Cyclic voltammetry: a fingerprint of
electrochemically active species
Valentin Mirčeski
Institute of Chemistry
Faculty of Natural Sciences and Mathematics
“Ss Cyril and Methodius” University, Skopje
Republic of Macedonia
1
Cyclic Voltammetry – a potentiodinamic transient voltammetry
1.2
0.05
1
0.04
0.6
0.03
0.4
0.2
0
0
0.5
t / s1
1.5
2
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.
+ e-
R⇄O
Current / A
0.8
0.02
forward scan
0.01
0
-0.4
-0.01
-0.3
-0.2
-0.1
0
0.1
reverse scan
0.2
0.3
0.4
-0.02
-0.03
-0.04
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!
RO+e
O+eR
Potential / V
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.
Typical features of a cyclic voltammogram
•Anodic peak current
•Cathodic peak current
•Anodic peak potential
•Cathodic peak potential
0.04
Ip,a (anod.
peak current)
0.03
Current / A
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.
0.05
Ep,a (anod.
peak potential)
0.02
0.01
0
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
-0.01
-0.02
-0.03
-0.04
Ip,c (cathod.
peak current)
Ep,c (cathod.
peak
potential)
Potential / V
3
Concentration profiles of a Cottrell experiment (explanation of the
previous slide)
1
t =0
t = 0,1 s
The thickness of the
diffusion layer
increases with time!
cR / c*R
t = 0,01 s
t = 0,001 s
t=1s
0,2
x
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.
4
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
•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
0.05
0.04
0.03
Current / A
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
Randles-Sevcik equation
0.02
0.01
Emid
0
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
-0.01
-0.02
-0.03
-0.04
Potential / V
0.3
0.4
Cyclic voltammograms of C60 and C70 in a toluene solution
6
Cyclic voltammograms of ferrocenedimethanol: reversible
electrode reaction
0.400
Ip = const.  v1/2
Ip,a/Ip,c = 0.095  0.019
Ep,a – Ep,c = 0.057 V
0.300
1,2
1
Ip,a/ mA
I / mA
0.200
0.100
0
0,8
0,6
0,4
0,2
0
0
-0.100
5
10
15
20
1/2
v / (mV/s)
-0.100
0
0.100 0.200 0.300 0.400 0.500 0.600 0.700
E/V
7
Totally irreversible electrode reaction
R  O + ne0.468
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.
0.5
0.4
electrochemically
irreversible
electrode
reaction
reversible
electrode
reaction
0.3
0.2
 k  1 0.1
 k 2
0
0.1
0.2
0.3
k0 is the standard rate constant and a is
the electron transfer coefficient.
 0.379 0.4
0.4
 0.5
0.2
0
Ek
0.2
0.4
0.5
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 = к/( 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 (к), 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.
0.468
0.5
The rate decreases
0.4
0.3
 k 1
 k 2
0.2
0.1
 k 3
 k 4
0
 k 5
0.1
0.2
0.3
 0.379 0.4
The rate decreases
0.4
 0.5
0.2
0
Ek
0.2
0.4
0.5
Dimensionless cyclic voltammograms, obtained by the simulation of the experiment. The dimensionless current
function  is defined as  = I (nFA)-1 (D)-1/2. The plot displays a comparison of the dimensionless current function 
for electrode reactions characterized with different rate.
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 
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 
The function , 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.:
 = 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 , or to both. For
instance, for a quasireversible electrode reaction, the dimensionless current function  decreases
by increasing the scan rate, however, the real current increases with the scan rate, as the scan rate
affects both kamp and . Obviously, the effect of the scan rate on the kamp prevails compared to the
simultaneous effect of the scan rate to . This is illustrated on the next slide.
10
Dimensionless vs. real cyclic voltammograms: variation of the
scan rate
15.735
20
0.367
scan rate increases
scan rate
increases
0.3
15
0.2
Ik 1
Real current
10
 k  2 0.1
Ik 2
Ik 3
 k 3
5
 k 4
Ik 4
Ik 5
Dimensionless
current function
 k 1
0
 k 5
0
0.1
scan rate increase
5
0.2
scan rate increase
 7.013 10
0.4
 0.5
0.2
0
Ek
0.2
 0.214 0.3
0.4
0.5
0.4
 0.5
0.2
0
Ek
0.2
0.4
0.5
Cyclic voltammograms obtained by the simulation of the experiment at a planar electode of a
dissolved redox couple. The dimensionless current function  is defined as  = I (nFA)-1 (D)-1/2.
