Electroanalytical Techniques
(chronoamperometry/
chronocoulometry)
ABC’s of Electrochemistry
July 12, 2011
Vedasri Vedharathinam
Center for Electrochemical Engineering
Research
Department of Chemical and Biomolecular
Engineering
2
Electrochemical
Engineering
Research Lab, Ohio
Overview
1. Electrochemistry basics
2. Electrochemical techniques
– Sweep method
• Cyclic voltammetry
– Step method (current or potential)
• Chronoamperometry
• Double step chronoamperometry
• Chronocoulometry
• Double step chronocoulometry
3
Basic concepts in electrochemistry
• Chemistry that deals with chemical reactions in a metal (e-
conductor) – solution (ion conductor) interface.
• Involves e- transfer between the elctrode and the elecrolyte or
species in solution.
• This transfer creates a current, the magnitude of which can
give us clues about the active species.
• Electrochemistry is based on Electron transfer reactions:
oxidation-reduction (redox) reactions.
• These reactions result in the generation of an electric current
(electricity) or caused by the application of an electric current.
– Chemical rection driven by an external voltage – electrolysis
– Chemical reaction producing voltage - battery
4
Applications
• Batteries
• Fuel cells
• Electrolysis
• Corrosion
• Industrial production of chemicals such as Cl2,
NaOH, F2 and Al
• Biological redox reactions
• Redox reactions employed in biological sensing
• Amperometric sensors
5
Terminology
• OXIDATION—loss of electron(s) by a species;
increase in oxidation number;
Fe2+ Fe3+ + e-
• REDUCTION—gain of electron(s); decrease in
oxidation number; decrease in oxygen;
Fe3+ + e- Fe2+
• OXIDIZING AGENT—electron acceptor; the
reagent is reduced
• REDUCING AGENT—electron donor; the reagent
is oxidized.
6
When a piece of metal is placed in a solution containing ions,
there is a charge separation across the boundary between the
metal and the solution. This sets up a potential , which cannot
be measured directly but requires a second half cell.
Electrochemical behavior of an electrode in
solution
7
Electrochemical cell
8
Electrochemical cell
2 – electrode cell 3 – electrode cell
• Interested in only one of the reactions, and
the electrode at which it occurs is called the
working (or indicator) electrode, coupled with
an electrode that approaches an ideal
nonpolarizable electrode of known potential,
called the reference electrode.
• Current is passed between the WE and CE
• Consistent, reliable and accurate.
• Used where measurement of the whole cell
voltage is significant (e.g. batteries, fuel cells,
super caps).
• where the counter electrode potential can be
expected not to drift over the course of the
experiment. E.g. systems with very low
currents and/or relatively short timescales
and which also have a well poised counter,
e.g. a micro working electrode and a much
larger silver counter electrode.
2-electrode vs. 3-electrode systems
9
• Requires a precise control of the
potential at the electrode.
• Three electrodes:
– Working electrode (WE),
– Counter electrode (CE)
– Reference electrode (RE).
• No current through RE ideally.
• RE is used to provide precise
control of potential at the WE, and
the current from WE to CE is
measured.
Three electrode cell
10
Faradiac process: Electron transfer causes oxidation and
reduction to occur. This process is governed by Faraday’s law.
Faradaic process
11
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Engineering
Research Lab, Ohio
Mass transport due to diffusion
12
In order to react a species at an electrode it needs to be
transported from bulk to surface.
Three principal mechanisms:
• Diffusion is the movement of molecules along a concentration
gradient, from an area of high concentration to an area of low
concentration.
• Migration is the transport of a charged species under the
influence of an electric field.
• Convection is the transport of species by hydrodynamic
transport (e.g. natural thermal motion and/or stirring).
Mass transport in electrochemistry
13
Current flow at Electrode Surface
The current that flows from a surface electrochemical
reaction can be defined as (using the example of reduction
of O):][ electroderedc OnFAki
F = 96485 Cmol-1. The amount of charge in C transferred for 1 mole of
reactant.
dt
dqi
To understand an electrochemical reaction it is necessary to have a feeling
for the concentration of the reactant [O] as a function of distance from
electrode and with respect to time as a reaction progresses.
