Environmental Engineering Reference
In-Depth Information
Using the current in amperes and the scan rate in volts per second, the differential
capacitance is obtained in farads. Assuming that the capacitance surface density is con-
stant, the capacitance gives a comparative idea of the extension of the electroactive area
(Figure 2.15).
EIS uses a small potential amplitude assuring that the measurement is made on a linear
system [130]. A typical Nyquist plot of a porous carbon (PC-CTAB) electrode is shown in
Figure 2.16.
The response at high frequencies shows a semicircle while the response at low frequen-
cies shows a line nearly parallel to the
y
-axis, suggesting that in this region the system
behaves as a simple double-layer capacitance. The small deviation at low frequency shows
100
50
0
-50
-100
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Electrode potential (V vs. Ag/AgCl)
FIGURE 2.15
Cyclic voltammogram of a porous carbon (PC-PDAMAC) in 1 M H
2
SO
4
. Scan rate = 50 mV/s.
12
ω
min
= 3 mHz
10
8
6
Semicircle
4
2
ω
max
= 50 kHz
0
0
2
4
6
8
10
12
Real impedance (ohms)
FIGURE 2.16
Impedance dependence with frequency. Nyquist plot of a porous carbon between 3 mHz and 50 kHz at 0.275
V
SCE
in 1.0 M H
2
SO
4
.
