  The thousandfold increase in data-collection speed enabled by
aberration-corrected optics allows us to overcome an electron microscopy
paradox - how to obtain atomic-resolution chemical structure in individual
nanoparticles, yet record a statistically significant sample from an
inhomogeneous population. This allowed us to map hundreds of Pt-Co
nanoparticles to show atomic-scale elemental distributions across different
stages of the catalyst aging in a proton-exchange-membrane fuel cell, and
relate Pt-shell thickness to treatment, particle size, surface orientation, and
ordering.