11
Decamethylferrocene: Quasireversible electrode reaction
Variation of the electrode kinetic
parameter k = к/(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
12
Revealing complex electrode mechanisms: electrochemistry of
cyclic hydroxylamine
CH3
O
0.000045
0.000040
0.000035
0.000030
0.000025
0.000020
I (A)
0.000015
0.000010
CMH PB pH 7.8
10 mV/s
25 mV/s
50 mV/s
100 mV/s
200 mV/s
350 mV/s
500 mV/s
O
H3C
CH3
N
H3C
CH3
OH
quasireversible
electrode reaction
0.000005
reversible
electrode reaction
0.000000
-0.000005
-0.000010
-0.000015
-0.000020
-1.5
-1.0
-0.5
0.0
E (V)
0.5
1.0
1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CMH): redox probe
used for detection of superoxide formation in living cells.
13
pH-dependency of CMH voltammetry
pH 4.2
pH 5.1
pH 6.2
pH 7
pH 7.8
pH 9
pH 9.8
pH 11
pH 12
0.025
0.020
0.015
0.000010
0.000005
v = 100 mV/s
pH 4 ~ 12
0.000000
-0.000005
-0.000010
-0.000015
-1.0
-0.5
0.0
0.5
1.0
1.5
14
Consecutive electron transfer: EE mechanism
R
+H+
+e -
N
OH
O–
CMH
CM-
+e -
N
O·
CM
N+
O
.
CMox
CMH PB pH7.8
0.000020
0.000015
R
-e -
-e -
-H+
N
R
100 mV/s
0.000010
0.000005
0.000000
-0.000005
-0.000010
-1.0
-0.5
0.0
0.5
1.0
15
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.
k
R ⇄ ne- + O  P
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
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 =
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 intramolecular reorganization to leucochrome;
•Leucochore is also electroactive compund. It can be oxidized at less
positive potentials than dopamine;
18
Electrochemical mechanism preceded by a chemical reaction: and CE
mechanism. Redox chemistry of 2-palmitoylhydroquinone – an artificial cellular
membrane calcium transporter
OH
O
OH
O
C15H31
OH
O
O
C15H31
OH
⇆
1
1
OH
2
O
O
C15H31
+
2H+
O
OH
C15H31
+ 2e
OH
O
⇆
2
O
C15H31
+ 2H+ + 2e
O
2
0.125x10-5
0.075x10-5
I/A
OH
C15H31
1
0.025x10-5
-0.025x10-5
-0.075x10
-5
-0.100
0
0.100
0.200
0.300
0.400
0.500
0.600
E vs Ag/AgCl (3 M KCl) / V
0.700
19
Revealing the mechanism by varying the scan rate
v = 10 mV s-1
v = 50 mV s-1
v = 100 mV s-1
-5
0.200x10
0.007
0.005
-5
0.100x10
0.150x10
-4
0.100x10
-4
0.050x10
-4
0
i/A
I/mA
I/A
0.0025
0
0
-0.0025
-5
-0.100x10
-0.005
-0.050x10
-4
-0.100x10
-4
-0.150x10
-4
-0.200x10
-4
-0.0075
-5
-0.200x10
-0.010
-0.250x10
-0.050
0.200
0.450
E/V
-0.300
0.700
-0.050
0.200
E/V
0.450
-4
-0.300
0.700
-0.050
0.200
0.450
0.700
E/V
v = 1 mV s-1
0.500x10
-6
0.400x10
-6
0.300x10
-6
0.200x10
-6
0.100x10
-6
i/A
-5
-0.300x10
-0.300
0
-0.100x10
-6
-0.200x10
-6
-0.300x10
-6
-0.400x10
-6
-0.500x10
-6
-0.300
-0.050
0.200
0.450
0.700
E/V
20
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.
21
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
22
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 > 0 and
a < 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.
23
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.
24
25
26