Identical-Location Transmission Electron Microscopy Study of Pt/C and Pt-Co/C Nanostructured Electrocatalyst Aging: Effects of Morphological and Compositional Changes on the Oxygen Reduction Reaction Activity

In this study, the activity for the oxygen reduction reaction (ORR) and the structural, morphological, and compositional changes of Pt, Pt3Co, and PtCo nanoparticles deposited on high surface area carbon (Vulcan XC72) are investigated after they were submitted to accelerated electrochemical aging tests. These tests consisted of stepping the potential in 1 min successively between potentials of 0.9 and 0.1 V vs. reversible hydrogen electrode (RHE) or between 0.9 and 0.6 V vs. RHE for 15 h in 1.0 M H2SO4 at 60 A degrees C. Transmission electron microscopy, identical-location transmission electron microscopy, X-ray energy-dispersive spectroscopy analyses, and in situ X-ray absorption spectroscopy are used to characterize the changes in the morphological, compositional, and in the Pt 5d electronic states before and after the electrochemical aging tests. The experimental results show that the Pt/C and Pt-Co/C electrocatalysts are modified upon aging, following changes in particle size, geometry, and composition. The 0.9- to 0.6-V protocol is more aggressive in reducing the ORR activity, and this seems to be strongly related to the carbon corrosion and not to changes in the metallic particles. After the 0.9- to 0.1-V aging procedure, the ORR activity of the Pt/C particles is improved, while that of Pt3Co/C particles only slightly changes. In the case of Pt/C, these effects are related to the increase of the particle sizes, surface reconstruction, and, therefore, smaller oxide formation, which potentially induces an increase of the ORR activity. On the contrary, on Pt3Co/C, these positive effects are counterbalanced by a detrimental (and large) effect of Co dissolution.