Classical density-functional theory of simple electrode-electrolyte interfaces

Abstract: This thesis is dedicated to the theoretical study of the electrolyte-electrode interface
with classical density-functional theory, with a focus on how the modeling of solvent
impacts the process quantities of electrolyte solutions. First, physical effects and relevant
parameter spaces for the electric double layers (EDLs), that consists of counter-ions near
a charged electrode, are deduced by discussing technical applications. Afterwards, the
framework of classical density-functional theory and observables from liquid-state theory
are introduced.
A historic overview of commonly applied EDL models is discussed and put into
a modern context, emphasizing the significance of the Stern layer for the theoretical
description of EDLs. The Stern layer is usually introduced by arguing that solvent particles
adsorbed on the electrode hinder ions from approaching the electrode at arbitrarily close
distances. The primitive model (PM) describes the electrolyte as charged hard-spheres
and includes an intrinsic Stern layer due to the hard-core interactions. In this thesis
it is examined how well the PM can reproduce experimentally measured differential
capacitances under the premise that ion diameters should reflect ion sizes determined
from scattering experiments and it is demonstrated that predicted capacitances are too
big and larger ion diameters or bigger Stern layers are necessary to map theoretical
prediction onto experimental data.
Based on previous observations this work proposes a new model extension for the
primitive model, where two diameters are attributed to each ion species. One determines
the hard ion-ion interactions and the other specifies the hard ion-wall interactions.
This new extension allows to consider the influence of the solvent by means of a radii-
independent Stern layer and hydration-shells. Within this model extension two process
quantities for EDLs are observed. First, the differential capacitance attains much closer
values to empirically determined ones, whereas the ion diameters as well as the Stern
distance are set to reasonable values that conform to sizes from scattering experiments of
ions and water molecules. Second, new effects for the reversible heat production during
charge-up of the EDL are discovered within this model extension if additionally neutral
hard solvent particles are added to the system. For example situations exists where one
electrode can be cooled whereas the other one is heated during the charging process.
However, the heat contributions of both electrodes average out, such that the predicted
effects could not be confirmed with the available experimental data.
Moreover, this work considers how to use more complex solvent particles by developing
a framework that allows to determine the mean-field electrostatic excess free-energy
functional of particles that are constructed from arbitrary charge distributions. A
comparison of the radial distribution function of a system of simple model particles
that consist of a point charge embedded within a spherical homogeneous charge density
of opposite charge, obtained from theory and Monte Carlo simulations, reveals that
the taken approach can not resolve structure in the system under bulk conditions. It
is demonstrated that the mean-field electrostatic functional actively suppresses the
formation of structure in the system, even if the charge distributions are embedded into
hard-spheres

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• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2024

Classification and Topics

• Keyword: Dichtefunktionalformalismus ; Dichtefunktionalformalismus ; Elektrische Doppelschicht ; Elektrolyt

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2024
• Creator: Pelagejcev, Philipp
• Contributor: Härtel, Andreas ; Thoss, Michael ; Arbeitsgruppe Statistische Physik Weicher Materie und Komplexer Systeme ; Albert-Ludwigs-Universität Freiburg. Fakultät für Mathematik und Physik

Further information

• DOI: 10.6094/UNIFR/258985
• URN: urn:nbn:de:bsz:25-freidok-2589855
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.