4.3 Trends in activity 63
Pt5Ln alloys are plotted as a function of the lattice parameter a, and on the top
axis, as a function of the nearest neighbour Pt-Pt distance (i.e. a=2) in the bulk
of the alloys. The activity of polycrystalline Pt is also plotted for comparison by
considering on the top axis the typical Pt-Pt distance in pure Pt. Interestingly,
a volcano shape relationship appears. The activities of Pt5Ca and Pt5Sr are also
inserted in the plot since they both exhibit the Cu5Ca type structure. Notably, their
position fits very well into the same trend.
jk / mA cm-2
Figure 4.3: Experimental ORR activity from Figure 4.2 of various Pt5Ln and Pt-alkaline
earths alloys vs. the lattice parameter a of the alloys as determined from XRD measurements.
On the top axis the nearest neighbour Pt-Pt distance in the bulk, which is assumed
to define the Pt-Pt distance in the Pt overlayer, is also indicated: dP tP t = a=2. The activity
of polycrystalline Pt, referred to its typical Pt-Pt lattice distance (top axis), is also reported.
Measurements and figure made by María Escudero-Escribano.
The appearance of a clear volcano trend is explained in terms of strain effects.
Both DFT calculations 54 and experimental results 31, 126, 132 agree
on the concept that compressive strain should weaken the binding energy of Pt
to oxygenated species such as O, OH or OOH. This weakening should result in a
higher ORR activity 16, 17, 19. In agreement with this concept, the reduction of
the lattice parameter a in Figure 4.3, or alternatively of the derived Pt-Pt nearest
neighbour distance, results in a higher ORR activity. This would explain the right
side of the volcano with activities that increase constantly until Pt5Tb. A similar re