Electrochemical charge transfer at a metallic electrode: a simulation study

Abstract

Part I Electrochemical charge transfer at a metallic electrode: a simulation study The factors which affect the rate of heterogeneous electron transfer at a metallic electrode in the context of Marcus theory are investigated through molecular dynamics simulations. The system consists of the ionic melt K3Eu2+ 0:5Eu3+ 0:5Cl5:5 sandwiched between two parallel plate platinum electrodes held at a preset electrical potential. The charges on the electrode atoms are variationally obtained through the method of Siepmann and Sprik [J. Chem. Phys. 102, 511 (1995)] which models the polarization of the electrode by the melt and maintains the condition of constant potential. A two-dimensional Ewald summation is employed to ensure that the absolute value of the potential is known, and the expressions derived by Kawata and Mikami [Chem. Phys. Lett. 340, 157 (2001)] are extended to allow for induced dipoles on the melt ions by their mutual interaction and the interaction with the electrode surface. The Marcus free energy curves are calculated for electron transfer events between a europium ion and the metallic electrode, and their dependence on the position of the redox ion and the applied potential examined. The system is consistently found to be in accord with the linear response regime. A moderately-ranged oscillatory character in the mean electrical (Poisson) potential is observed extending into the fluid, which is in marked disagreement with the predictions of existing mean-field (Gouy-Chapman) predictions. These oscillations are found not to be reflected in the calculated Helmholtz reaction free energy, which indicates that the Poisson potential is not the appropriate potential for discussions of the kinetics of electrode processes. The strong dependence of the reorganization energy on the position of the redox ion is traced to the image charge effect, and appears insensitive to the polarizability of the anion. Following the evolution of the Eu{Cl radial distribution function throughout a redox process reveals that the bond length in the transition complex is exactly in between those of the ground state reactant and product complexes. The potentials of mean force for the approach of a Eu2+ and Eu3+ ion to the electrode calculated through umbrella sampling are found to be in quantitative agreement with those calculated through the position-dependence of the respective concentration profiles. A method to parameterize a model of the interactions between the melt ions and the electrode surface from ab initio density functional theory calculations is described. The method is used to obtain a suitable interaction model for a system consisting of a LiCl liquid electrolyte and a solid aluminium electrode. The electrolyte is found to exhibit a potential-driven phase transition which involves the commensurate ordering of the electrolyte ions with the electrode surface; this leads to a maximum in the differential capacitance as a function of applied potential. Away from the phase transition the capacitance was found to be independent of the applied potential. Part II Are dipolar liquids ferroelectric? The observation of a very sharp low frequency spike in the hyper-Rayleigh spectrum (HRS) of strongly dipolar fluids, such as acetonitrile and water, has been interpreted as reflecting a very slowly relaxing component in the transverse dipole density. This suggestion is at variance with the expectation of dielectric theory for an isotropic fluid and has led to the speculation that the slow relaxation is associated with the reorganization of ferroelectric domains. Very large-scale molecular dynamics simulation ( 28000 molecules) have been carried out using a 3-site potential model of acetonitrile. The simulated fluid shows no suggestion of strong dipole correlations and domain structure. The dipole density correlations behave as predicted by normal dielectric theory and their spectra do not show the low-frequency feature seen in the HRS. In order to examine the characteristics of the spectra which would be seen in a ferroelectric domain, the acetontrile model was transmuted to more closely resemble a Stockmayer-like fluid with the same dipole density and a ferroelectric phase was observed. In this phase the dielectric spectra show (i) a high-frequency spectral feature due to librational motion of the molecules within a domain, and (ii) slowly-relaxing longitudinal and transverse polar modes, again at variance from the experimental HRS characteristics.