  Proton exchange membrane fuel cells hold promise as energy conversion devices
for hydrogen-based power generation and storage. However, the slow kinetics of
the oxygen reduction at the cathode imposes the need for highly active
catalysts, typically Pt or Pt-based, with a large available area. The scarcity
of Pt increases deployment and operational cost, driving the development of
novel highly-active material systems. As an alternative, Rh-doped PtNi
nanoparticle has been suggested as promising oxygen reduction catalyst, but the
3D distributions of constituent elements in the nanoparticles have remained
unclear, making it difficult to guide property optimization. Here, a
combination of advanced microscopy and microanalysis techniques is used to
study the Rh distribution in the PtNi nanoparticles, and Rh surface segregation
is revealed, even with an overall Rh content below 2 at. %. Our findings
suggest that doping and surface chemistry must be carefully investigated to
establish a clear link with catalytic activity can truly be established.
