DISSERTATION

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Results and Discussion
In order to find the capacitance minimum in the Cd-E curve, potentiodynamic electrochemical
impedance spectroscopy (PDEIS) was used. This technique consists of a set of EIS
measurements performed for a desired range of potentials. Initially, EIS was performed for all
solutions to identify the suitable frequency range for the determination of the pzc. This was
done to verify whether the system can be fitted to the RC equivalent circuit and that there are
no artifacts in the frequency range of interest. Very high frequencies were excluded since a high
frequency region of the impedance spectra is very sensitive to the cell geometry 79 . To be
specific, the solution resistance is smaller for the electrode edge than for the center of the
electrode, leading to a mixed behavior of the system which cannot be fitted with a simple RC
circuit. In order to minimize this behavior, a setup that assures homogeneous distribution of
current lines was constructed, where the counter electrode completely surrounds the working
electrode and the reference electrode is placed below them (Chapter 5.4, Figure 5.4). Moreover,
very low frequencies were also avoided and with this any possible faradaic side-reactions.
Furthermore, a capacitive bridge (shunt capacitor 80 ) was implemented and a Pb/PbF2 reference
electrode was used that shows high impedance 81 in the measuring setup to eliminate possible
instrumental artifacts due to very low ionic strengths and high frequencies. After these
optimization steps Nyquist plots with the characteristic behavior for a system with the solution
resistance and the double layer capacitance connected in series (Figure 3.13) were obtained. In
an ideal case, a Nyquist plot of this system should consist of a vertical line with the intercepting
the real axis at the solution resistance. However, as shown in Figure 3.13 a frequency dispersion
which is common for polycrystalline electrodes is observed. There are various theories
discussing the reasons for this phenomena, among which are the roughness of polycrystalline
electrodes, that creates inhomogeneities in the current density along the surface, or adsorption
effects 77,82 .
After performing PDEIS for all solutions in the chosen frequency range and the potential range
from -0.2 to 0.7 V (vs. Ag/AgCl/3 M KCl), the determination of Cd was done by extracting the
imaginary impedance component from the Nyquist plots for each sampled potential value and
calculating Cd values using the following equations for the RC equivalent electric circuit:
Z = R −
j
ωC d
(3.3)
3.2 Importance of knowing the surface 44