A computational study of the interfacial structure and capacitance of graphene in BMIMPF6 ionic liquid (original) (raw)
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Structure and Capacitance of Electrical Double Layers at the Graphene–Ionic Liquid Interface
Applied Sciences
Molecular dynamics simulations are carried out to investigate the structure and capacitance of the electrical double layers (EDLs) at the interface of vertically oriented graphene and ionic liquids [EMIM] + /[BF 4 ] −. The distribution and migration of the ions in the EDL on the rough and non-rough electrode surfaces with different charge densities are compared and analyzed, and the effect of the electrode surface morphology on the capacitance of the EDL is clarified. The results suggest that alternate distributions of anions and cations in several consecutive layers are formed in the EDL on the electrode surface. When the electrode is charged, the layers of [BF 4 ] − anions experience more significant migration than those of [EMIM] + cations. These ion layers can be extended deeper into the bulk electrolyte solution by the stronger interaction of the rough electrode, compared to those on the non-rough electrode surface. The potential energy valley of ions on the neutral electrode surface establishes a potential energy difference to compensate the energy cost of the ion accumulation, and is capable of producing a potential drop across the EDL on the uncharged electrode surface. Due to the greater effective contact area between the ions and electrode, the rough electrode possesses a larger capacitance than the non-rough one. In addition, it is harder for the larger-sized [EMIM] + cations to accumulate in the narrow grooves on the rough electrode, when compared with the smaller [BF 4 ] −. Consequently, the double-hump-shaped C-V curve (which demonstrates the relationship between differential capacitance and potential drop across the EDL) for the rough electrode is asymmetric, where the capacitance increases more significantly when the electrode is positively charged.
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 Letters, 2011
Molecular simulations reveal that the shape of differential capacitance (DC) versus the electrode potential can change qualitatively with the structure of the electrode surface. Whereas the atomically flat basal plane of graphite in contact with a room-temperature ionic liquid generates camel-shaped DC, the atomically corrugated prismatic face of graphite with the same electrolyte exhibits bell-shaped behavior and much larger DCs at low double-layer potentials. The observed bell-shaped and camel-shaped DC behavior was correlated with the structural changes occurring in the double layer as a function of applied potential. Therefore, the surface topography clearly influences DC behavior, suggesting that attention should be paid to the electrode surface topography characterization in the studies of DC to ensure reproducibility and unambiguous interpretation of experimental results. Furthermore, our results suggest that controlling the electrode roughness/structure could be a route to improving the energy densities in electric double-layer capacitors.
Differential capacitance of the double layer at the electrode/ionic liquids interface
Physical Chemistry Chemical Physics, 2010
The differential capacitance of the electrical double layer at glassy carbon, platinum and gold electrodes immersed in various ionic liquids was measured using impedance spectroscopy. We discuss the influence of temperature, the composition of the ionic liquids and the electrode material on the differential capacitance/potential curves. For different systems these curves have various overall shapes, but all include several extremes and a common minimum near the open circuit potential. We attribute this minimum to the potential of zero charge (PZC). Significantly, the differential capacitance generally decreases if the applied potential is large and moving away from the PZC. This is attributed to lattice saturation [A. A. Kornyshev, J. Phys. Chem. B, 2007, 111, 5545] effects which result in a thicker double layer. The differential capacitance of the double layer grows and specific adsorption diminishes with increasing temperature. Specific adsorption of both cations and anions influences the shapes of curves close to the PZC. The general shape of differential capacitance/potential does not depend strongly on the identity of the electrode material.
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.
Model of electrical double layer structure at semi-metallic electrode/ionic liquid interface
Electrochimica Acta, 2021
In this study, we developed a model for the electrical double layer (EDL) structure formed at the semimetallic electrode/ionic liquid (IL) interface. To reveal the origin of experimentally observed "U-like" nature of capacitance (C)-potential (E) curve at a glassy carbon (semi-metallic) electrode in ILs, the expression of the capacitance of metallic electrode was modified by taking into account the capacitance of the characteristic space charge layer of semiconductor electrode. The concept of density-of-state of semimetal and the various physicochemical parameters of ions including size and concentration of practically available ILs were considered for the simulation of C-E curve. We succeeded to reproduce the U-like C-E curve at glassy carbon electrode/IL interface in simulation trials with three commercial ILs. The success of simulating the experimental C-E curve for the metallic electrode by the same strategy to exploit the relevant theory and the properties of ILs further validated the present attempt. Our analysis unveils that the capacitances of both compact and diffuse layers significantly contribute to the total capacitance for the EDL formed at semi-metallic electrode/IL interface, while the diffuse layer mainly contributes to the total capacitances of the metallic electrode in ILs. This study also opens a novel route to determine the compressibility factor of IL by simulating experimental C-E curve at metallic electrode.
Journal of Electroanalytical Chemistry, 2008
Impedance spectra (50 kHz-1 Hz) were acquired and used to obtain the differential capacitance at the interfaces between 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6] ionic liquid and three different electrode materials (Hg, Pt, and glassy carbon (GC)) as a function of the applied potential. The electrocapillary curve for the Hg/[BMIM][PF6] interface was obtained from drop time measurements, from which the potential of zero charge was calculated to be À0.39 V (Ag wire). The potential of zero charge is 0.30 V less negative than the potential of differential capacitance minimum. This disagreement suggests that the differential capacitance minimum is not related to a classical diffuse layer minimum. Additional support to this conclusion was obtained from positive temperature coefficient for the differential capacitance in contrast to the negative temperature predicted by the classic Gouy-Chapman model. The results do not support the recent model predictions of bell shaped capacitance curves for room temperature ionic liquids, RTILs.
Physical Chemistry Chemical Physics, 2011
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 understanding of the structure-capacitance relationships for the EDLs of these systems. Here we present a theoretical framework termed ''counter-charge layer in generalized solvents'' (CGS) for describing the structure and capacitance of the EDLs in neat RTILs and in RTILs mixed with different mass fractions of organic solvents. Within this framework, an EDL is made up of a counter-charge layer exactly balancing the electrode charge, and of polarized generalized solvents (in the form of layers of ion pairs, each of which has a zero net charge but has a dipole moment-the ion pairs thus can be considered as a generalized solvent) consisting of all RTILs inside the system except the counter-ions in the counter-charge layer, together with solvent molecules if present. Several key features of the EDLs that originate from the strong ion-ion correlation in RTILs, e.g., overscreening of electrode charge and alternating layering of counter-ions and co-ions, are explicitly incorporated into this framework. We show that the dielectric screening in EDLs is governed predominantly by the polarization of generalized solvents (or ion pairs) in the EDL, and the capacitance of an EDL can be related to its microstructure with few a priori assumptions or simplifications. We use this framework to understand two interesting phenomena observed in molecular dynamics simulations of EDLs in a neat IL of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]) and in a mixture of [BMIM][BF 4 ] and acetonitrile (ACN): (1) the capacitance of the EDLs in the [BMIM][BF 4 ]/ACN mixture increases only slightly when the mass fraction of ACN in the mixture increases from zero to 50% although the dielectric constant of bulk ACN is more than two times higher than that of neat [BMIM][BF 4 ]; (2) the capacitance of EDLs near negative electrodes (with BMIM + ion as the counter-ion) is smaller than that near positive electrodes (with BF 4 À as the counter-ion) although the closest approaches of both ions to the electrode surface are nearly identical.