Electrochemical interface between an ionic liquid and a model metallic electrode

Abstract

A molecular dynamics simulation model for an electroactive interface in which a metallic electrode is maintained at a preset electrical potential is described. The model, based on earlier work of Siepmann and Sprik [J. Chem. Phys. 102, 511 (1995)], uses variable charges whose magnitudes are adjusted on-the-fly according to a variational procedure to maintain the constant potential condition. As such, the model also allows for the polarization of the electrode by the electrolyte, sometimes described by the introduction of image charges. The model has been implemented in a description of an electrochemical cell as a pair of parallel planar electrodes separated by the electrolyte using a two-dimensional Ewald summation method. The method has been applied to examine the interfacial structure in two ionic liquids, consisting of binary mixtures of molten salts, chosen to exemplify the influences of dissimilar cation size and charge. The stronger coordination of the smaller and more highly charged cations by the anions prevents them from approaching even the negatively charged electrode closely. This has consequences for the capacitance of the electrode and will also have an impact on the rates of electron transfer processes. The calculated capacitances exhibit qualitatively the same dependence on the applied potential as has been observed in experimental studies.

Water at an electrochemical interface—a simulation study

“…Specifically, when the electrode potential has the values V 0 = 0.27 V, V 0 = 1.36 V, and V 0 = 2.72 V, the dielectric constant for the bulk water is = 75.07, = 61.50, and = 57.30 respectively. The low voltage result is in reasonable agreement with direct simulation studies (68±5.8 [31]), which is good confirmation that the potential and the response of the water molecules to it are correct (see also reference [16]). The reduction in the apparent dielectric constant at higher voltages is consistent with a saturation effect [32], note that the potential difference of ∆Ψ 1.1 V, obtained with V 0 =2.72 V, across our virtual capacitor of width 41.5Å is equivalent to an electric field of 2.6 × 10 8 Vm −1 .…”

“…The methodology for the simulation of the electrodes is described in great detail in ref [16], where it is shown that the electrodes become polarised in the presence of a charge in the solution region in a way which corresponds to the classical image-charge response. We illustrate this response in figure 3 where the charge induced on the electrode atoms by the instantaneous configuration of the charges in solution is shown by colour-coding the electrode atoms.…”

“…However, the time and length scales involved in the relaxation of the solution in the vicinity of the electrodes (which we will characterise below) are far too long to allow a full self-consistent description of the screening of the electrode potential within an ab initio scheme [15]. Recently we introduced a way of incorporating some of the essential physical effects necessary for a realistic description of the interfacial charge-transfer process into a simulation which uses interaction potentials and therefore enables simulations of much larger time and length scales than is possible ab initio [16]. In particular, model metallic electrodes maintained at a constant electrical potential may be introduced into such a simulation, following a technique introduced by Siepmann and Sprik [17].…”

Simulation of Adsorption Processes at Metallic Interfaces: An Image Charge Augmented QM/MM Approach

“…For values of α larger than The total sum of the image charges for the given test system is in the order of 10 −2 . This sum is expected to converge to zero when reaching the macroscopic continuum limit 15,18 , i.e. in the limit of an infinite metal slab (see also Figure S2).…”

“…The Gaussian charge method developed by Siepmann and Sprik 15 treats the image charges as dynamical variables and realizes the mutual modification of induced and adsorbate charges. The Siepmann-Sprik scheme has been successfully applied to water/platinum interfaces [15][16][17] as well as ionic liquids at platinum 18 and graphene 19,20 electrodes within a purely classical frame.…”