Comparative Investigation on the Activity Degradation Mechanism of Pt/C and PtCo/C Electrocatalysts in PEMFCs during the Accelerate Degradation Process Characterized by an in Situ X‑ray Absorption Fine Structure

The activity degradation mechanism of Pt/C and PtCo/C electrocatalysts in proton-exchange membrane fuel cells during the accelerated degradation test (ADT) was directly investigated by the in situ X-ray absorption fine structure (XAFS). Compared to Pt/C, the PtCo/C electrocatalyst possesses higher Pt 5d-orbital vacancy, low level of oxygen species chemisorption on the surface Pt atoms and shorter Pt–Pt bond length (R), and consequently an enhanced oxygen reduction reaction (ORR) activity. During the ADT process, a drastic drop in ORR activity after 30 000 ADT cycles can be observed in both the electrocatalysts in spite of their different activity degradation mechanisms. For Pt/C, XAFS results revealed that the oxidation of Pt during the ADT process induced the structure transformation from the partial-coverage surface PtO layer to the full-coverage surface and partial-coverage subsurface PtO layer, resulting in the loss of the Pt active site and consequently the decrease in the ORR activity. For PtCo/C, the oxidation of the Co atom induced the attenuation of strain and ligand effects, and the dissolution and segregation of CoO caused the increase in Pt outer layer thickness during the ADT process, mainly determined the decrease in ORR activity.