3.2 Electrochemical characterization 41
activity enhancements for Pt5La and Pt5Ce are 3.4 and 3.7. Alloys of Pt and late
transition metals prepared under similar conditions (Pt3Fe, Pt3Co and Pt3Ni) exhibited
a considerably lower activity, with about a factor of 2 enhancement relative
to polycrystalline Pt 37, 73. It is important to specify that the measured activity
of polycrystalline Pt is in agreement with the values recently proposed by reputable
groups under the same conditions 44, 75, 116.
3.2.1 Stability test
Once stable CVs were obtained and the ORR activity was measured, accelerated
stability tests were conducted in order to test the long-term durability of the samples.
10 000 consecutive cycles between 0.6 and 1.0 V vs. RHE in an O2-saturated
0.1 M HClO4 electrolyte at 100 mVs1 were chosen as a standard stability test
(see Section 2.5.1 for further details). Kinetic currents of Pt5La and Pt5Ce were
measured before and after cycling; in both cases they experienced remarkably low
losses: around 8% for Pt5La and 10% for Pt5Ce. This is a good evidence of the
long-term electrochemical stability of these alloys. Figure 3.3 shows the ORR activity
of Pt5La and Pt5Ce: the activity of pure polycrystalline Pt and of Pt3Y is also
plotted for comparison. Both activities before and after stability test are reported.
Interestingly, after 10 000 cycles both alloys maintain a specific activity which is
more than 3 times higher than for pure Pt.
Although a complete stability test should be carried out on large surface area
catalysts and possibly in a fuel cell, the results on polycrystalline Pt5La and Pt5Ce
are a good indication of the resistance to de-alloying, which is considered as the
main cause of degradation for alloys of Pt and late transition metals 76. Recently,
Todoroki et al. studied the stability of Pt-enriched Ni/Pt(111) catalysts whose initial
activity is 8 times higher than for pure Pt(111). After a stability test of only
1 000 cycles between 0.6 and 1.0 V vs. RHE in analogous conditions, they measured
activity losses close to 75% 48. State-of-the-art Pt-Co ORR catalysts also
exhibited a substantial degradation under fuel-cell operating conditions, primarily
due to de-alloying 76, 77. The high electrochemical stability of Pt5La and Pt5Ce
suggests them as promising materials for the cathode catalysts of PEMFCs. This
motivates the interest towards the fabrication of large surface area catalysts based
on these alloys and towards more extended stability tests under the technologically
relevant conditions of a fuel cell. However, it should be anticipated that the
chemical (or electrochemical) synthesis of Pt-La or Pt-Ce alloyed nanoparticles will
be particularly challenging due to the high reactivity of the lanthanides and to the
difficulties of reducing them to a metallic state.