Towards understanding the structure and capacitance of electrical double layer in ionic liquids

Maxim V. Fedorov, Alexei A. Kornyshev

Research output: Contribution to journalArticle

284 Citations (Scopus)

Abstract

In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding Values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable over-screening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.). are suppressed by the high electrode polarizations, following the onset of the so-called 'lattice saturation effect'. The maximum of the capacitance coincides with the p.z.c., but it is true only for this 'symmetric' system. If sizes of cations and anions are different the maximum will be shifted away front the p.z.c., and generally the shape of the capacitance curve could be more complicated. 

LanguageEnglish
Pages6835-6840
Number of pages6
JournalElectrochimica Acta
Volume53
Issue number23
DOIs
Publication statusPublished - 1 Oct 2008

Fingerprint

Ionic Liquids
Ionic liquids
Capacitance
Electrodes
Mean field theory
Anions
Cations
Negative ions
Positive ions
Ions
Polarization
Molecular dynamics
Conservation
Screening
Computer simulation

Keywords

  • ionic liquids
  • double layer
  • molecular dynamics
  • electrified interfaces
  • concentrated electrolytes
  • mean-field theory
  • differential capacitance
  • molecular dynamics
  • interfaces
  • simulation
  • spectroscopy
  • relaxation

Cite this

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abstract = "In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding Values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable over-screening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.). are suppressed by the high electrode polarizations, following the onset of the so-called 'lattice saturation effect'. The maximum of the capacitance coincides with the p.z.c., but it is true only for this 'symmetric' system. If sizes of cations and anions are different the maximum will be shifted away front the p.z.c., and generally the shape of the capacitance curve could be more complicated. ",
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Towards understanding the structure and capacitance of electrical double layer in ionic liquids. / Fedorov, Maxim V.; Kornyshev, Alexei A.

In: Electrochimica Acta, Vol. 53, No. 23, 01.10.2008, p. 6835-6840.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Towards understanding the structure and capacitance of electrical double layer in ionic liquids

AU - Fedorov, Maxim V.

AU - Kornyshev, Alexei A.

PY - 2008/10/1

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N2 - In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding Values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable over-screening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.). are suppressed by the high electrode polarizations, following the onset of the so-called 'lattice saturation effect'. The maximum of the capacitance coincides with the p.z.c., but it is true only for this 'symmetric' system. If sizes of cations and anions are different the maximum will be shifted away front the p.z.c., and generally the shape of the capacitance curve could be more complicated. 

AB - In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding Values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable over-screening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.). are suppressed by the high electrode polarizations, following the onset of the so-called 'lattice saturation effect'. The maximum of the capacitance coincides with the p.z.c., but it is true only for this 'symmetric' system. If sizes of cations and anions are different the maximum will be shifted away front the p.z.c., and generally the shape of the capacitance curve could be more complicated. 

KW - ionic liquids

KW - double layer

KW - molecular dynamics

KW - electrified interfaces

KW - concentrated electrolytes

KW - mean-field theory

KW - differential capacitance

KW - molecular dynamics

KW - interfaces

KW - simulation

KW - spectroscopy

KW - relaxation

U2 - 10.1016/j.electacta.2008.02.065

DO - 10.1016/j.electacta.2008.02.065

M3 - Article

VL - 53

SP - 6835

EP - 6840

JO - Electrochimica Acta

T2 - Electrochimica Acta

JF - Electrochimica Acta

SN - 0013-4686

IS - 23

ER -