layer.3 Essentially, the larger rare earth atoms deform the closely packed structure of Pt,
allowing a larger number of Pt atoms to fit onto the Pt overlayer than on bulk Pt.
In Fig. 5 we demonstrate that the activity of PtxY catalysts is strongly correlated to the Pt-Pt
distance in the particles, similar to dealloyed PtCux.20 An increase in compression brings about
an increased ORR activity, relative to Pt, via slightly weakened OH binding to the surface. On
the other hand, PtxY nanoparticles should be inherently more stable than PtCux and other Pt-late
transition metal alloys against continued dealloying under reaction conditions.3,8,11,49 Dealloying
via diffusion of Y through the bulk and the Pt overlayer will be hindered by the strong
interaction between Y and Pt.8 The moderate stability we report here could be improved even
further through optimisation of the catalyst composition and by suitable annealing.38 This will be
a subject of future studies.
In conclusion, the mass activity of the catalyst, at 3.05 AmgPt
-1 at 0.9 V is, within the range of
experimental error, as high as the best reported in the literature. By using mass selected
nanoparticles, not only do we provide a proof-of-concept that nanoparticles of Pt-Y alloys are
highly active, but we also shed scientific insight into the underlying mechanism of their
enhanced activity. It turns out that their superior performance is due to a compressive strain
exerted by the catalyst core onto the surface. Our work provides a strong impetus for the
implementation of PtxY in fuel cells. Nevertheless, the methods used here do not allow for mass
production. The next step is to develop the chemical synthesis of this catalyst in bulk quantities,
where the Y content may also be optimised.
I.C. and I.S. conceived the experiments. P.H-F. performed the electrochemical experiments.
F.M., D.N.M., C.E.S., P.M. and A.N. performed the UHV-based experiments. D.F., A.B. and
A.M.W performed the XAS measurements. D.D. performed the microscopy. P.H.F. designed the
figures. P.H-F. wrote the first draft of the paper. All authors discussed the results and commented
on the manuscript.