Ab Initio Atomistic Thermodynamics for Fuel Cell Catalysis - Fuel Cell Catalysis: A Surface Science Approach - page 146

Figure 5.8 Calculated g 0 versus electrode potential Df SCE (referenced to an SCE) curves for
Au(100) in 0.01 M HClO 4 . The crossing between the curves indicates the electrode potential at
which the surface reconstruction is lifted [Au(100)-hex ! Au(100)-(1 1)].
E z (Df). This relation can be used to plot g 0 (E z ) from Fig. 5.7 as a function of the elec-
trode potential, g 0 [E z (Df)], for different electrolytes and concentrations, depending
on which experimental capacity measurements have been used for the integration.
Since these measurements were performed with an SCE, we have added a correspond-
ing subscript to the electrode potential.
Figure 5.8 shows the resulting curves for 0.01 M HClO 4 , which now exhibits a
crossing at around 0.6 V, in good agreement with experiment. Since the reconstructed
Au(100)-hex surface is stable below Df SCE 0.6 V, the crossing of the curves indi-
cates the lifting of surface reconstruction. Since only electronic effects and no specific
ion adsorption have been considered in these calculations, from the agreement of the
calculated transition potential around 0.6 V with the experimentally measured value of
0.55 V, one might conclude that the lifting of the Au(100)-hex surface reconstruction
is exclusively caused by the surface charging. However, experimentally measured
cyclic voltammetry curves were required as external input, which might reduce the
predictive ability of the calculations. As the occurrence of any specific ion adsorption
should influence the experimental cyclic voltammetry curves, from these kinds of
studies, specific adsorption cannot be excluded. Instead, the results provide evidence
that surface charging alone plays an important role in destabilizing the hexagonal
reconstructed Au(100) surface at positive potentials.
The previous example of lifting the Au(100)-hex surface reconstruction showed
that, even without specific adsorption of electrolyte ions, the electronic effects induced
by the electrode potential might cause significant changes to the surface morphology
of the electrode. Although the potential-induced changes to the surface free energy are
only about 0.005 eV/ ˚ 2 , this is comparable to differences in the surface stabilities of
clean metal surfaces. This might be of particular relevance for electrocatalytic