trend in activity. However, the most active alloys experience higher activity losses
during stability tests, suggesting that high levels of compression might not favour
the long-term stability of the Pt overlayers. This hypothesis is supported by Density
Functional Theory (DFT) calculations and by AR-XPS. A model for the quantitative
estimate of the Pt overlayer thickness from AR-XPS measurements indicates a
correlation between the thickening of the Pt overlayers and the activity losses, supporting
the concept that more compressed overlayers have lower physical stability.
The application of these materials in a fuel cell requires the fabrication in
nanoparticulate form. Through the combination of a gas aggregation technique and
a time-of-flight mass spectrometer size-selected Pt-Y nanoparticles are produced.
With a mass activity of 3.05 A mg1
Pt at 0.9 V vs. RHE, 9 nm Pt-Y nanoparticles are
among the most active ORR catalysts ever reported, although they lose 37 % of this
activity after stability test. Similar to the case of polycrystals, after immersion in
the acidic electrolyte and testing the active phase consists of a Pt shell surrounding
an alloyed core. Also in this case the compressed Pt-Pt distance explains the ORR
activity enhancement of these catalysts.
The deposition of these 9 nm Pt-Y nanoparticles on the cathode side of a Membrane
Electrode Assembly (MEA), part of a specifically prepared fuel cell, allows
AP-XPS measurements under operation conditions. As a consequence of potential
cycling, Y oxidizes due to the dealloying process which is observed in-situ. The adsorbed
species can be also probed and correlated to the electrochemical potential.
Near the open circuit potential (OCP) conditions the oxygenated species consist, to
a good extent, of non-hydrated OH, similar to the case of pure Pt nanoparticles.