Environmental Engineering Reference
In-Depth Information
reactions, since the surface morphology strongly influences reaction energetics and
even entire reaction mechanisms.
In this example, we have focused on the surface excess charge term in (5.18) and
(5.19); the next example will show that the potential is able to modify not only the
electrode structure, but also its composition.
5.3.2 Electro-Oxidation of Pt Electrodes
Experimentally, cyclic voltammetry (CV) is one of the commonly used techniques to
study the processes occurring at electrode surfaces as function of the electrode poten-
tial. With respect to the growth of oxides on metal electrodes by electro-oxidation, it
allows one to distinguish between pronounced potential regions, which can be
assigned to initial interface charging, followed by surface oxidation, and finally
oxide formation at higher positive electrode potentials [Clavilier et al., 1996;
Vielstich et al., 2003; Jerkiewicz et al., 2004; Climent et al., 2006]. Although different
experimental techniques, such as CV, electrochemical quartz crystal nanobalance
(EQCN), and Auger electron spectroscopy (AES) have been employed in order to
obtain a better understanding of these structural changes, even for the “standard”
system of a Pt electrode in an aqueous solution (e.g., H
2
SO
4
), there is still an ongoing
debate regarding the geometry and composition on the atomic scale.
Figure 5.9 schematically shows the process of electro-oxidation of a polycrystalline
Pt electrode with increasing electrode potential. Focusing on the part of the cyclic
voltammogram that is marked by a dashed box, there is (almost) no current density
at low positive electrode potentials, which indicates the absence of any charge transfer
reaction at the electrode. Instead, the increasing electrode potential lowers the elec-
tronic Fermi level of the electrode, leading to an accumulation of positive excess
charge at the electrode surface, accompanied by a structural reorganization within
the electrolyte. As a consequence, the water molecules close to the electrode surface
orient their dipoles according to the excess surface charge. This scenario is comparable
to what we have already discussed in the previous example, where we have seen that
surface excess charge densities around 0.02 e/
˚
2
might cause significant structural
changes at transition metal electrodes.
Further increase of the potential causes stronger interaction between the water
dipoles and the electrode, finally leading to the onset of surface oxidation. Since
this process involves charge transfer, in this potential range a finite current density
can be observed (see Fig. 5.9). Whether this is caused by adsorption of OH
-
or O
2-
is still under debate. While the presence of OH
-
is commonly accepted
[Angerstein-Kozlowska et al., 1973; Dickinson et al., 1975], from recent EQCN
and CV measurements Jerkiewicz et al. [2004] calculated a molecular weight of the
adsorbing species of 15.8 g/mol and concluded the presence of atomic oxygen
only. As a consequence of the surface oxidation, the electrolyte now sees a different
(shielded ) electrode surface, causing changes in the structure of the electrolyte.
In this potential region, it is mainly the term N
n
m
n
in (5.18) and (5.19) (with n ¼
H
2
O) that contributes to the stability, because water is the source for oxygen adsorbed
on the surface.
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