Electrocatalysis and Catalyst Screening from Density Functional Theory Calculations - Fuel Cell Catalysis: A Surface Science Approach - page 70

Figure 3.9 The relative activity of Pt and Pt skins on Pt 3 X alloys as a function of the adsorp-
tion energy of O .

particles do not have the same shape, this could be part of the reason. This explanation
is supported by experiments on Pt 3 Ni single-crystal (111) surfaces that show an
activity exactly where the model predicts it to be (Fig. 3.9). We note that we
cannot, in general, expect the model to predict experiments this well, and the perfect
agreement is most likely fortuitous. Future single-crystal experiments on some of the
other alloys could give a more complete picture of how good the correlation is between
the model and the experiments.
Applying the Tafel equation with U ORR

Max , we obtain the polarization curves for Pt
and Pt 3 Ni (Fig. 3.10). The experimental polarization curves fall off at the transport
limiting current; since the model only deals with the surface catalysis, this part of
the polarization curve is not included in the theoretical curves. Looking at the low
current limit, the model actually predicts the relative activity semiquantitatively.
We call it semiquantitative since the absolute value for the prefactor on Pt is really
a fitting parameter.

3.4.4 Polarization Curves for OH / Water Adsorption
The potential-determining structure for the ORR on Pt is OH . It is therefore interest-
ing to compare, in more detail, models of OH adlayer structures with experimental
results. Experiments of the OH coverage as function of the potential on Pt(111) and
Pt 3 Ni(111) are found in [Stamenkovic et al., 2007a]. Our model translates a free
energy to a potential, which means that the coverage as function of the binding
energy—or vice versa—has to be found.