108 Towards in-situ XPS measurements
liquid electrolyte flow complicates the removal of the Y oxide or solvated Y ions
from the surface. The presence of these oxidized species was already found to
reduce the ORR performance of alloys of Pt and late transition metals 77, 177.
This suggests that, as in the case of Pt3Co 3, 82, a proper acid leaching procedure
should be optimized and performed prior to use of these particles in a fuel cell.
This would prevent the accumulation of oxidized Y that could possibly poison the
catalytic active sites or the membrane. Figure 6.4 also shows the presence of a
small Si contamination; this was occasionally observed on some particular spots
and found to increase during electrochemical measurements. This is expected to
be a contamination from either the carbon substrate or the carbon/nafion mixture.
After ECfrontside: vacuum
Y oxidizedY metallic (27%)
frontside: 1 torr O2
in 10 torr O2
Y metallic (20%)
Normalized intensity (a. u.)
Binding energy / eV
Figure 6.5: Comparison of the Y 3d XPS region after electrochemical cycling in 5 Torr
H2O pressure (top) and after electrochemical cycling reaching higher potentials in 10 Torr
H2O pressure (bottom). A further increase of the oxidized component is observed. The
percentages of metallic Y with respect to the total Y, and the pressure conditions of anode
and cathode side during acquisition of the XPS signals are indicated.
Figure 6.5 provides a further evidence of dealloying over a second spot of the