsequent EDL layers are not identified here. Once the EDL is divided into separate layers, g i in layer i can be obtained from the integration of ion number density as shown in Fig. 1b. We next compute the separation of counter-ion and co-ion sub-layers, D i , in each EDL layer i. In the EDL model described in Fig. 5a, the counter/co-ion sub-layer in each EDL layer and the charges are placed at discrete locations but represent ions distributed continuously in each EDL layer. Consequently, we must next determine the effective location z eff of the counter/co-ion sub-layers in each of the EDL layers identified above. z eff of the ions within each ...
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... is smaller than that at the positive electrode side (BF 4 À ion as counter- ion) although the closest approaches of the BMIM + ion and BF 4 À ion to electrode surfaces are nearly the same. Since we are interested in the capacitance, which reflects the variation of the EDL structure as an electrode becomes electrified, we use the scheme shown in Fig. 5b to describe the EDLs. To this end, we first determine the distribution of the effective counter- charges and the effective co-charges. Since the effective counter-charge at position z, r n,counter-charge (z) stands for the addition of counter-ions or removal of co-ions at this position as the electrode surface charge density increases ...
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... value. However, since the polarization of the generalized solvents in each EDL layer, as indicated by g i D i , decreases as the layer is located farther away from the electrode, the first layer plays the most important role in determining the EDL capacitance. With all four EDL layers nearest to electrodes included, using the EDL model shown in Fig. 5b and the accompanying capacitance model (eqn (5)), the EDL capacitance can be reproduced within 15% from that computed in MD simulations by using eqn (1). This suggests that, on one hand, the EDL capacitance is governed mainly by the polarization of RTILs (or generalized solvents within the CGS framework) within a short distance (B1 nm) ...
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... 5 Overscreening is often observed during the charging of EDLCs when co-ions near the interface trap undesired counter-ions. 6 This unusual double layer structure 7,8 is different from the commonly used Gouy-Chapman-Stern model. 9 The camellike differential capacitance curve caused by such unusual double layer structure has a significant impact on the energy storage of the EDLC. ...
... -(8) into Equation(3), the distribution of ion concentration at any given time can be expressed as it was widely reported that the effect of overscreening on bulk concentrations was negligible.8,14,29 Thus, the distribution function of ion concentration can be truncated within a finite number of layers, expressed as ...
... -(8) into Equation(3), the distribution of ion concentration at any given time can be expressed as it was widely reported that the effect of overscreening on bulk concentrations was negligible.8,14,29 Thus, the distribution function of ion concentration can be truncated within a finite number of layers, expressed as ...
Overscreening was commonly observed during charging/discharging cycles of electric double layer capacitors(EDLCs). This study presented a two-dimensional continuum electrochemical thermal model with a distribution function for concentration considering overscreening to investigate how overscreening affected the dynamic formation of electric double layer (EDL) near electrodes in macroscale. The results indicated that ion distribution was co-affected by EDL formation and overscreening near the electrode. Correspondingly, layered space charge density distribution resulted in a thicker diffuse layer, whose thickness was determined by the sum of co-ion and counter-ion diameters. Additionally, the diffuse layer capacitance was proportional to the reciprocal of counter-ion diameter, while the total capacitance was proportional to a linear combination of counter-ion and the larger ion diameters. Moreover, the shift from endothermic to exothermic for the total heat generation rate was dominated by heat of mixing, and was caused by the local ion mixing due to overscreening, resulting in local entropy increase. The results of this study can be used to further investigate the effect of overscreening on ion transient transport dynamic and heat generation rates.
... Because the dielectric layer acts as a capacitor in the electrowetting system, solid-liquid interfacial energy is reduced and θ decreases as voltage increases. In contrast, Fig. 4(c) shows an increase of the interaction energy between the RTIL and the solid surface due to strong ion-ion correlation in the RTIL and electrostatic interaction between ions and the charged surface [44,45]. Second, a relatively large driving voltage (∼50-200 V) is needed to achieve the electrowetting, except that an ultralow voltage electrowetting was reported for RTILs on an octadecyltrichlorosilane-coated silicon wafer [46]. ...
