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Ionic liquid interface at an electrode : Simulations of electrochemical properties using an asymmetric restricted primitive model

Lu, Hongduo LU ; Nordholm, Sture; Woodward, Clifford E. and Forsman, Jan LU (2018) In Journal of Physics Condensed Matter 30(7).
Abstract

We use Monte Carlo simulations of a coarse-grained model to investigate structure and electrochemical behaviours at an electrode immersed in room temperature ionic liquids (RTILs). The simple RTIL model, which we denote the asymmetric restricted primitive model (ARPM), is composed of monovalent hard-sphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hard-sphere size (d), but also the charge displacement (b), is identical for all species, i.e. the monovalent RTIL ions are fully described by only two parameters (d, b). In earlier work, it was demonstrated that the ARPM can capture typical static RTIL properties in bulk solutions with remarkable accuracy. Here, we investigate its behaviour at an... (More)

We use Monte Carlo simulations of a coarse-grained model to investigate structure and electrochemical behaviours at an electrode immersed in room temperature ionic liquids (RTILs). The simple RTIL model, which we denote the asymmetric restricted primitive model (ARPM), is composed of monovalent hard-sphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hard-sphere size (d), but also the charge displacement (b), is identical for all species, i.e. the monovalent RTIL ions are fully described by only two parameters (d, b). In earlier work, it was demonstrated that the ARPM can capture typical static RTIL properties in bulk solutions with remarkable accuracy. Here, we investigate its behaviour at an electrode surface. The electrode is assumed to be a perfect conductor and image charge methods are utilized to handle polarization effects. We find that the ARPM of the ionic liquid reproduces typical (static) electrochemical properties of RTILs. Our model predicts a declining differential capacitance with increasing temperature, which is expected from simple physical arguments. We also compare our ARPM, with the corresponding RPM description, at an elevated temperature (1000 K). We conclude that, even though ion pairing occurs in the ARPM system, reducing the concentration of 'free' ions, it is still better able to screen charge than a corresponding RPM melt. Finally, we evaluate the option to coarse-grain the model even further, by treating the fraction of the ions that form ion pairs implicitly, only through the contribution to the dielectric constant of the corresponding dipolar (ion pair) fluid. We conclude that this primitive representation of ion pairing is not able to reproduce the structures and differential capacitances of the system with explicit ion pairs. The main problem seems to be due to a limited dielectric screening in a layer near the electrode surface, resulting from a combination of orientational restrictions and a depleted dipole density.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
coarse-grained, differential capacitance, ionic liquid, Monte Carlo simulation
in
Journal of Physics Condensed Matter
volume
30
issue
7
publisher
IOP Publishing
external identifiers
  • scopus:85041455121
ISSN
0953-8984
DOI
10.1088/1361-648X/aaa524
language
English
LU publication?
yes
id
58dd477b-d76a-4b22-a02f-2f5f738fc6a7
date added to LUP
2018-02-21 12:47:47
date last changed
2018-05-29 11:12:31
@article{58dd477b-d76a-4b22-a02f-2f5f738fc6a7,
  abstract     = {<p>We use Monte Carlo simulations of a coarse-grained model to investigate structure and electrochemical behaviours at an electrode immersed in room temperature ionic liquids (RTILs). The simple RTIL model, which we denote the asymmetric restricted primitive model (ARPM), is composed of monovalent hard-sphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hard-sphere size (d), but also the charge displacement (b), is identical for all species, i.e. the monovalent RTIL ions are fully described by only two parameters (d, b). In earlier work, it was demonstrated that the ARPM can capture typical static RTIL properties in bulk solutions with remarkable accuracy. Here, we investigate its behaviour at an electrode surface. The electrode is assumed to be a perfect conductor and image charge methods are utilized to handle polarization effects. We find that the ARPM of the ionic liquid reproduces typical (static) electrochemical properties of RTILs. Our model predicts a declining differential capacitance with increasing temperature, which is expected from simple physical arguments. We also compare our ARPM, with the corresponding RPM description, at an elevated temperature (1000 K). We conclude that, even though ion pairing occurs in the ARPM system, reducing the concentration of 'free' ions, it is still better able to screen charge than a corresponding RPM melt. Finally, we evaluate the option to coarse-grain the model even further, by treating the fraction of the ions that form ion pairs implicitly, only through the contribution to the dielectric constant of the corresponding dipolar (ion pair) fluid. We conclude that this primitive representation of ion pairing is not able to reproduce the structures and differential capacitances of the system with explicit ion pairs. The main problem seems to be due to a limited dielectric screening in a layer near the electrode surface, resulting from a combination of orientational restrictions and a depleted dipole density.</p>},
  articleno    = {074004},
  author       = {Lu, Hongduo and Nordholm, Sture and Woodward, Clifford E. and Forsman, Jan},
  issn         = {0953-8984},
  keyword      = {coarse-grained,differential capacitance,ionic liquid,Monte Carlo simulation},
  language     = {eng},
  month        = {01},
  number       = {7},
  publisher    = {IOP Publishing},
  series       = {Journal of Physics Condensed Matter},
  title        = {Ionic liquid interface at an electrode : Simulations of electrochemical properties using an asymmetric restricted primitive model},
  url          = {http://dx.doi.org/10.1088/1361-648X/aaa524},
  volume       = {30},
  year         = {2018},
}