Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 295

a large potential loss for the ORR. The activity-and-barrier plots illustrate why the
RDS may vary with reaction conditions, or may not even exist. They also hint at
the important role that the elementary steps (9.3) and (9.4) play directly in the ORR.
9.3.4 Palladium Alloy Electrocatalysts
As already mentioned, considerable ongoing research is focused on replacing Pt
altogether without sacrificing ORR activity. Recent reports [Fernandez et al.,
2005a, b; Lee et al., 2006; Mustain et al., 2007; Raghuveer et al., 2005; Savadogo
et al., 2004; Shao et al., 2006a, c, 2007b; Tarasevich et al., 2007; Wang W et al.,
2007; Zhang L et al., 2007] have indicated that binary Pd-M alloys (M ¼ Cr, Co,
Fe, Ni, Ti) and ternary ones were comparable to, or slightly better than commercial
Pt catalysts. These Pd-M catalysts have attracted considerable attention because they
are comparatively inexpensive and have very high methanol tolerance, making them
promising cathode catalytic materials for hydrogen PEMFCs and especially direct
methanol fuel cells (DMFCs).
Theoretical calculations and experimental data demonstrated that, upon annealing
at elevated temperatures, the Pd-M alloys, like the Pt-M alloys, undergo Pd segre-
gation, in which Pd atoms migrate to the surface to form a pure Pd skin on the bulk
alloys [Bozzolo et al., 2003; Rousset et al., 1996; Ruban et al., 1999]. The Pd lattice
also contracts upon alloying with a 3d metal, which generates compressive strains in
the Pd skin. A good correlation was found between specific activity and the Pd - Pd
interatomic distance expressed as a function of Fe content (Fig. 9.21). The highest
ORR activity was obtained for Pd 3 Fe, which has a Pd - Pd bond distance of 0.273 nm,
smaller than that in Pd by 2.3%. Similar results were obtained for Pd - Co alloys
[Suo et al., 2007; Wang W et al., 2007].
DFT calculations show that, like Pt 3 Co alloy surfaces, the electronic structure of the
Pd skin is modified by strain and the underlying alloy, which in turn modifies the
Figure 9.21 Specific activity and Pd - Pd bond distance calculated from XRD data against the
concentration of Fe in Pd-Fe/C electrocatalysts treated at 500 8C. (Reproduced with permission
from Shao et al. [2006c].)