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Title: Structure and dynamics of electrical double layers in organic electrolytes

Abstract

The organic electrolyte of tetraethylammonium tetrafluoroborate (TEABF{sub 4}) in the aprotic solvent of acetonitrile (ACN) is widely used in electrochemical systems such as electrochemical capacitors. In this paper, we examine the solvation of TEA{sup +} and BF{sub 4}{sup -} in ACN, and the structure, capacitance, and dynamics of the electrical double layers (EDLs) in the TEABF{sub 4}-ACN electrolyte using molecular dynamics simulations complemented with quantum density functional theory calculations. The solvation of TEA+ and BF4- ions is found to be much weaker than that of small inorganic ions in aqueous solutions, and the ACN molecules in the solvation shell of both types of ions show only weak packing and orientational ordering. These solvation characteristics are caused by the large size, charge delocalization, and irregular shape (in the case of TEA+ cation) of the ions. Near neutral electrodes, the double-layer structure in the organic electrolyte exhibits a rich organization: the solvent shows strong layering and orientational ordering, ions are significantly contact-adsorbed on the electrode, and alternating layers of cations/anions penetrate ca. 1.1 nm into the bulk electrolyte. The significant contact adsorption of ions and the alternating layering of cation/anion are new features found for EDLs in organic electrolytes. These features essentiallymore » originate from the fact that van der Waals interactions between organic ions and the electrode are strong and the partial desolvation of these ions occurs easily, as a result of the large size of the organic ions. Near charged electrodes, distinct counter-ion concentration peaks form, and the ion distribution cannot be described by the Helmholtz model or the Helmholtz + Poisson-Boltzmann model. This is because the number of counter-ions adsorbed on the electrode exceeds the number of electrons on the electrode, and the electrode is over-screened in parts of the EDL. The computed capacitances of the EDLs are in good agreement with that inferred from experimental measurements. Both the rotations (ACN only) and translations of interfacial ACN and ions are found to slow down as the electrode is electrified. We also observe an asymmetrical dependence of these motions on the sign of the electrode charge. The rotation/diffusion of ACN and the diffusion of ions in the region beyond the first ACN or ion layer differ only weakly from those in the bulk« less

Authors:
 [1];  [1];  [1];  [1];  [2]
  1. ORNL
  2. Clemson University
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Center for Computational Sciences
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program
OSTI Identifier:
980722
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Chemistry Chemical Physics; Journal Volume: 12; Journal Issue: 20
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACETONITRILE; ADSORPTION; AQUEOUS SOLUTIONS; BEVERAGES; CAPACITANCE; CAPACITORS; DENSITY; DIFFUSION; DISTRIBUTION; DYNAMICS; ELECTRODES; ELECTROLYTES; ELECTRONS; FUNCTIONALS; INTERACTIONS; IONS; LAYERS; MOLECULES; PEAKS; SHAPE; SHELLS; SOLVATION; SOLVENTS; STOWING

