  Surface strain plays a key role in enhancing the activity of Pt-alloy
nanoparticle oxygen reduction catalysts. However, the details of strain effects
in real fuel cell catalysts are not well-understood, in part due to a lack of
strain characterization techniques that are suitable for complex supported
nanoparticle catalysts. This work investigates these effects using strain
mapping with nanobeam electron diffraction and a continuum elastic model of
strain in simple core-shell particles. We find that surface strain is relaxed
both by lattice defects at the core-shell interface and by relaxation across
particle shells caused by Poisson expansion in the spherical geometry. The
continuum elastic model finds that in the absence of lattice dislocations,
geometric relaxation results in a surface strain that scales with the average
composition of the particle, regardless of the shell thickness. We investigate
the impact of these strain effects on catalytic activity for a series of Pt-Co
catalysts treated to vary their shell thickness and core-shell lattice
mismatch. For catalysts with the thinnest shells, the activity is consistent
with an Arrhenius dependence on the surface strain expected for coherent strain
in dislocation-free particles, while catalysts with thicker shells showed
greater activity losses indicating strain relaxation caused by dislocations as
well.
