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

Fedorov, Maxim V.; Georgi, N.; Kornyshev, Alexei A.

doi:10.1016/j.elecom.2009.12.019

Cited by 286 publications

(187 citation statements)

References 16 publications

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“…Thus, we expect that they will also play a role in establishing the camel shape of the C D . That this is the case, is seen from both simulations (Lamperski & Zydor , 2007;Vatamanu et al , 2010) (Fedorov et al , 2010;Trulsson et al , 2010), play an important role in determining their properties. In this work, we will use a model for ILs wherein the molecular ions interact via dispersion, as well as electrostatic, forces and the cation has an oligomeric structure consistent with imidazolium ions.…”

Section: Other Approachesmentioning

confidence: 67%

A Classical Density Functional Theory of Ionic Liquids

2011

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Classical Density Functional Theory of Ionic Liquids 3 Other approachesILs are fascinating from a theoretical standpoint. In order to treat ILs, a theory must account for both electrostatic and steric correlations. Attempts to develop such a theory have appeared in the literature. These include the lattice based model by Kornyshev and co-workers (Fedorov et al. , 2008;Kornyshev , 2007). In another lattice approach by Lauw and coworkers (Lauw et al. , 2009), both cations and anions were modelled by a star-like architecture, with each monomer subject to an independently specified dielectric response. Under certain circumstances, these models were able to qualitatively reproduce the so-called "camel-shaped" differential capacitance, C D , profile observed in imidazolium ILs at carbon electrodes (Islam et al. , 2009;Lockett et al. , 2008a). This behaviour is an indication of subtle structuring effects at charged surfaces and is a stern test for theory. Although camel-shaped C D profiles are predicted, qualitative discrepancy against experimental results remains. This is not surprising since these crude theories lack a proper description of electrostatic correlations despite accounting for steric contributions. Charge-charge correlations have been shown to be important in interfacial phenomena of highly coupled electrostatic fluids. As with dispersion forces, ionic correlations are always attractive and lower the total energy. Thus, we expect that they will also play a role in establishing the camel shape of the C D . That this is the case, is seen from both simulations (Lamperski & Zydor , 2007;Vatamanu et al. , 2010) (Fedorov et al. , 2010;Trulsson et al. , 2010), play an important role in determining their properties. In this work, we will use a model for ILs wherein the molecular ions interact via dispersion, as well as electrostatic, forces and the cation has an oligomeric structure consistent with imidazolium ions. This model has also been adopted by us in previous recent publications (Forsman et al. , 2011;Trulsson et al. , 2010), and employs the methods of polymer free energy density functional theory (DFT) (Woodward , 1991). The result will be a simple, albeit approximate DFT for ILs. We have already introduced the salient features of the theory in a previous publication (Forsman et al. , 2011). Here, we will present a much more thorough treatment, emphasizing the relation to theories of simpler fluids. We will also extend the scope of the study with some emphasis on temperature dependence and the effects of ion-specific adsorption. The IL-DFT that we will discuss here is an off-lattice theory, whose accuracy can be straightforwardly tested against computer simulations of the same molecular model. We will begin with a discussion of relevant earlier work, in order to introduce the essential mechanistic components that will be combined in our density functional theory for ionic liquids. Classical density functional theory (DFT) of simple fluidsThe ideal gas law is known and used by most physicists and chemist...

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Section: Other Approachesmentioning

confidence: 67%

A Classical Density Functional Theory of Ionic Liquids

2011

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“…As seen in Figure 2a, the shape of the C−V curve for neat EMImTFSI is concave up within the 2 V potential window studied, which is in agreement with the shape predicted by models of nonspherical RTIL ions. 23 When EMImTFSI is diluted with DCE to 75 mol%, the capacitance increases globally as shown by the vertical shift in the C−V curve and features a shallow "U"-shape within the given potential window, with a pronounced C min . As EMImTFSI is further diluted to 36 mol%, the slopes of the anodic and cathodic branches of the C−V curve increase, and the curve assumes a sharper "V"- …”

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confidence: 99%

Anomalous Capacitance Maximum of the Glassy Carbon–Ionic Liquid Interface through Dilution with Organic Solvents

