Ab Initio Atomistic Thermodynamics for Fuel Cell Catalysis - Fuel Cell Catalysis: A Surface Science Approach - page 151

dependence of G to be rather small, and the contributions from configurational and
vibrational entropy to G will be mostly compensated by the g bulk

Pt term on the
right-hand side of (5.28). Therefore, the DFT energies, which mimic T ¼ 0, can be
used instead.
This finally leads to the (T, a, Df) phase diagram shown in Fig. 5.10 (Plate 5.1).
The plot in Fig. 5.10a shows the modified interfacial free energy g 00 of different adsor-
bate overlayers as a function of the water chemical potential Dm H 2 O and the electrode
potential. Since the most stable structures have the lowest interfacial free energy,
Fig. 5.10b shows the view to the bottom of the phase diagram. Each shaded area cor-
responds to a different stable adsorbate structure. Since, according to (5.2) and (5.3),
Dm H 2 O can be related to temperature and activity, for a water activity a H 2 O ¼ 1 (ideal
solution), we have added the corresponding temperature scale to Fig. 5.10b and
marked T ¼ 298K by a horizontal dashed line. This line crosses the Df¼ 0 line (ver-
tical dashed line) in the area, which corresponds to the clean Pt(111) surface with no
oxygen on the surface. Keeping these a H 2 O and T conditions and increasing the elec-
trode potential shows that in the range from 0.95 to 1.20 V, a (2 2) oxygen overlayer
structure with 0.25 monolayer (ML) coverage is dominant. In the case that the elec-
trode potential exceeds 1.20 V, our calculations predict the formation of Pt bulk
oxide. As discussed before and shown in Fig. 5.9, the experiments by Jerkiewicz
et al. [2004] found the surface oxidation to occur from 0.85 to 1.1 V, followed by
oxide formation above 1.1 V.
The agreement between our phase diagram and the experimental CV curve is
surprisingly good, considering the simplicity of our interface model and the fact
that polycrystalline Pt has been used experimentally. Furthermore, for our studies
we have used Pt(111), whereas the experiments were performed with polycrystalline
Pt. One of the reasons for the agreement might be the absence of specific ion adsorp-
tion on the electrode. Moreover, there might be a kinetic barrier to oxide formation,

Figure 5.10 (T, a, Df) phase diagram for the electrochemical oxidation of Pt(111) in an
aqueous electrolyte. (a) Modified interfacial free energy g 00 as a function of Dm H 2 O and the
electrode potential Df. (b) View to the bottom of the phase diagram. In addition, the temperature
scale corresponding to a H 2 O ¼ 1(p ¼ 1atm) is given on the right side of the phase diagram.