Porous hollow PtNi/C nanoparticles (NPs), an highly active electrocatalyst for the oxygen reduction reaction (ORR)¹, were successfully synthesized on different carbon supports. The latter plays an essential role in the NPs synthesis, providing a way to control their morphology and Ni content. The physico-chemical properties of the different carbon supports were crucial during accelerated stress tests simulating start-stop events at a PEMFC cathode (i.e. step potentials between 1.0 and 1.5 V vs. RHE) and were discussed by the means of identical-location transmission electron microscopy (IL-TEM) and electrochemistry (see. Figure 1). It was observed that electrocatalysts supported on graphene nanosheets and carbon xerogel suffer from NPs detachment and carbon complete corrosion while CO_surf forms on carbon blacks supports (i.e. incomplete oxidation of the surface). The degradation of the carbon support resulted in mass activity and specific activity losses, as the result of the increase of the resistance of the catalytic layers (ΔR_k).