Room-temperature ionic liquids (RTILs) are intriguing fluids that have drawn much attention in applications ranging from tribology and catalysis to energy storage. With strong electrostatic interaction between ions, their interfacial behaviors can be modulated by controlling energetics of the electrified interface. In this work, we report atomic-force-microscope measurements of contact angle hysteresis (CAH) of a circular contact line formed on a micron-sized fiber, which is coated with a thin layer of conductive film and intersects an RTIL-air interface. The measured CAH shows a distinct change by increasing the voltage U applied on the fiber surface. Molecular dynamics simulations were performed to illustrate variations of the solidlike layer in the RTIL adsorbed at the electrified interface. The integrated experiments and computations demonstrate a new mechanism to manipulate the CAH by rearrangement of interfacial layers of RTILs induced by the surface energetics.
... Some works considered gap widths in the range 5.9−6.6 nm as large enough. 4,15,76 Fewer tackled the effect of EDL interferences using nano-and subnano-meter . Green, blue, white, red, yellow, and pink licorices represent carbon, nitrogen, hydrogen, oxygen, sulfur, and fluorine atoms. ...
... For less severe confinements (d ≥ 10 nm), a bulk region forms in the gap center and EDLs (or ionic layers) span over roughly 3−4 nm from electrode surfaces, which is in a reasonable agreement with a previous simulation work. 76 At such gaps between electrodes, the amount of ionic species needed for the formation of EDLs is reached, and therefore, increasing d further does not substantially modify them. This explains the nearly Langmuir-like behavior observed in liquid contributions to differential capacitances at the d-ranges where EDLs form. ...
... Electrolyte concentration is another important property that affects both capacitance and operating potential [25][26][27][28]. Electrolyte properties also strongly affect supercapacitor performance, and capacitance depends on ion saturation in the electrolyte, its electrochemical potential window that dictates the operating potential, as well as the interactions between the electrolyte and other device components [7,[25][26][27][28][29][30][31][32][33][34][35]. ...
Supercapacitors are high power electrochemical energy storage systems that are attractive candidates for use in high power applications. Yet, their widespread adoption has been restricted due to their relatively low energy density. Improving the energy storage performance of supercapacitors is linked to rationally optimizing the key descriptors that affect capacitance. This work presents a systematic approach based on a hybrid artificial neural network (ANN) and genetic algorithm (GA) integrated with the Big M method to efficiently and rationally design carbon-based supercapacitors with improved energy storage performance. By performing structural and electrochemical characterization on systems we fabricate, we experimentally validate the robustness and generalizability of the developed ANN-GA framework. This study takes a step towards the rational design of supercapacitors by implementing the hybrid ANN-GA framework as an optimization tool to provide guidelines for rationally tuning material properties and operational conditions for improved capacitance.
... and~7.0 µF cm −2 , was recognized by Feng and colleagues [14]. Moreover, Liu et al. recently delineated a positive solvent effect on the electrode capacitance [15]. ...
... Using MD simulations, our work revealed that a slight decrement in dipole moment benefited an obvious enhancement of capacitance and kinetics, which agreed well with previous studies [15]. However, our work also showed conflicting observations with previously reported simulation results [13,14]. Specifically, the polarity-dependent charging mechanism (from pure counterion adsorption, via ion exchange, to coion desorption) has never been reported before. ...
Solvents have been considered to show a profound influence on the charge storage of electric double-layer capacitors (EDLCs). However, the corresponding mechanisms remain elusive and controversial. In this work, the influences of solvent dipole moment on the EDL structures, kinetic properties, and charging mechanisms of graphene-based EDLCs are investigated with atomistic simulations. Specifically, electrolyte structuring is conspicuously modulated by solvents, where a sharp increment of capacitance (~325.6%) and kinetics (~10-fold) is documented upon the slight descent of polarity by ~33.0%. Unusually, such an impressive enhancement is primarily attributed to the suppressed interfacial electric fields stimulated by strong-polarity solvents in the proximity of electrodes, which goes beyond the previously observed issues that stemmed from the competitive interplays between ions and solvents. Moreover, a distinctive polarity-dependent charging mechanism (i.e., from pure counterion adsorption to coion desorption) is identified, which for the first time delineates the pivotal role of solvent polarity in manipulating the charge storage evolutions. The as-obtained findings highlight that exploiting the solvent effects could be a promising avenue to further advance the performances of EDLCs.
