Electrocatalysis and Catalyst Screening from Density Functional Theory Calculations - Fuel Cell Catalysis: A Surface Science Approach - page 63

TABLE 3.1 Calculated Adsorption Free Energies (in eV) for OH, O, and H on Ni, Pt,
and Au(111) Surfaces at Standard Conditions (T 5 298K, pH 5 0, U 5 0V)
OH þ e 2 þ H þ O þ 2(e 2 þ H þ ) H -(e 2 þ H þ )

Pt:
DE

1.05 1.57 20.33

DE w. water 0.46 1.54 20.33

DG w. water 0.81 1.59 20.09

Ni:
DE 0.13 0.34 20.51

DG w. water 20.11 0.36 20.27

Au:
DE

1.49 2.75 0.21

DG w. water 1.25 2.77 0.45

The binding energy determines the potential at which the surface is reduced by
binding hydrogen, the potential window where the surface is free of any adsorbates,
and the potential at which the surface starts being oxidized by OH or oxygen.
These potentials are material-dependent: gold is free of absorbates in a large potential
window, but nickel, on the other hand, goes directly from being reduced to oxidized,
and Pt is somewhere in between these two extremes. See Table 3.1 and Fig. 3.3.

Figure 3.3 Phase diagram showing the free energy for different surface structures for water at
pH ¼ 0 in contact with Au(111), Pt(111), and Ni(111). The figure is based on the free energy
values in Table 3.1. All free energies are shown relative to those of the clean surface with the
water bilayer.