122 Conclusions and perspectives
non-hydrated species, in particular non-hydrated OH, similar to what was reported
for pure Pt nanoparticles.
The observed trends in activity and stability will need to be confirmed by a few
more experiments on polycrystalline alloys. In all cases they already indicate various
promising materials for application at the cathode of PEMFCs. The studies on
polycrystalline alloys did not really allow to investigate the effect of composition on
the catalytic performance of these alloys. However, we anticipate that it will probably
be hard to overcome the stability of some ordered intermetallic phases and
these will put strict requirements in terms of alloy stoichiometry. The comparison
with polycrystalline alloys of Pt and late transition metals might elucidate the role
of more negative heats of alloy formation on stability, since this was the original
reason motivating our search of new materials for catalyzing the ORR. However,
definitive conclusions will probably require a stability comparison between similarly
At the current level of knowledge, a better understanding of the catalytic properties
of these new materials will most probably require the study of single crystals.
Studies that were already initiated in the case of the Pt-Y system 101 (Paper 4
in the end of this thesis). Single crystals of these intermetallic compounds would
allow, not only to achieve a more precise characterization, but most importantly to
select single ordered facets and distinguish their behaviour in terms of activity and
stability. Some of them might exhibit outstanding activities, similar or higher than
that of Pt3Ni(111) 39. In this perspective the realization of Pt-skin structures and
their stabilization in acidic electrolytes will probably be the main challenge. The
completion of the EC-XPS setup will help to characterize these single crystal alloys
and to investigate their electronic structures by XPS and UPS.
Similar challenges will also concern the Pt-Y (or Pt-rare earth) nanoparticles.
Also in this case the improvement of their catalytic activity and stability will probably
require an enhanced level of ordering. On the basis of the progresses achieved
on large surface area catalysts of Pt and late transition metals, the optimization
of acid leaching and annealing procedures is expected to stabilize more ordered
structures, with advantages both in terms of activity and stability. Tuning the morphology
of the particles in order to select particular facets and active sites of these
ordered structures might bring further advances.
Finally, the biggest challenge will probably be the development of an economically
viable and scalable process for the production of nanoparticles based on
these new alloys. Various synthesis methods are currently under investigation. The
production of thin films in perspective of an application onto nanostructured substrates
might represent another promising alternative towards the fabrication of
large surface area catalysts.