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