Bozym

Uralcan

Limmer

et al. 2015

J. Phys. Chem. Lett.

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We use electrochemical impedance spectroscopy to measure the effect of diluting a hydrophobic room temperature ionic liquid with miscible organic solvents on the differential capacitance of the glassy carbon−electrolyte interface. We show that the minimum differential capacitance increases with dilution and reaches a maximum value at ionic liquid contents near 5− 10 mol% (i.e., ∼1 M). We provide evidence that mixtures with 1,2-dichloroethane, a lowdielectric constant solvent, yield the largest gains in capacitance near the open circuit potential when compared against two traditional solvents, acetonitrile and propylene carbonate. To provide a fundamental basis for these observations, we use a coarse-grained model to relate structural variations at the double layer to the occurrence of the maximum. Our results reveal the potential for the enhancement of double-layer capacitance through dilution. R oom temperature ionic liquids (RTILs) are regarded as the next-generation electrolytes for electrochemical double-layer capacitors (EDLCs), as their low volatility and wide electrochemical windows (>4 V) can lead to safer devices with greater energy density.1 However, the low conductivity of RTILs limits the rate performance of EDLCs and yields devices with suboptimal power density.2 To overcome this limitation, in top-performing EDLCs, 50 wt % acetonitrile (AN) is added to the RTIL electrolyte (∼1 M), as the bulk conductivity of the mixture is greater than that of the neat RTIL due to a lower viscosity.3−5 Such RTIL/solvent mixtures are electrochemically stable up to 4 V and have also shown promise in devices operating at high temperatures. 6,7 Additionally, RTILs offer processing advantages over traditional solid salt-based electrolytes, namely, they can be evaporatively consolidated with the active electrode material from a volatile phase (e.g., organic solvent) to form dense electrodes.3,8 Though improvements in rate performance have been achieved using these mixtures, the effect of diluting RTILs with organic solvents on the doublelayer capacitance of the electrode−electrolyte interface remains unclear.9−12 Understanding the effect of RTIL dilution with organic solvents on the double-layer capacitance is critical in the design of RTIL/solvent combinations that maximize capacitance and, correspondingly, improve the energy density of EDLCs.A large body of work exists regarding the theory and characterization of neat RTILs; yet, only a few reports have studied RTIL/solvent mixtures. 13 Recently, several computational works have suggested that the addition of nonionic solvent (to ∼1 M) to RTILs has an insignificant effect on the double-layer capacitance. 9,10 This claim runs counter to the Gouy−Chapman-Stern (GCS) theory of the electrochemical double-layer (EDL) as a decrease in capacitance with dilution is expected, due to a decreasing ion concentration.14 Furthermore, two previous experimental studies suggest a maximum in capacitance with dilution. 15,16 Liu et al. show a maximum massspecific capacitance at...

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“…Experiments (e.g [6][7][8][9][10]), theories (e.g [11][12][13]) and simulations (e.g [14][15][16][17][18][19], or a recent review [20] ) have shed light on the structure of bulk ionic liquids near charged surfaces. Those pioneering works revealed the effect of ion size in determining the interfacial capacitance, and established that there is a region of closely packed ions near a highly charged surface, followed by a gradual (potentially oscillatory) decay of ion density into the bulk.…”

Section: Introductionmentioning

confidence: 99%

Unravelling nanoconfined films of ionic liquids

Lee

Vella

Perkin

et al. 2014

The Journal of Chemical Physics

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The confinement of an ionic liquid between charged solid surfaces is treated using an exactly solvable 1D Coulomb gas model. The theory highlights the importance of two dimensionless parameters: the fugacity of the ionic liquid, and the electrostatic interaction energy of ions at closest approach, in determining how the disjoining pressure exerted on the walls depends on the geometrical confinement. Our theory reveals that thermodynamic fluctuations play a vital role in the "squeezing out" of charged layers as the confinement is increased. The model shows good qualitative agreement with previous experimental data, with all parameters independently estimated without fitting.

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

Cited by 286 publications

References 16 publications

A Classical Density Functional Theory of Ionic Liquids

A Classical Density Functional Theory of Ionic Liquids

Anomalous Capacitance Maximum of the Glassy Carbon–Ionic Liquid Interface through Dilution with Organic Solvents

Unravelling nanoconfined films of ionic liquids

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