Chemistry Reference
In-Depth Information
R
Z
W
C
9.4
Equivalent electrical circuit for the electrochemical cell described
in section 9.2.2 with palladium electrodes of 314 mm
2
, electrically
in contact with each other through an electrolyte solution over a
distance of 112 mm at
T
=
298.0 K.
solution; the capacitor represents the capacity of the electrode-electrolyte
interface. Finally, there is also the Warburg Impedance (
Z
W
) in series with
the resistance and in parallel with the capacitance. This element represents
the diffusion of Na
+
and Cl
-
ions in solution.
From Fig. 9.2, it can be seen that in the high-frequency range the imagi-
nary impedance and real impedance are small at the onset of the experi-
ment. By decreasing the frequency, the imaginary impedance first starts to
grow, reaches a maximum and then decreases again to zero. The real imped-
ance increases with decreasing frequency and ends up at a constant value.
These changes of real and imaginary impedance result in a typical semi-
circle corresponding to the equivalent electrical circuit shown in Fig. 9.4.
Despite the fact that the imaginary impedance is small at the onset of the
experiment, it is the main parameter because the phase angle shift shows a
value around 90° for an electrolyte concentration of 1 ¥ 10
-4
mol l
-1
, which is
typical for pure capacitive behaviour. This result indicates that the electri-
cal current going through the electrochemical cell is flowing exclusively
through the capacitive part. Indeed, this element behaves as a good con-
ductor at high frequencies because its charge and discharge rate is
extremely high.
At lower frequencies (range of 1 MHz-5 ¥ 10
4
Hz) its conductive behav-
iour decreases because of decreasing charge and discharge rate; therefore
a competition between the capacitive and the resistive element in the equiv-
alent electrical circuit starts to occur, and a fraction of the current flows
through the resistor element. This is in correlation with the increase of the
real impedance, but is also in correlation with an increase of the imaginary
impedance because the capacitor becomes less conductive with decreasing
frequency. In addition, curve 4 in Fig. 9.3.b shows that an increasing frac-
tion of the electrical current is flowing through the resistive element
because the phase angle shift is decreasing, indicating an increase in resis-
tive behaviour.
