DISSERTATION

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Results and Discussion
According to the GC model of the double layer 43 , Cd consists of two contributions (Figure 3.12,
a), namely the capacitance of the compact layer (Ci) and the capacitance of the diffuse double
layer (Cddl):
1
= 1 + 1
(3.2)
C d C i C ddl
Figure 3.12. Schematic representation of a) potential profile at the electrode surface, and
b) expected behavior of Cd in dependence on the applied potential and the ionic strength
according to the GCS theory. Φ represents the potential distribution and d distance from
the electrode surface. Figures adapted from 43 .
This means that Cd is determined by the lower value of these two components. In theory, Ci
does not depend on the ionic strength nor on the potential. On the other hand, Cddl depends on
both of these values under certain conditions. When the applied potential is far away from the
pzc, Cd is determined by Ci regardless on the ionic strength. However, when the potential
approaches the pzc (Figure 3.12, b), Cddl becomes significant at high ionic strength and the term
1/Cddl in Equation (3.2) becomes negligible. In that case, Cd is again determined by Ci and only
a constant capacitance of Ci is observed. However, at very low ionic strength Cddl is small and
the term 1/Cddl becomes significant as compared with 1/Ci. Cd is then determined by Cddl, which
is represented as a minimum in the Cd-E curve. It should be noted, that this model is based on
certain assumptions like the independence of Ci on the ionic strength and the potential, that
result in some discrepancies in real systems. In reality, a Cd-E curve does not necessarily follow
the predicted ideal U-shape.
3.2 Importance of knowing the surface 43