A “counter-charge layer in generalized solvents” framework for electrical double layers in neat and hybrid ionic liquid electrolytes (original) (raw)
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The importance of ion size and electrode curvature on electrical double layers in ionic liquids
Physical Chemistry Chemical Physics, 2011
Room-temperature ionic liquids (ILs) are an emerging class of electrolytes for supercapacitors. We investigate the effects of ion size and electrode curvature on the electrical double layers (EDLs) in two ILs 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and 1-butyl-3methylimidazolium hexafluorophosphate [BMIM][PF 6 ], using a combination of molecular dynamics (MD) and quantum density functional theory (DFT) simulations. The sizes of the counter-ion and co-ion affect the ion distribution and orientational structure of EDLs. The EDL capacitances near both planar and cylindrical electrodes were found to follow the order: [BMIM][Cl] (near the positive electrode) 4 [BMIM][PF 6 ] (near the positive electrode) E [BMIM][Cl] (near the negative electrode) E [BMIM][PF 6 ] (near the negative electrode). The EDL capacitance was also found to increase as the electrode curvature increases. These capacitance data can be fit to the Helmholtz model and the recently proposed exohedral electrical double-cylinder capacitor (xEDCC) model when the EDL thickness is properly parameterized, even though key features of the EDLs in ILs are not accounted for in these models. To remedy the shortcomings of existing models, we propose a ''Multiple Ion Layers with Overscreening'' (MILO) model for the EDLs in ILs that takes into account two critical features of such EDLs, i.e., alternating layering of counter-ions and co-ions and charge overscreening. The capacitance computed from the MILO model agrees well with the MD prediction. Although some input parameters of the MILO model must be obtained from MD simulations, the MILO model may provide a new framework for understanding many important aspects of EDLs in ILs (e.g., the variation of EDL capacitance with the electrode potential) that are difficult to interpret using classical EDL models and experiments.
The Journal of Physical Chemistry C, 2009
We experimentally observed for the first time a bell-shaped (convex parabolic) differential capacitance versus potential (C dl-E) curve, which is expected according to the theory of Kornyshev given for the electrical double layer (EDL) of metal electrode/ionic liquid (IL) interface, at platinum and gold electrodes in four different [quaternary ammonium, imidazolium, and pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion-based] ILs with cations and anions of similar sizes. The C dl-E curves measured at a glassy carbon (nonmetallic) electrode in the same set of ILs were found to be U-shaped, in contrast to those obtained at platinum and gold electrodes. The present study corroborates the so-called Kornyshev's model of the EDL at metal electrode/IL interfaces and at the same time demands a theoretical model for the nonmetallic electrode/ IL interface. The EDL formation in ILs is discussed.
The Journal of Physical Chemistry C, 2011
Room-temperature ionic liquids (RTILs) have attracted significant attention in fundamental and applied research over the past decade due to their unique physicochemical properties, such as wide electrochemical potential window, excellent thermal stability, nonvolatility, relatively inert nature, and good ionic conductivity, which make them exceptionally useful in diverse electrochemical devices. 1À4 Electrochemistry is usually done at the interface, and the rate of the reaction is significantly dependent on the structure and dynamics of the interface. 5À8 Hence, the study of the interfacial properties of the RTILs is the key to understanding their functional performance in technological applications, for instance in capacitor and solar cell.
The Journal of Physical Chemistry C, 2008
The interfaces formed at glassy carbon electrodes in three low-temperature ionic liquids (1-methyl-3ethylimidazolium chloride, emimCl; 1-methyl-3-butylimidazolium chloride, bmimCl; and 1-methyl-3hexylimidazolium chloride, hmimCl) were investigated by cyclic voltammetry and impedance spectroscopy. The potential dependence of the differential double layer capacitance was measured at several temperatures between 80 and 140°C, and the temperature response was found to be broadly similar to that obtained with high-temperature molten salts. The differential capacitance/potential curves have a minimum and two side branches. The minimum corresponds to the point of zero charge. The differential capacitance increases in the order hmimCl < bmimCl < emimCl because the double layer is thinner when imidazolium (Rmim) cations with shorter alkyl chain lengths are used. The impedance spectra and capacitance curves indicate that cations are adsorbed at the open-circuit potential and that their surface excess concentration increases with negative polarization. Adsorption of the cation becomes stronger as the length of the alkyl chain decreases. Adsorption of chloride anions occurs at positive potentials and is weakest with bmimCl. The increase in the differential capacitance with temperature is most probably due to ion association within the double layer, which diminishes as temperature increases. The electrochemical window narrows as the temperature increases but is almost unaffected by the length of the alkyl chain of the Rmim cation.
The charging kinetics of electric double layers (EDLs) is closely related to the performance of a wide variety of nanostructured devices including supercapacitors, electro-actuators, and electrolyte-gated transistors. While room temperature ionic liquids (RTIL) are often used as the charge carrier in these new applications, the theoretical analyses are mostly based on conventional electrokinetic theories suitable for macroscopic electrochemical phenomena in aqueous solutions. In this work, we study the charging behavior of RTIL-EDLs using a coarse-grained molecular model and constant-potential molecular dynamics (MD) simulations. In stark contrast to the predictions of conventional theories, the MD results show oscillatory variations of ionic distributions and electrochemical properties in response to the separation between electrodes. The rate of EDL charging exhibits non-monotonic behavior revealing strong electrostatic correlations in RTIL under confinement. Recent experimental investigations of EDLC have Nano Research 2015, 8(3): 931-940
Electrical Double Layer in Mixtures of Room-Temperature Ionic Liquids
The Journal of Physical Chemistry C, 2009
The interfacial structure at the Hg electrode in mixtures of room-temperature ionic liquids (1-ethyl-3methylimidazolium tetrafluoroborate (EMIBF 4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF 4)) has been studied for the first time by the measurement of surface tension and differential capacitance. Surface tension, charge density on the electrode surface, and capacitance decrease with increasing the addition of OMIBF 4 in EMIBF 4 , which has been found to be the consequence of the preferential adsorption of the octyl group on the Hg surface and increasing extent of heterogeneity of the mixtures. The continuous widening of the electrocapillary maxima (ECM) with increasing addition of OMIBF 4 in EMIBF 4 also supports the above reasoning. In accordance with the ECM, the potential corresponding to the minimum of the capacitance-potential curve is assigned as the potential of zero charge (PZC). The probable cause for the appearance of the PZC at the minimum of the capacitance-potential curve has been discussed.
Journal of Molecular Liquids, 2017
A classical density functional theory has been used to study the structure and phase behavior of the electrical double layer of a dense ionic liquid. The model for IL consists of a trimer cation (with a charged head and two neutral segments) and a monomer anion. The effect of dispersion interactions on the density profile and differential capacitance curve has been investigated. Increasing the contribution of dispersion interactions leads to a camelshape differential capacitance curve. In the case of bell-shape curve, the maximum of the differential capacitance increases with decreasing the dispersion forces. These observations are related to the depletion or accumulation of ions near electrode with zero or low surface charge density.
Double layer in ionic liquids: capacitance vs. temperature from atomistic simulations
2022
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 actua...