Kompleksni soedinenija

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Cyclic voltammetry a fingerprint of
electrochemically active species
Valentin Mirceski Institute of Chemistry
Faculty of Natural Sciences and Mathematics Ss
Cyril and Methodius University, Skopje Republic
of Macedonia
2
Cyclic Voltammetry a potentiodinamic transient
voltammetry
forward scan
reverse scan
Variation of the electrode potential in the
course of the experiment. The rate of potential
variation in time is called scan rate (v (V/s)),
which represents the critical time of the
experiment.
R ? O e-
Cyclic voltammetry (CV) is the most frequently
used technique. Almost any electrochemical study
starts with application of CV. From the features
of the cyclic voltammogram, one can deduce
thermodynamic, kinetic and mechanistic
characteristics of the electrode reaction!
The outcome of the experiment is presented as an
I-E curve, called cyclic voltammogram. By
convention, the positive current reflects
oxidation, whereas the negative current
represents reduction reaction.
3
Typical features of a cyclic voltammogram

• Anodic peak current
• Cathodic peak current
• Anodic peak potential
• Cathodic peak potential

Ip,a (anod. peak current)
Ep,a (anod. peak potential)
The peak-like shape of the voltammetric curves of
both forward and reverse scan are consequence of
the exhaustion of the diffusion layer adjacent to
the electrode with the electroactive material.
With time, the thickness of the diffusion layer
increases, thus the flux (i.e., the current)
decreases with time. That is why the current
commences decreasing after reaching the peak of
the current. The expansion of the diffusion layer
with time is shown on a next slide.
Current / A
Ip,c (cathod. peak current)
Ep,c (cathod. peak potential)
Potential / V
4
Concentration profiles of a Cottrell experiment
(explanation of the previous slide)
The thickness of the diffusion layer increases
with time!
Concentration profiles. Variation of the
concentration of electroactive species with the
distance x measured from the electrode surface at
different times of the chronoamperometric
experiment.
5
Electrode mechanism revealed by cyclic
voltammetry Reversible electrode reaction

• Reversible electrode reaction of R (R O ne)
  species dissolved in the electrolyte solution
  undergoing oxidation at the electrode surface
  means that the voltammogram is affected by the
  mass transfer regime only, and the redox species
  at the electrode surface obey the Nernst
  equation E E0 RT/nF ln (cO/cR). The peak
  current
• Ip,a (2,69 ? 105) n3/2 AcD1/2
  v1/2 Randles-Sevcik equation

n number of electrons in the electrode reaction
A electrode surface area c concentration of
redox active species dissolved in the electrolyte
solution D diffusion coefficient of the redox
active species v scan rate (the speed of
variation of the potential with time
Emid
Current / A

• The ratio of the peak currents is equal to 1
  Ip,c/Ip,a 1 and it is independent on the scan
  rate.
• Ep,a Ep,c RT/nF (? 59 mV/n)
• The peak potentials are independent on the scan
  rate
• The mid-peak potential (Emid (Ep,a Ep,c)/2 is
  equal to the formal (standard) potential E0 of
  the redox couple R/O

Potential / V
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Cyclic voltammograms of C60 and C70 in a toluene
solution
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Cyclic voltammograms of ferrocenedimethanol
reversible electrode reaction
Ip const. ? v1/2
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Totally irreversible electrode reaction
R ? O ne-
electrochemically irreversible electrode reaction
reversible electrode reaction
The plot compares CV of a reversible (blue) with
a very slow (irreversible) electrode reaction.
For a very slow irreversible electrode reaction,
the peak potential and current of the forward
scan are deffined with the eqs below. In some
cases, the reverse peak cannot be even observed,
and the voltammogram of a irreversible electrode
reaction contains only one CV peak.
k0 is the standard rate constant and a is the
electron transfer coefficient.
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Quasireversible electrode reaction
Quasireversible electrode reaction is controlled
by both mass transfer regime and the kinetics of
the electrode reaction. The critical parameter
that controls voltammetric characteristics, is
defined as k ??/(? D)1/2, where ? nFv/RT .
Hence, the dimensionless kinetic parameter k
unifies the diffusion coefficient (D) as a
parameter representing the mass transfer, the
standard rate constant (??), as a parameter
controlling the rate of the electrode reaction,
and the scan rate (v), as a parameter controlling
the time available for the electrode reaction.
The rate decreases
The rate decreases
Dimensionless cyclic voltammograms, obtained by
the simulation of the experiment. The
dimensionless current function Y is defined as Y
I (nFA)-1 (?D)-1/2. The plot displays a
comparison of the dimensionless current function
Y for electrode reactions characterized with
different rate.
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Dimensionless current function vs. real current

