Electrocatalysis and Catalyst Screening from Density Functional Theory Calculations - Fuel Cell Catalysis: A Surface Science Approach - page 78

simple strategy is effective. The scheme permits the estimation of the activity and stab-
ility of a large number of transition metal alloys with reasonable computational effort.

1. Determine the smallest possible set of catalytic descriptors (binding energies,
activation barriers, pre-exponential factors, etc.) that is needed to determine
trends in the catalytic activity of the reaction of interest. This determination
can be made, for example, by finding the smallest set of descriptors that corre-
lates well with empirical activity data. Alternatively, microkinetic models
[Cortright and Dumesic, 2001] or Sabatier analyses [Bligaard et al., 2004]
can be used to develop rate expressions as functions of selected groups of
descriptors. The latter approach is exactly the technique that was used to devel-
oped activity-based descriptions of the ORR earlier in this chapter.
2. Determine the values of the descriptors from step 1 that yield optimal catalytic
activity. This determination can, again, be made empirically, via microkinetic
modeling, or via Sabatier analysis.
3. Select a suitable pool of transition metal alloys and, using DFT techniques,
evaluate the values of the catalytic descriptors on these alloys. A more approxi-
mate, but still very useful, alternative is to estimate the descriptors by some form
of linear interpolation [Andersson et al., 2006; Jacobsen et al., 2001].
4. Using the results of steps 2 and 3, find the alloys with the highest predicted
activity.
5. Using results from the DFT calculations, combined with databases of segre-
gation energies, estimate the stability of the alloys in working reaction

environments.
6. Test the best candidates experimentally.

3.6.3 Electrochemical Activity Model for the HER

The development of our HER model uses many of the basic principles developed for
the ORR [Nørskov et al., 2004, 2005], and only a brief overview of the approach will
be repeated here. The focus is on predicting trends in the HER exchange current den-
sity. As in the case of the ORR, the free energy of surface intermediates is calculated,
including the effects of solvation (found to be negligible for hydrogen adsorption
energies) and electrode potential (we note, however, that such potential effects are
by definition absent for exchange current determinations). These free energies are
then used to evaluate rates from simple mechanistic models; below, we consider the
well-known Tafel - Volmer mechanism:

H þ þ e þ ! H
H ! 2 H 2 (g) þ

A single catalytic descriptor, the free energy of hydrogen adsorption DG H , turns out to
be sufficient to predict trends in the exchange current density. For very exothermic
hydrogen adsorption (DG H , 0), the coverage of adsorbed hydrogen will be high,