Cyclic voltammetry is a versatile electroanalytical technique used to study electrochemical reactions. It involves scanning the potential of a stationary working electrode using a triangular waveform. This technique can provide information about redox reaction kinetics, thermodynamics, and coupled chemical reactions. The current measured during the potential scan produces a voltammogram that reveals details about the electroactive species. Reversible systems show well-defined peaks that follow theoretical relationships, while irreversible systems have broader peaks that shift with scan rate. Cyclic voltammetry is useful for investigating organic and inorganic reactions, as well as probing reaction mechanisms and coupled chemical steps.

1. Cyclic Voltammetry : A Popular
Tool for Investigating
Electrochemical Reactions
Presented By
MD. ADIL (Ph.D.)
Energy Science and Engineering Department
IIT Bombay, India

2. • One of the most versatile electroanalytical technique for the study of electroactive species
where the voltage is in excess as predicted by Nernst Equation
• It involves linear scanning of potential of a stationary electrode using a triangular
waveform
• Organic chemist used
• Inorganic chemist used it for
 It to evaluate the bio-synthetic reaction pathways
 To study of electrochemically generated free radical
 To study the effect of ligand on the oxidation and reduction potential
of central metal ion in complex and multinuclear clusters
Cyclic Voltammetry?

3. Provides information on
 The kinetics of coupled reaction
 The kinetics of the heterogeneous electron transfer reaction
 The thermodynamic of redox reaction

4. When does signal arises?
 Firstly, the potential of the working electrode is
control by reference electrode
 The controlling potential across the two electrode
is the excitation signal
 Excitation signal of C.V is a linear potential scan with
a triangular waveform
A voltammogram is obtained by measuring current
at the working electrode during the potential scan
Nutshell:
Cause : applied potential
Effect : obtained current

5. Working electrode : Pt wire
Electrolyte : 6.0 mM K3Fe(CN)6 in 1(M) KNO3 in H2O
Potential Window : +0.8 (V) to -0.2 (V)
Switching potential towards positive scan : -0.15 (V)
FeIII(CN)6
-3 + e FeII(CN)6
-4
 When the electrode potential turns sufficiently
negative the cathodic current starts at “b”
 When the electrode turns sufficiently oxidant
FeII(CN)6
-4 FeIII(CN)6
-3 + e
Nutshell
forward scan : FeIII(CN)6
-3 + e FeII(CN)6
-4
Reverse scan : FeII(CN)6
-4 FeIII(CN)6
-4 + e

6. Nernst Equation explanation
E = E
o
FeIII(CN)6
-3
FeII(CN)6
-4
+ 0.059 log FeIII(CN)6
-3
FeII(CN)6
-4
 Initial E value is sufficiently more positive than Eo to maintain high concentration of FeIII in solution
providing negligible current
 As potential moves to positive value, the conversion of FeIII to FeII starts providing current in the circuit
 Logarithmic relationship between E and can be seen in the rapid shout up of current value
where E = E
o
i.e FeIII
FeII
= 1
FeIII
FeII

7. Electron transfer rate constant
The current value depends on two steps in the overall process
 The movement of electroactive species to the electrode surface
 The electron transfer reaction
kf = k˚ exp {
−𝜶𝒏𝑭
𝑹𝑻
(E-E°) }
n = number of electron per molecule
T = temperature in kelvin
R = universal gas constant
F = Faraday constant
E° = formal reduction potential
α = transfer coefficient
k˚ = standard heterogeneous
electron-transfer rate constant
Its value depends on the reaction between an species
and electrode surface
α arises because a fraction of energy that is put into the system
lowers the activation energy barrier
α 0 to 1
Depends on the shapes of the free energy surfaces for the reactant and the product

8. Why current rises to a peak and decay?
 Experimental solution is unstirred
 Mode of mass transport – Diffusion
 This relative slow mode of mass transport cannot maintain a steady state concentration
profile in the region close to electrode
Results to
• Depletion zone grows
• Average distance travel by the ions increases
• Rate of mass transport decreases
 Dependence on mass transport ,and the fact that finite rate for the reverse
electron transfer process is possible this prevents the current to rise exponentially with potential
Eventually the mass transport step becomes the r.d.s and current rises to peak
 With the reaction the concentration gradient decreases, rate of mass transport
decreases causing current to decay
 Beyond the peak ,current value depends on time, independent of potential
I α t -0.5
e
e
electrode
Electron transfer rate constant for backward scan
kf = k˚ exp {
(𝟏 − 𝜶)𝒏𝑭
𝑹𝑻
(E-E°) }

9. Formal reduction potential
• It is usually reported as average of forward and backward peak potential for the redox couple
• It is accurate if the electron transfer process is reversible and the diffusion coefficient of the
oxidized and reduced species are same
What is mean by reversibility?
For an electrochemist, it means that the reaction is fast enough to maintain the concentration
of the oxidized and reduced species in equilibrium with each other at the surface of electrode
Equilibrium ratio at a given potential from Nernst Equation
E = E
o
-
𝑹𝑻
𝒏𝑭
ln
[𝑹]
[𝑶]
( )x = 0

