52 Pt-La and Pt-Ce alloys: the active chemical phase
of strong corrosion of this alloy.
jk / mA cm-2
U / V vs. RHE
Figure 3.9: Tafel plot (a) representing the kinetic current densities of Pt3La, Pt5La and
polycrystalline Pt as a function of potential. Corresponding ICP-MS analysis (b) of the
amount of dissolved La and Pt from Pt3La and Pt5La electrodes in 0.1 M HClO4 electrolytes.
The same measurement was performed on a clean 0.1 M HClO4 electrolyte and produced
much lower signals of La and Ce (negligible on the chosen scale). Measurements performed
by María Escudero-Escribano and Arnau Verdaguer-Casadevall.
Contrary to these results, Kim and co-workers 99 observed a considerable
enhancement of specific activity relative to pure Pt on their Pt3La thin films: as well
as high stability during potential cycling. Possible reasons for the corrosion of our
Pt3La polycrystal and a comparison with Pt5La, together with a broader discussion
about the stability of these surfaces, will be assessed in the following section.
As for the other alloys, analogous XPS characterization was performed on
Pt3La. For the close similarity of the spectra with those of Pt5La (see Figure 3.4)
the results are described briefly. The fitted spectra of Pt3La as-prepared (i.e. during
sputter-cleaning), air-exposed and after ORR testing are presented in Figure 3.10.
Similar to Pt5La the La 3d XPS peaks of the sputtered sample are completely metallic
and very different from those measured by Kim and co-workers for thin films
of the same stoichiometry (i.e. Pt3La) 99. After air-exposure the level of oxidation
was also similar to the one of Pt5La, demonstrating the same tendency of
La atoms towards oxidation. After ORR testing small La 3d signals could still be
measured and appeared to be completely metallic, indicating that La2O3 dissolved
in the electrolyte. For all spectra the energy position and relative intensity of the
fn components was similar to Pt5La (see table of Appendix A). Due to the fast corrosion
no long-term stability tests were conducted on Pt3La. At the same time, no
AR-XPS depth profiles will be presented for Pt3La due to the increased roughness
that could easily affect such measurements.