  We report a rapid solution-phase strategy to synthesize alloyed PtNi
nanoparticles which demonstrate outstanding functionality for the oxygen
reduction reaction (ORR). This one-pot co-reduction colloidal synthesis results
in a monodisperse population of single-crystal nanoparticles of rhombic
dodecahedral morphology, with Pt enriched edges and compositions close to
Pt1Ni2. We use nanoscale 3D compositional analysis to reveal for the first time
that oleylamine (OAm)-aging of the rhombic dodecahedral Pt1Ni2 particles
results in Ni leaching from surface facets, producing aged particles with
concave faceting, an exceptionally high surface area and a composition of
Pt2Ni1. We show that the modified atomic nanostructures catalytically
outperform the original PtNi rhombic dodecahedral particles by more than 2-fold
and also yield improved cycling durability. Their functionality for the ORR far
exceeds commercially available Pt/C nanoparticle electrocatalysts, both in
terms of mass-specific activities (up to a 25-fold increase) and intrinsic
area-specific activities (up to a 27-fold increase).