... They interpreted that, at intermediate ion concentrations, the solvent molecules surrounding ions enhance charge-density fluctuation near the electrode surface. This observation is consistent with earlier studies using MD simulations [58,76] commonly pointed out that the added solvents on RTILs reduce both the charge intensity and effective length of overscreening layer. Polymer electrolytes [43] are emerging targets of interest, too, which have potential to be further optimized for their capacitive properties. ...
Electrostatic interactions play a dominant role in charged materials systems. Understanding the complex correlation between macroscopic properties with microscopic structures is of critical importance to develop rational design strategies for advanced materials. But the complexity of this challenging task is augmented by interfaces present in the charged materials systems, such as electrode–electrolyte interfaces or biological membranes. Over the last decades, predictive molecular simulations that are founded in fundamental physics and optimized for charged interfacial systems have proven their value in providing molecular understanding of physicochemical properties and functional mechanisms for diverse materials. Novel design strategies utilizing predictive models have been suggested as promising route for the rational design of materials with tailored properties. Here, an overview of recent advances in the understanding of charged interfacial systems aided by predictive molecular simulations is presented. Focusing on three types of charged interfaces found in energy materials and biomacromolecules, how the molecular models characterize ion structure, charge transport, morphology relation to the environment, and the thermodynamics/kinetics of molecular binding at the interfaces is discussed. The critical analysis brings two prominent field of energy materials and biological science under common perspective, to stimulate crossover in both research field that have been largely separated.
... The layered interfacial structure of IL allows viewing the oppositely charged IL layers as series-connected capacitors [66]. By considering only the first two IL layers to have a significant effect on the u IL , then it can be estimated using the bilayer (BL) model [67], which defines it as: ...
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.
... On the basis of this observation, Feng et al. proposed a counter-charge layer framework to analyze the structure of adsorbed liquids. 189 Upon increasing the potential, the overscreening effects vanish and they are replaced by lattice saturation. 190 The first adsorbed layer saturates in counterions, and the following layers start to exchange ions with the bulk. ...
Electrochemical double-layer capacitors (EDLCs) are devices allowing the storage or production of electricity. They function through the adsorption of ions from an electrolyte on high-surface-area electrodes and are characterized by short charging/discharging times and long cycle-life compared to batteries. Microscopic simulations are now widely used to characterize the structural, dynamical, and adsorption properties of these devices, complementing electrochemical experiments and in situ spectroscopic analyses. In this review, we discuss the main families of simulation methods that have been developed and their application to the main family of EDLCs, which include nanoporous carbon electrodes. We focus on the adsorption of organic ions for electricity storage applications as well as aqueous systems in the context of blue energy harvesting and desalination. We finally provide perspectives for further improvement of the predictive power of simulations, in particular for future devices with complex electrode compositions.
... The layered interfacial structure of IL allows viewing the oppositely charged IL layers as seriesconnected capacitors [59]. By considering only the first two IL layers to have a significant effect on the IL , then it can be estimated using the interfacial bilayer (IBL) model [60], which defines it as: ...
In this study, we investigated the graphene-ionic liquid (EMImBF4) interface to clarify the effects of ambient temperature and potential on the differential capacitance. We complemented molecular dynamics simulations with density functional theory calculations to unravel the electrolyte and electrode contributions to the differential capacitance. As a result, we show: (1) the relation of characteristic saddle points of the capacitance-potential curve to the structural changes; (2) the smearing effect of temperature on the local structure and, consequently, on the capacitance; (3) rationalization of these observations with the interfacial bilayer model; and, finally, (4) how quantum capacitance correction dampens the influence of temperature and improves the agreement with the experimental data. 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.
... The behaviour of the first layers of confined ionic liquids could be crucial not only for describing the mobility properties of ILs but also their structural ones. Molecular dynamic simulations 10 , performed on microporous carbon electrodes, show that the first adsorbed ionic layer inside micropores is formed by one ionic species only, whose total charge balances exactly that of the electrode and it is not compensated also by the second ionic layer, according to the over-screening effect or Coulombic ordering [30][31][32][33] . ...
