58 Experimental Methods
3.3.1 Impedance Spectroscopy
In order to facilitate comparison between experimental groups and setups it is
important to correct for the Ohmic losses. Ohmic losses come from resistance
in wires, connections, sample and the electrolyte. In fact, the ionic conductivity
of the electrolyte is a dominant factor and it is therefore an advantage to keep
a similar distance between working and counter electrodes for a series of experiments.
The magnitude of the Ohmic losses can be evaluated by impedance
spectroscopy, where the impedance is measured as a function of AC current
frequency. A simple equivalent circuit model of an electrochemical interface is
comprised of a resistor and a constant phase element in parallel. The capacitative
contribution to the impedance vanishes at high frequencies (the imaginary
component is approximately zero) and the resistance can be determined as the
real component of the impedance. An example of such a measurement can be
seen in gure 3.11.
ImpedanceforMnO2onAupc
Nyquistplot
~100Hz
Fit
Measured
Ohmicdropinsetup
020406080100
60
40
20
0
~10000Hz~1Hz
-Im(Z)/ W
Re(Z)/ W
Figure 3.11: Nyquist plot of an impedance measurement for MnO2 on a gold
polycrystalline disk. The Ohmic drop in the experimental setup is found as the high
frequency intercept with the 1st axis. A range of 200 kHz to 1 Hz was measured.
In practice, the Ohmic losses can be accounted for before or after measuring.
The measured potential is easily corrected using Ohms law.
U
= j R (3.7)
Where U
is the potential change due to Ohmic losses, j the measured
current (not normalised) and R the resistance. It is possible to correct for 85 %
of the Ohmic losses using a feature in the potentiostat. It is important to keep in
mind that at high currents the remaining 15 % can actually result in rather large
discrepancies between the nominally applied potential and the actual applied