1. Electrochemistry 1
1
The Basic of the basic
1. Interface;
2. Thermodynamics & Kinetics;
3. Overpotential.
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Electrochemistry 1
1. Interface
Electrochemistry is the study of reactions in which
charged particles (ions and/or electrons) cross the
interface between two phases of matter, such as the
interface between a solid and a liquid (=electrode
/electrolyte).
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Electrochemistry 1
1. Interface
Electrode processes take
place within an electric
double layer.
Electric double layer that is a
transition region between two
phases consists of (1) an inner
monomolecular layer and (2)
an outer diffuse region.
Between the inner molecular
layer and outer diffuse layer,
and (3) a layer intermediate
between inner molecular
layer and outer diffuse layer
exists.
(1) (3)
(2)
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Electrochemistry 1
1. Interface
(1) an inner monomolecular
layer of adsorbed molecules
or ions in which a very large
potential gradient is
produced (e.g. 1 volt across
1 angstrom that
corresponds to 100
MV/cm).
(1) (3)
(2)
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Electrochemistry 1
1. Interface
(2) an outer diffuse region
that compensates for any
local charge unbalance that
gradually merges into the
completely random
arrangement of the bulk
solution (charge unbalance,
namely, the violation of
electronuetrality, can be
temporarily/locally
produced but it will be
nuetralized/compensated.).
(1) (3)
(2)
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Electrochemistry 1
1. Interface
(3) a layer intermediate
between inner molecular
layer and outer diffuse layer
exists. In this intermediate
layer, excess charges are
solvated or weakly bonded
with counter ions.
(1) (3)
(2)
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Electrochemistry 1
2. Electrode thermodynamics and kinetics
Thermodynamics:
Thermodynamic equation ΔG=-nFE refers to the
movement of n moles of charge across the cell potential
E. The value of ΔG expresses the maximum useful energy
that a system can give the surroundings. This quantity can
only be perfectly extracted from the system under the
limiting conditions of a reversible change, which implies
zero current. The more rapidly the cell operates, the less
electrical energy it can supply.
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Electrochemistry 1
2. Electrode thermodynamics and kinetics
Kinetics:
(1) If the redox reaction steps of an electrode reaction are
rapid enough, then its potential is equal to the equlibrium
potential (the electrode will be non-polarizable).
(2) If, on the other hand, an equlibrium is established only
slowly due to a kinetic inhibition of steps involved in an
electrode reaction, then the electrode will be polarizable:
in order to induce the reaction to proceed in a given
direction, the kinetic inhibition of the reaction must be
overcome by applying a high overpotential.
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Electrochemistry 1
3. Overpotential
Within the theory of thermodynamic, the Nernst
equation should predict what electrode reaction will take
place. According to the Nernst equation, the hydrogen
evolution potential as a function of the concentration of
proton [H+] is E=0.000-0.059*log(1/[H+]).
At pH=7, E=-0.414 V. Therefore, only metals whose
reduction potentials are less negative than -0.41 V should
be reduced and plate out at the cathode in the
electrolysis of aqueous solution of electrolytes. This
means that it should not be possible to reduce metal ions
such as Zn2+ (E0=-0.76 V) from aqueous solution.
However, some such metals including Zn do plate out of
aqueous solution of electrolytes.
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Electrochemistry 1
3. Overpotential
The Nernst equation is a thermodynamic equation that
tells nothing about kinetics. For example, the evolution of
H2 at some cathode surfaces in some aqueous solutions
of electrolytes are too slow to occur at the potentials
given by the Nernst equation and only take place at
higher voltages (it needs overpotentials). Activation
overpotentials for the evolution of H2 on Zn, graphite,
and glassy carbon electrodes are -0.77 V, -0.62 V, and
more negative than -0.62 V, respectively. Carbon fibers
are also sp2 carbons, and its activation overpotentials are
usually more negative than that of graphite but less
negative than those of glassy carbons.
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Electrochemistry 1
3. Overpotential
The cell overpotential is
considered to be composed
of a number of independent
contributions: (1) ohmic
drop; (2) activation
overpotential; and (3)
diffusion overpotential.
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Electrochemistry 1
3. Overpotential
(1) Ohmic drop between
electrodes results from the
fact that the electrolyte
solution has a finite
conductivity;
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Electrochemistry 1
3. Overpotential
(2) activation overpotential
at one or both electrodes
arising from kinetic
inhibition of one of the
steps involved in the
electrode reaction
(desolvation of the reactive
ion, chemisorption of the
reaction product, etc.);
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Electrochemistry 1
3. Overpotential
(3) diffusion overpotential
at one or both electrodes
due to the presence of
concentration gradients in
the vicinity of the electrode
surface. As a result of
electrochemical reaction,
the concentration at the
electrode surface no longer
have their equilibrium
values.
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Electrochemistry 1
3. Overpotential
(3) (diffusion overpotential.
continued) If migration
through the electric double
layer is very rapid, then
diffusion from the bulk of
the solution towards the
electrode will be unable to
replenish the ions at the
double layer quickly enough
and a concentration
gradient will result.
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Electrochemistry 1
3. Overpotential
Detailed information
(1) Ohmic (IR) drop
Polarization measurements include a so-called ohmic
potential drop through a portion of the electrolyte
surrounding the electrode, through a metal- reaction
product film on the surface, or both.
An ohmic potential drop always occurs between the
working electrode and the reference electrode. This
contribution to polarization is equal to IR , where I is the
current density, and R is the resistance.
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Electrochemistry 1
3. Overpotential
Detailed information
(1) Ohmic (IR) drop
If copper is made cathode in a solution of dilute CuSO4 in
which the activity of cupric ion is represented by α(Cu+2),
then the potential φ1, in absence of external current, is
given by the Nernst equation,
φ1=0.34+(0.059/2)*log[α(Cu+2)].
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Electrochemistry 1
3. Overpotential
Detailed information
(2) Activation overpotential
Activation polarization is caused by a slow electrode
reaction. The reaction at the electrode requires an
activation energy in order to proceed. The most
important example is that of hydrogen ion reduction at a
cathode, H++e−→0.5H2 . For this reaction, the polarization
is called hydrogen overpotential. Overpotential is defined
as the polarization (= potential change) of an equilibrium
electrode that results from current flow across the
electrode/solution interface. Hydrogen overpotential can
vary with metal, current density, etc.
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Electrochemistry 1
3. Overpotential
Detailed information
(3) Diffusion overpotential
When current flows, copper is deposited on the
electrode, thereby decreasing surface concentration of
copper ions to an activity α(Cu+2)s. The potential φ2 of the
electrode becomes,
φ2=0.34+(0.059/2)*log[α(Cu+2)s].
Since α(Cu+2)s is less than α(Cu+2), the potential of the
polarized cathode is less positive than in the absence of
external current. The difference of potential, φ2−φ1, is the
concentration polarization, equal to
φ1-φ2=(0.059/2)*log[α(Cu+2)s/α(Cu+2)].
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Electrochemistry 1
3. Overpotential
Detailed information
(3) Diffusion overpotential
The larger the current, the smaller the surface
concentration of copper ion, or the smaller the value of
α(Cu+2)s, thus the larger the corresponding polarization.
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Electrochemistry 1
3. Overpotential
Note
The product, IR, decays simultaneously with shutting off
the current, whereas concentration polarization and
activation polarization usually decay at measurable rates.
Concentration polarization decreases with stirring,
whereas activation polarization and IR drop are not
affected significantly with stirring.