10. Now the question is how fast is the fast enough?
• Many redox reaction looks reversible when the voltage is scanned slowly
• But at higher scan rate ΔEp appears greater than
58
𝑛
mV
Reversibility is a matter of degree and depends on the stress that is applied to the system
Two researchers Matsuda and Ayabe indicated that any deviation from reversibility
becomes imperceptible if ko is greater than the numerical value of 0.3ϒ0.5
There are some instruments that can scan upto 103 Vs-1 , so any electron transfer reaction
With rate constant ˃ 10 cms-1 will be reversible even in the very fast experiment

11. Reversible System
• Peak current for a reversible couple at 25o C is given by Randles – Sevcik equation
  1/2
1/2
3/2
5
p ν
ACD
n
10
x
2.69
i 
n = number of electrons
A = electrode area (cm2)
C = concentration (mol/cm3)
D = diffusion coefficient (cm2/s)
ν = potential scan rate (V/s)
 Ip is directly proportional to C
 Ip is directly proportional to ν0.5
For a simple reversible fast couple reaction
ip/ic ≈ 1
Formal reduction potential for electrochemical reversible couple
𝐸𝑝𝑎 + 𝐸𝑝𝑐
2
Eo
=

12. Reversible System
• The separation between peak potentials
V
n
0.059
E
E
ΔE pc
pa
p 


• Used to determine the number of electrons transferred in the electrochemical reaction
• For a fast n electrons transferred reaction
59
𝑛
𝑚𝑉
ΔEp =
• Epa and Epc remains independent of the scan rate

13. Irreversible System
 Systems having sluggish electron transfer reaction
 Individual peaks are reduced in size and are widely separated
 It is characterized by shift in the peak potential with scan rate


















1/2
a
1/2
o
a
o
p
RT
Fν
αn
ln
D
k
ln
0.78
F
αn
RT
E
E
    1/2
1/2
1/2
a
5
p ν
ACD
αn
n
10
x
2.99
i 
α = transfer coefficient
na = number of electrons involved in a charge transfer step
ko = standard heterogeneous rate constant (cm/s)
ip is proportional to C but lower depending on the value of α

14. Quasi-reversible Systems
 Current is controlled by both charge transfer and mass transport
 Exhibit larger separation in peak potentials compared
to reversible systems
 Shape depends on heterogeneous rate constant and scan rate
 Exhibits irreversible behavior at very fast scan rates
 It is characterized by ΔP >
59
𝑛
𝑚𝑉

15. Peak current and coupled chemical reaction
 The height of the peak current can be used to
determine the concentration of reactant in the bulk solution
 There are some inorganic ions, metal complexes, and organic compounds which
undergo electron transfer reactions without the making or breaking of covalent bonds
 CV provides the capability for generating a species during the forward scan, probing its
fate with the reverse scan and subsequent cycles
 The real forte of this CV technique is the analysis
of homogenous chemical reaction that are
coupled to the electron transfer reaction
 This analysis is based on the height of anodic
and cathodic peaks
Peak
current

16. Consider a charge transfer reaction followed by irreversible chemical reaction
O + ne R R Z
k
 If conversion of R to Z is fast and the potential is scanned slowly
RESULT Smaller anodic peak
Extreme Case
k is very fast relative to the scan rate
RESULT anodic peak disappear
k5 > k4 ……. > k1
k1
k2
k3
k4
k5

17.  Two researchers Nicholson and Shain demonstrated that the ratio of
𝐼𝑝
𝐼𝑐
can be
estimated by rate constant of chemical step and the time ‘Ꞇ’ spent between E1/2 and Eλ
Plot of theoretical relationship
E1/2 the midpoint between the peaks at a very high scan rate
where the chemical reaction does not have enough time consume R
Eλ switching potential

18. Electrochemical mechanisms involving coupled chemical reactions
1 Reversible electron transfer, no chemical complications
2 Reversible electron transfer followed by a reversible chemical reaction
3 Reversible electron transfer followed by an irreversible chemical reaction
4 Reversible chemical reaction preceding an reversible electron transfer
5 Reversible chemical reaction preceding an irreversible electron transfer
6 Reversible electron transfer followed by an irreversible regeneration of starting material
7 Irreversible electron transfer followed by an irreversible regeneration of starting material

19. 1 O + ne R
2 O + ne R R Z
k1
k-1
3 O + ne R R Z
k
4 Z O
k1
k-1
O + ne R
5 Z O
k1
k-1
O + ne R
6 O + ne R R + Z O
7 O + ne R R + Z O

20. How to fix the baseline for peak current measurement ?
For forward scan it is fairly simple
It can be obtained by recording the background current without
the analyte under same conditions
For reverse scan
Electrolysis for the forward scan still contributes to
the total current until the scan has passed the foot of
the forward wave again.

21.  The baseline curve for the reverse scan
can be obtained by stopping the forward
scan at the switching potential
First approach
Second approach
 To stop the scan at a convenient spot
(at least 35 mV past Ep for the forward scan) and hold the
potential until the current is relatively constant.
Nicholson formula
𝑖𝑝𝑎
𝑖𝑝𝑐
=
𝑖𝑝𝑎 𝑜
𝑖𝑝𝑐
+
0.485 𝑖λ 𝑜
𝑖𝑝𝑐
+ 0.086

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