Mass transport in electrochemistry
Diffusion
14
Fick’s first law quantifies the movement of a species (under diffusion
control) with respect to distance x from an electrode with the flux, J.
x
ODJ oo
][
2
2 ][][
x
OD
t
Oo
More important is to understand how surface concentration changes as
function of time:
1st law
2nd law
Diffusion limited electrode reaction & Fick’s law
15
Solving Fick’s second law (for planar electrode boundary conditions), and then substituting gives the
Cotrell equation:
[O] is now the bulk concentration of O.
electredc OnFAki
t
DOnFAi
][
Diffusion limited electrode reaction & Fick’s law
16
Concentration verses distance
above the electrode before voltage stepConcentration verses distance
above the electrode just after pulse
x
ODJ oo
][
i ∝ J
Fe3+ + e- → Fe2+
(reduction)
Diffusion limited electrode reaction & Fick’s law
Current behavior with time
17
Electrochemical techniques -
Voltammetry
18
Electrochemical techniques
19
Electrochemical techniques
Voltammetry
Voltammetry: measurement of current (I)
as a function of applied potential (E) over
a time. Under condition with polarization
(η). Negligible consumption of analyte.
– Amperometry: measure I at a fixed E
– Potentiometry: measure E when I 0,
no polarization
– Coulometry: measure C, polarization is
compensated, all analyte is consumed.
Commonly uses three electrodes
- Working electrode
- Auxillary electrode
- Reference electrode
20
Electrochemical techniques
Why use voltammetry?
• Handles high salt concentrations better than
chromatographic instrumentation
• Can differentiate between ionic species
Example: Ni2+ Ni3+
• Extremely low detection limits – high sensitivity
• Can detect a wide range of species
21
Electrochemical techniques
Types of Voltammetry
1. Sweep methods
• Cyclic voltammetry
• Linear sweep voltammetry
• Rotating disk electrode
2. Step and pulse methods
• Step voltammetrya. Chronoamperometry
b. Chronocoulometry
c. Chronopotentiometry
• Pulse voltammetrya. Normal pulse voltammetry
b. Differential pulse voltammetry
c. Square wave voltammetry
22
What can be learnt from voltammetry?
• Mechanism of electrode reaction.
• Concentration of oxidative or reductive
species: useful for making a sensor.
• Determination of Diffusion coefficent ofelectroactive species, D.
• Rate constant.
• Type of reaction mechanism
23
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Cyclic Voltammetry
24
Electrochemical techniques – Sweep method
Cyclic voltammetry
Applied waveform
Resulting voltammogram
25
Electrochemical techniques – Sweep method
Cyclic voltammetry
For a reversible electrochemical reaction the CV recorded has certain
well defined characteristics.
I. The voltage separation between the current peaks is
I. The positions of peak voltage do not alter as a function of voltage
scan rate
II. The ratio of the peak currents is equal to one
Ia / ic = 1
I. The peak currents are proportional to the square root of the scan
rate
26
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Engineering
Research Lab, Ohio
a. Chronoamperometry
b. Double step chrono amperometry
c. Chronocoulometry
d. Double step chronocoulometry
Step methods
27
Types
Electrochemical techniques – Step methodW
E p
ote
nti
al
WE
po
ten
tial
Cu
rren
t
Ch
arg
eC
urr
en
tW
E p
ote
nti
al
t = 0
t = 0
t = 0
Chronoamperometry
Chronopotentiometry
Chronocoulometry
time
time
time
time
time
time
28
Chronoamperometry
Electrochemical techniques – Step method
Voltage applied to cell begins at V1 where no reaction occurs
and is stepped up to V2 causing electrode process to begin
and a current spike results.
V2
V1
WE
po
ten
tia
l
time
Stable R species
29
Chronoamperometry
Faradaic current under diffusion controlled conditions is related directly to the
concentration gradient, ∂Ci / ∂x, evaluated at x = 0. Thus, as the slope of the
concentration profile for Ox decreases with time following the potential step, so will the
observed current.
Electrochemical techniques – Step method
30
Current drops off with time according to the Cottrell
equation since material must diffuse to the
electrode surface in order to react.
Chronoamperometry
time
i
i ∝ 1 / √t
t
DOnFAi bulk
][
Electrochemical techniques – Step method
V1
WE
po
ten
tia
l
time
31
• Perform a potential step measurement.
• Ignore current before potential step.
