Electrochemical Electron Transfer: From Marcus Theory to Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 40

The strength of the chemical interaction with the metal, can be expressed by the
quantity D, which decays roughly exponentially with distance; typically, the decay
lengths are of the order of 1 ˚ [Koper and van Santen, 1999]. Combining the results
of quantum chemical calculation with the potential of mean force, the potential energy
surface of the ion as a function of the separation and the solvent coordinate can be con-
structed at the potential of zero charge (PZC) [Schmickler, 1995]—for other electrode
potentials, one would need the distribution of the electric potential in the double layer,
which is not well known. For the case of the adsorption of an iodide ion, such a plot is
shown in Fig. 2.7. At the PZC, adsorption of the ion is favored owing to a strong
chemical interaction, but to get from the bulk towards the surface, the ion has to over-
come a barrier, whose height is mainly determined by the partial desolvation. In the
adsorbed state, the ion is partially discharged, so the minimum is situated near
q ¼ 20.5. Far from the electrode, the surface shows a high lying valley centered
near q ¼ 0 corresponding to the uncharged atom. Obviously, at the PZC, the atom
is in an energetically unfavorable state.
Similar surfaces have been constructed for other reactions, but this example suffices
to illustrate the important point that ion transfer, just like electron transfer, is governed
by the interplay of the interactions with the solvent and the electrode.
Figure 2.7 Potential energy surface for the adsorption of an iodide ion on Pt(111)
[Schmickler, 1995] as a function of the solvent coordinate q and the distance x from the
electrode.