Ionic liquid interface at an electrode : Simulations of electrochemical properties using an asymmetric restricted primitive model
(2018) In Journal of Physics: Condensed Matter 30(7). Abstract
We use Monte Carlo simulations of a coarsegrained 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 hardsphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hardsphere 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 coarsegrained 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 hardsphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hardsphere 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 coarsegrain 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.
(Less)
 author
 Lu, Hongduo ^{LU} ; Nordholm, Sture ; Woodward, Clifford E. and Forsman, Jan ^{LU}
 organization
 publishing date
 20180123
 type
 Contribution to journal
 publication status
 published
 subject
 keywords
 coarsegrained, differential capacitance, ionic liquid, Monte Carlo simulation
 in
 Journal of Physics: Condensed Matter
 volume
 30
 issue
 7
 article number
 074004
 publisher
 IOP Publishing
 external identifiers

 pmid:29300174
 scopus:85041455121
 ISSN
 09538984
 DOI
 10.1088/1361648X/aaa524
 language
 English
 LU publication?
 yes
 id
 58dd477bd76a4b22a02f2f5f738fc6a7
 date added to LUP
 20180221 12:47:47
 date last changed
 20210929 01:58:40
@article{58dd477bd76a4b22a02f2f5f738fc6a7, abstract = {<p>We use Monte Carlo simulations of a coarsegrained 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 hardsphere ions, all of the same size, in which the charge is asymmetrically placed. Not only the hardsphere 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 coarsegrain 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>}, author = {Lu, Hongduo and Nordholm, Sture and Woodward, Clifford E. and Forsman, Jan}, issn = {09538984}, 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/10.1088/1361648X/aaa524}, doi = {10.1088/1361648X/aaa524}, volume = {30}, year = {2018}, }