• In cyclic voltammetry, the current of any
  electrode mechanism can be defined as
• I (nFA) (v)1/2(nF/RT)1/2 D 1/2 Y
• The product (nFA) (v)1/2(nF/RT)1/2 D 1/2
  contains only constants for given electrode
  reaction and experimental conditions, and it
  could be considered as amperometric constant
  kamp. Hence,
• I kamp Y
• The function Y, could be reviled by simulations
  (mathematical modeling) only. It is specific for
  each electrode mechanism. Usually, it is a
  function of many other parameters, as electrode
  potential, rate constants, electron transfer
  coefficient, diffusion coefficients etc.
• Y f(E, k0, a, D)
• Care must be taken in reading the literature in
  regard whether particular discussion refers to
  the properties of the real current I, or to the
  dimensionless current function Y, or to both. For
  instance, for a quasireversible electrode
  reaction, the dimensionless current function Y
  decreases by increasing the scan rate, however,
  the real current increases with the scan rate, as
  the scan rate affects both kamp and Y. Obviously,
  the effect of the scan rate on the kamp prevails
  compared to the simultaneous effect of the scan
  rate to Y. This is illustrated on the next slide.

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Dimensionless vs. real cyclic voltammograms
variation of the scan rate
scan rate increases
scan rate increases
Dimensionless current function
Real current
scan rate increase
scan rate increase
Cyclic voltammograms obtained by the simulation
of the experiment at a planar electode of a
dissolved redox couple. The dimensionless current
function Y is defined as Y I (nFA)-1 (?D)-1/2.
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Decamethylferrocene Quasireversible electrode
reaction

• Variation of the electrode kinetic parameter k
  ??/(?D)1/2 by altering the scan rate of the
  experiment. Increasing the scan rate, the
  parameter k decreases. As a consequence, the peak
  potential separation increases, indicating a
  quasirevrsible electrode reaction. Let us recall
  that for a reversible electrode reaction the peak
  potential separation should be independent on the
  scan rate.

scan rate increase
scan rate increase
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Revealing complex electrode mechanisms
electrochemistry of cyclic hydroxylamine
quasireversible electrode reaction
reversible electrode reaction

• 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpy
  rrolidine (CMH) redox probe used for detection
  of superoxide formation in living cells.

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pH-dependency of CMH voltammetry

pH 4.2
v 100 mV/s
0.025

pH 5.1

pH 6.2
0.020

pH 7

pH 7.8
0.015

pH 9

pH 9.8
0.000010

pH 11

pH 12
0.000005
pH 4 12
0.000000
-0.000005
-0.000010
-0.000015
-1.0
-0.5
0.0
0.5
1.0
1.5
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Consecutive electron transfer EE mechanism
R
R
R
-
-e
-
-e
-H
e
-
H
-
e
N

N
N
N

OH

O
O
O
CMH
CM-
CMox
CM.
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Electrode reaction coupled with a following
irreversible chemical reaction - EC mechanism

• Cyclic voltammetryu is an excellent method for
  studying chemical reactions coupled with the
  electrode reaction. If the chemical reaction
  follows the electrode reaction, then the
  mechanism is termed as an EC mechanism, where the
  symbol E refers to the electrode reaction, while
  the symbol C refers to the follow up chemical
  reaction.
• In the scheme above, initially, in the solution,
  R species are present only. At the electrode
  surface, the reversible electrode reaction R O
  ne- is taking place. However, the product of the
  electrode reaction. i.e., the O species, are
  additionally involved in the chemical reaction
  leading to the electrochemically inactive final
  product P. The latter chemical reaction is
  attributed with a rate constant k. The latter
  chemical reaction takes place only in the
  vicinity of the electrode surface, i.e., in the
  thin diffusion layer close to the electrode in
  which O species are present due to previous
  electrode reaction. As O species are lost in the
  chemical reaction, in the reverse scan of the
  cyclic voltammogram, the reverse current is
  diminishing proportional to the rate of the
  chemical reaction. The shape of the cyclic
  voltammograms depends on the kinetic parameter
• kchem k/ ?, which unifies the rate of the
  chemical reaction (k) and the available time of
  the voltammetric experiment represented by the
  scan rate ? nFv/RT

