Chemistry Reference
In-Depth Information
2.3
Cyclic voltammetry
In cyclic voltammetry, the potential applied to the working electrode is
varied linearly (Fig. 2.1) between potentials
E
1
and
E
2
,
E
2
being a potential
more positive (for oxidation) or negative (for reduction) than the peak
maximum observed for the oxidation/reduction reaction concerned. At
E
2
,
the voltage scan is reversed back to
E
1
or to another end potential value,
E
3
.The application of this type of potential ramp can be done in a number
of ways, varying the starting potential
E
1
, the reverse potential
E
2
, the end
potential
E
3
and the scan rate. The latter is the rate that is applied to vary
the potential as a function of time, commonly represented in V s
-1
or
mV s
-1
.
The resulting current measured while scanning the potential from
E
1
to
E
2
and back to the initial potential,
E
1
, is shown in Fig. 2.2 for a reversible
redoxsystem as a function of time, and in Fig. 2.3 in the more common way.
Note that the scan rate (in this case 10 mV s
-1
) is the main relation between
the way of data presentation in Fig. 2.2 and Fig. 2.3.
The voltammogram shown in Fig. 2.3 is characterised by a peak potential
E
p
,a potential corresponding to the point where the measured current
reaches it maximum value
I
p
.For a reversible system, the peak current is
given by:
12
()
[2.21]
I
=
0 4463
.
n
FD
A
a
c
p
where
I
p
is the peak current (A),
n
is the number of electrons exchanged
in the concerned reaction, F is the Faraday constant (Q mol
-1
),
A
is the
E
2
E
3
E
1
Time (s)
-
E
2
2.1
Variation of the applied potential in cyclic voltammetry. Typical tri-
angle wave between two potentials followed by an opposite trian-
gle (.........) or by another type of potential ramp (______).
