We aim to elucidate the relationship between oxidation state and stability of electrocatalyst for automotive proton exchange membrane fuel cells (PEMFCs) by operando and in situ characterization. The structural evolution of Pt nanoparticles will be tracked in a representative device under accelerated protocols able to capture the effect of low potential excursions, which occur in the application during the stops and were recently recognized as critical for Pt nanoparticles growth and dissolution. The mechanisms (oxidation/reduction, morphological reconstruction, dissolution) rising under the cathodic scan to low potential (<0.4 V) will be elucidated by W/SAXS techniques. The study of different catalyst coated membranes and operating PEMFC conditions can clarify the electrochemical drivers that lead the degradation in this process. The work helps to improve aging mitigation strategies and Pt degradation model, and define PEMFC operational boundaries in dependence on material properties.