Citation Formats

Huang, Jingsong, Sumpter, Bobby G, Meunier, Vincent, Qiao, Rui, and Feng, Guang. Structure and dynamics of electrical double layers in organic electrolytes. United States: N. p., 2010. Web.
Huang, Jingsong, Sumpter, Bobby G, Meunier, Vincent, Qiao, Rui, & Feng, Guang. Structure and dynamics of electrical double layers in organic electrolytes. United States.
Huang, Jingsong, Sumpter, Bobby G, Meunier, Vincent, Qiao, Rui, and Feng, Guang. Fri . "Structure and dynamics of electrical double layers in organic electrolytes". United States. doi:.
@article{osti_980722,
title = {Structure and dynamics of electrical double layers in organic electrolytes},
author = {Huang, Jingsong and Sumpter, Bobby G and Meunier, Vincent and Qiao, Rui and Feng, Guang},
abstractNote = {The organic electrolyte of tetraethylammonium tetrafluoroborate (TEABF{sub 4}) in the aprotic solvent of acetonitrile (ACN) is widely used in electrochemical systems such as electrochemical capacitors. In this paper, we examine the solvation of TEA{sup +} and BF{sub 4}{sup -} in ACN, and the structure, capacitance, and dynamics of the electrical double layers (EDLs) in the TEABF{sub 4}-ACN electrolyte using molecular dynamics simulations complemented with quantum density functional theory calculations. The solvation of TEA+ and BF4- ions is found to be much weaker than that of small inorganic ions in aqueous solutions, and the ACN molecules in the solvation shell of both types of ions show only weak packing and orientational ordering. These solvation characteristics are caused by the large size, charge delocalization, and irregular shape (in the case of TEA+ cation) of the ions. Near neutral electrodes, the double-layer structure in the organic electrolyte exhibits a rich organization: the solvent shows strong layering and orientational ordering, ions are significantly contact-adsorbed on the electrode, and alternating layers of cations/anions penetrate ca. 1.1 nm into the bulk electrolyte. The significant contact adsorption of ions and the alternating layering of cation/anion are new features found for EDLs in organic electrolytes. These features essentially originate from the fact that van der Waals interactions between organic ions and the electrode are strong and the partial desolvation of these ions occurs easily, as a result of the large size of the organic ions. Near charged electrodes, distinct counter-ion concentration peaks form, and the ion distribution cannot be described by the Helmholtz model or the Helmholtz + Poisson-Boltzmann model. This is because the number of counter-ions adsorbed on the electrode exceeds the number of electrons on the electrode, and the electrode is over-screened in parts of the EDL. The computed capacitances of the EDLs are in good agreement with that inferred from experimental measurements. Both the rotations (ACN only) and translations of interfacial ACN and ions are found to slow down as the electrode is electrified. We also observe an asymmetrical dependence of these motions on the sign of the electrode charge. The rotation/diffusion of ACN and the diffusion of ions in the region beyond the first ACN or ion layer differ only weakly from those in the bulk},
doi = {},
journal = {Physical Chemistry Chemical Physics},
number = 20,
volume = 12,
place = {United States},
year = {Fri Jan 01 00:00:00 EST 2010},
month = {Fri Jan 01 00:00:00 EST 2010}
}
  • Room-temperature ionic liquids (RTILs) have received significant attention as electrolytes due to a number of attractive properties such as their wide electrochemical windows. Since electrical double layers (EDLs) are the cornerstone for the applications of RTILs in electrochemical systems such as supercapacitors, it is important to develop an understanding of the structure capacitance relationships for these systems. Here we present a theoretical framework termed counter-charge layer in generalized solvents (CGS) for describing the structure and capacitance of the EDLs in neat RTILs and in RTILs mixed with different mass fractions of organic solvents. Within this framework, an EDL is mademore » up of a counter-charge layer exactly balancing the electrode charge, and of polarized generalized solvents (in the form of layers of ion pairs, each of which has a zero net charge but has a dipole moment the ion pairs thus can be considered as a generalized solvent) consisting of all RTILs inside the system except the counter-ions in the counter-charge layer, together with solvent molecules if present. Several key features of the EDLs that originate from the strong ion ion correlation in RTILs, e.g., overscreening of electrode charge and alternating layering of counter-ions and co-ions, are explicitly incorporated into this framework. We show that the dielectric screening in EDLs is governed predominately by the polarization of generalized solvents (or ion pairs) in the EDL, and the capacitance of an EDL can be related to its microstructure with few a priori assumptions or simplifications. We use this framework to understand two interesting phenomena observed in molecular dynamics simulations of EDLs in a neat IL of 1-butyl-3- methylimidazolium tetrafluoroborate ([BMIM][BF4]) and in a mixture of [BMIM][BF4] and acetonitrile (ACN): (1) the capacitance of the EDLs in the [BMIM][BF4]/ACN mixture increases only slightly when the mass fraction of ACN in the mixture increases from zero to 50% although the dielectric constant of bulk ACN is more than two times higher than that of neat [BMIM][BF4]; (2) the capacitance of EDLs near negative electrodes (with BMIM+ ion as the counter-ion) is smaller than that near positive electrodes (with BF4as counter-ion) although the closest approaches of both ions to the electrode surface are nearly identical.« less
  • We evaluate the accuracy of local-density approximations (LDAs) using explicit molecular dynamics simulations of binary electrolytes comprised of equisized ions in an implicit solvent. The Bikerman LDA, which considers ions to occupy a lattice, poorly captures excluded volume interactions between primitive model ions. Instead, LDAs based on the Carnahan–Starling (CS) hard-sphere equation of state capture simulated values of ideal and excess chemical potential profiles extremely well, as is the relationship between surface charge density and electrostatic potential. Excellent agreement between the EDL capacitances predicted by CS-LDAs and computed in molecular simulations is found even in systems where ion correlations drivemore » strong density and free charge oscillations within the EDL, despite the inability of LDAs to capture the oscillations in the detailed EDL profiles.« less
  • The structure and charging kinetics of electrical double layers (EDLs) at interfaces of NaCl solutions and planar electrodes are studied by molecular dynamics (MD) and Poisson Nernst Planck (PNP) simulations. Based on the MD results and prior experimental data, we show that counterion packing in planar EDLs does not reach the steric limit at electrode voltages below 1 V. In addition, we demonstrate that a PNP model, when complemented with a Stern model, can be effectively used to capture the overall charging kinetics. However, the PNP/Stern model can only give a qualitative description of the fine features of the EDL.
  • No abstract prepared.