k
R ? ne- O ? P
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• Typical voltammetric response of an EC mechanism
  observed for different values of the ratio kchem
  k/ ?. When kchem is big value (kchem 500),
  the rate of the follow-up chemical reaction is
  big, almost the complete amount of O species are
  consumed in the chemical reaction, thus, in the
  reverse scan of the cyclic voltammogram, no
  reduction current could be seen, as no O species
  remained to be reduced back to R species. When
  the chemical parameter is moderate (e.g., kchem
  0.01), the reverse scan is still formed, as
  significant fraction of O species are still
  present in the diffusion layer as the follow-up
  chemical reaction is significantly slow.
  Obviously, for given k, the parameter kchem could
  be made big or small depending on the scan rate
  of the experiment (i.e., the available time for
  the chemical reaction), which is an excellent
  tool for studying and estimating the kinetics of
  the chemical reaction.

kchem
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EC mechanism of dopamine predicting the
evolution of the consecutive cyclic voltammograms

• Dopamine/dopamine o-quinone is a reversible redox
  couple
• Dopamine o- quinone is chemically unstable and
  undergoes intra-molecular reorganization to
  leucochrome
• Leucochore is also electroactive compund. It can
  be oxidized at less positive potentials than
  dopamine

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Electrochemical mechanism preceded by a chemical
reaction and CE mechanism. Redox chemistry of
2-palmitoylhydroquinone an artificial cellular
membrane calcium transporter
2
1
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Revealing the mechanism by varying the scan rate
v 10 mV s-1
v 100 mV s-1
v 50 mV s-1
0.007
0.005
0.0025
0
I/mA
-0.0025
-0.005
-0.0075
-0.010
-0.300
-0.050
0.200
0.450
0.700
E / V
v 1 mV s-1
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Voltammetry of an a surface confined redox
couple Reversible electrode reaction

• Reversible electrode reaction of a thin film

This is typical, idealized response of a
reversible electrode reaction of a thin film
immobilized on the electrode surface. The shape
of the voltammogram dislikes strongly from the
typical wave-shaped voltammogram of a dissolved
redox couple. For a thin film, there is no mass
transfer and the exhaustion of the redox active
material in the course of the potential scan
causes a peak like shape of voltammetric branches
of the cyclic voltammogram.
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• The current of a reversible surface (thin-film)
  electrode reaction is defined as
• Here Go, and GR are surface concentration of O
  and R species, GT Go GR is the total surface
  concentration, E? is the formal potential, and
  bo and bR are adsorption constants related with
  the Gibbs free energy of adsorption.
• Peak current is defined as
• Peak potential is defined as
• In addition, the following is valid Ip,c
  Ip,a, and Ep,c Ep,a and the half-peak width is
• The charge consumed in a course of a single
  potential scan (calculated as a surface under the
  single voltammetric peak) is

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Interactions between particles of the immobilized
polymer on the electrode surface

• If there are interactions between immobilized
  species on the electrode surface, the shape of
  the voltammogram change as shown below. The
  interactions are quantified by the numerical
  interaction parameter a linked with the Frumkin
  adsorption isotherm. The values on the plot refer
  to the interaction parameter. When a 0, there
  are no interactions. For a gt 0 and a lt 0, the
  interactions are attractive or repulsive between
  immobilized species on the electrode surface.
  Obviously the broadening and narrowing of the
  voltammetric peaks are linked to the repulsive
  and attractive interactions, respectively.

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Guasireversible electrode reaction of a surface
confined redox couple

• If the charge transfer within the film is slow,
  the redox equilibrium does not prevail, hence
  instead of having so called reversible
  electrochemical behaviour, the system behaves as
  a quasireversible electrode process. Thus, the
  rate of the charge transfer controls the
  voltammetric response, which is obviously seen
  from the shape of the voltammograms. The numbers
  on the plots correspond to the surface standard
  rate constant k0 (s-1). Thus, as the rate of the
  charge transport within the film is becoming
  slower, the separation between the cathodic and
  anodic CV peaks increases proportionally together
  with the decrease of the peak currents.

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