Large enhancement of capacitance driven by electrostatic image forces.
2011-04
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Large enhancement of capacitance driven by electrostatic image forces.
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2011-04
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Abstract
The purpose of this thesis is to examine the role of electrostatic images in determining
the capacitance and the structure of the electrostatic double layer (EDL) formed at the
interface of a metal electrode and an electrolyte. Current mean field theories, and the
majority of simulations, do not account for ions to form image charges in the metal
electrodes and claim that the capacitance of the double layer cannot be larger than that
of the Helmholtz capacitor, whose width is equal to the radius of an ion. However, in
some experiments, and simulations where the images are included, the apparent width
of the capacitor is substantially smaller.
Monte Carlo simulations are used to examine the interface between a metal electrode
and a room temperature ionic liquid (RTIL) modeled by hard spheres (the “restricted
primitive model”). Image charges for each ion are included in the simulated electrode.
At moderately low temperatures the capacitance of the metal/RTIL interface is so large
that the effective thickness of the electrostatic double-layer is up to 3 times smaller
than the ion radius. To interpret these results, an approach is used that is based on
the interaction between discrete ions and their image charges, which therefore goes
beyond the mean-field approximation. When a voltage is applied across the interface,
the strong image attraction causes counterions to condense onto the metal surface to
form compact ion-image dipoles. These dipoles repel each other to form a correlated
liquid. When the surface density of these dipoles is low, the insertion of an additional
dipole does not require much energy. This leads to a large capacitance C that decreases
monotonically with voltage V , producing a “bell-shaped” C(V ) curve. In the case of
a semi-metal electrode, the finite screening radius of the electrode shifts the reflection
plane for image charges to the interior of the electrode resulting in a “camel-shaped”
C(V ) curve, which is parabolic near V = 0, reaches a maximum and then decreases.
These predictions are in qualitative agreement with experiment.
A similarly simple model is employed to simulate the EDL of superionic crystals.
In this case only small cations are mobile and other ions form an oppositely charged
background. Simulations show an effective thickness of the EDL that may be 3 times smaller than the ion radius. The weak repulsion of ion-image dipoles again plays a central
role in determining the capacitance in this theory, which is in reasonable agreement
with experiment.
Finally, the problem of a strongly charged, insulating macroion in a dilute solution
of multivalent counterions is considered. While an ideal conductor does not exist in the
problem, and no images are explicitly included, simulations demonstrate that adsorbed
counterions form a strongly correlated liquid of at the surface of the macroion and acts
as an effective metal surface. In fact, the surface screens the electric field of distant
ions with a negative screening radius. The simulation results serve to confirm existing
non-mean-field theories.
Description
University of Minnesota Ph.D. dissertation. April 2011. Major: Physics. Advisor: Boris Shklovskii. 1 computer file (PDF); x, 106 pages, appendices A-B.
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Loth, Matthew Scott. (2011). Large enhancement of capacitance driven by electrostatic image forces.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/104688.
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