• Linearise Cottrell equation
Plot 1 / i2 vs tSlope = /n2F2A2[O]2D
• Slope will give the value of ―D”
t
DOnFAi bulk
][
DOAFn
t
i bulk
22222 ][
.1
Chronoamperometry - estimation of diffusion co-efficient
i, A
t-1/2, s-1/2
Cottrell plot
Electrochemical techniques – Step method
32
• If the diffusion coefficient of an electroactive species is known, or
has been calculated, the diffusion layer thickness can be estimated
using this equation:
• The diffusion layer extends into the bulk solution more and more
slowly after application of a potential step. Hence for a molecule with
a diffusion coefficient of 1 x 10-10 m2s-1, the diffusion layer thickness
is around 20 mm after 1 second.
• The fraction of molecules oxidised or reduced can also be estimated
by calculating the volume of a hemispherical diffusion layer around a
circular electrode as a fraction of the total solution.
Dtl
Chronoamperometry - estimation of diffusion layer thickness
Electrochemical techniques – Step method
33
Double potential step Chronoamperometry
Electrochemical techniques – Step method
To study the chemical reactions which follow electron transfer
E: O + e R
C: R X
E: X + e P
Unstable R species
FORWARD STEP
REVERSE STEP
34
Double potential step Chronoamperometry
Electrochemical techniques – Step method
Oxidised species
Reduced species
FORWARD STEP REVERSE STEP
35
Double potential step Chronoamperometry
Electrochemical techniques – Step method
kt
t
t = 400ms
t = 200ms
t = 300ms
J. Phys. Chem., 1965, 69 (1), pp 30–40
Theoretical working curves for double
potential chronoamperometry applied to EC
mechanism
Kinetic plot for double potential
chronoamperometry
Slope = rate constant
36
– Measurement of surface area
– Measurement of diffusion co-efficient
– Determination of heterogeneous rate constant
– Determination of diffusion layer thickness
– Evaluation of ECE mechanisms
Chronoamperometry - Applications
t
DOnFAi bulk
][
Electrochemical techniques – Step method
37
Measuring instantaneous currents is not easy.
dt
t
CADnti
2/12/1
*
O
2/1
OF
2/1
2/1
O
2/1
OF2
tCADnQ
0 t
Ef
Ei
i
t
Q
t
dt
Chronocoulometry
Electrochemical techniques – Step method
38
Chronocoulometry
Electrochemical techniques – Step method
Q t( ) =2nFADO
1/2CO
* t1/2
p 1/2+Qc
Q t( ) = FnACbD
pt
æ
èçö
ø÷
1/2
dt + ic dtòò
Qc – response in the absence of reactant
(i.e only supporting electrolyte)
Linear plot with Qc intercept, slope
proportional to concentration of
reactantP
ote
ntia
lC
urre
nt
Charg
e
E2
E1
Qc
0
00
ic
Cottrell current
time
39
For O + n e- R, plot Q vs. t1/2
Q
QDL
QDL
t1/2
If plot linear, the reaction is Diffusion Controlled
Chronocoulometry
Electrochemical techniques – Step method
Q t( ) =2nFADO
1/2CO
* t1/2
p 1/2+Qc
Charge due to cottrell current
Interfacial capacitance charge
40
Chronocoulometry
Electrochemical techniques – Step method
What if redox species (O) is adsorbed on electrode surface?
Q t( ) =2nFADO
1/2CO
* t1/2
p 1/2+Qc +Qads
Q
QadsQDL (blank - only S.E)
t1/2
OF AnQads
O the quantity of adsorbed reactants
Charge flowing into the interfacial
capacitance when the electrode
potential is stepped from E1 to E2
Extra charge produced by the
adsorbed reactant
41
Double potential step Chronocoulometry
Electrochemical techniques – Step method
• Measuring Qc is not a problem –
adsorbed species produces little or no
change in the value of Qc
• But, adsorbed species produces
significant Qc values, so that the
evaluated Qc in the absence of reactant
(blank) do not apply.
SOLUTION
Double potential chronocoulometry
time
0
Qr
Pote
ntia
lC
urre
nt
Charg
e
E2
E1t f
t r
42
Q
2
Double potential step Chronocoulometry
Electrochemical techniques – Step method
Forward
reverse
Qt<t =Qc + nFAGO +2nFADO
1/2CO
* t1/2
p 1/2
Qt>t =Qc +2nFADO
1/2CO
*
p 1/2t 1/2 + t -t( )
1/2- t1/2( )
43
Qads + Qc
Qc
If R
not
adsorbed!
For adsorption:Qf vs. t
sec1/2
)(
.
r
t
Q
vsQ
Get Qads by subtraction.
Double potential step Chronocoulometry
Electrochemical techniques – Step method
Thank you !
Questions????