Atomic-Scale Compositional Mapping and 3-Dimensional Electron Microscopy of Dealloyed PtCo₃Catalyst Nanoparticles with Spongy Multi-Core/Shell Structures

Advanced electron microscopy techniques, i.e., high angle annular dark field (HAADF) imaging, elemental mapping based on electron energy loss spectroscopy (EELS), and three-dimensional (3D) reconstructions from electron tomography, have been used to characterize the complex spongy core-shell structures observed in a high-performing fuel cell catalyst, a dealloyed PtCo 3 catalyst supported on high surface carbon (or D-PtCo 3 /HSC). HAADF imaging showed that multiple dark-contrast patches were randomly decorated on the D-PtCo 3 nanoparticles, a type of image that had previously been reported for other catalysts and had been referred to as a spongy structure. In this work, EELS-based elemental mapping revealed that not all of the dark-contrast patches were simple holes (as had been reported for other catalysts). Rather, in this material, while some of the dark-contrast patches corresponded to voids or divots, others were Co-rich cores. The probability of a dark-contrast patch in an HAADF being a Co-rich core was statistically determined to be 58% ± 15%. 3D tomography further confirmed the existence of voids and divots in the multi-core/shell-structured D-PtCo 3 nanoparticles. Possible formation mechanisms of the spongy multi-core/shell structure are discussed.