Hollow bimetallic nanostructures are perfect systems to unravel the ageing mechanisms of both Pt-based alloys and highly defective nanostructures used in proton-exchange membrane fuel cells (PEMFC) cathodes, since the mobility of their surface and bulk atoms leads to detectable chemical (i.e. Ni dissolution) and physical (i.e. decrease of the density of structural defects, collapse of the nanostructure, etc.) changes. In this study, we precisely and dynamically monitored these physicochemical changes on porous hollow PtNi/C nanoparticles during an ageing procedure composed of 5,000 potential cycles with linear ramp between 0.6 and 1.0 or 1.1 V vs. RHE by using (i) synchrotron operando wide and small angle X-ray scattering (WAXS and SAXS), (ii) scanning transmission electron microscopy (STEM) in combination with X-ray energy dispersive spectroscopy (X-EDS) and (iii) electrochemical measurements. The synchrotron operando WAXS and SAXS results dynamically correlated the structural changes of the hollow NPs both at the atomic (Ni depletion, lattice parameter relaxation, variation in the density of structural defects, etc.) and nanometric (restructuring of the nanoparticles/collapse of the hollow nanostructure) level. They revealed that the collapse of the hollow nanostructure was always accompanied by a significant loss of the Ni content. The 0.6 – 1.1 V vs. RHE ageing protocol resulted in a more severe depreciation of the ORR associated with a larger restructuration of the nanoparticles than the 0.6 – 1.0 V vs. RHE ageing protocol, thus providing evidence that a critical potential exists for the stability of highly defective nanoalloys.