Insights into depth-dependent degradation of Pt/C fuel cell catalysts from operando X-ray scattering

Long-term durability remains a critical challenge for Proton Exchange Membrane Fuel Cells (PEMFCs). While degradation mechanisms have been identified, their quantitative contributions to performance loss have not been conclusively determined. Here, we systematically investigate the degradation of two Pt/C PEMFC catalysts with distinct average particle sizes using a newly developed operando cell that combines X-ray total scattering with Pair Distribution Function analysis, X-ray Diffraction, and Small-Angle X-ray Scattering (SAXS). By combining these techniques, particularly through a sophisticated Monte Carlo analysis of the SAXS data, depth- and time-resolved particle size distributions are obtained, enabling direct tracking of particle size evolution throughout the catalyst layer. Coupled with a physico-statistical model, this approach enables the identification and quantitative separation of system-specific degradation pathways, including particle agglomeration, coalescence, and Ostwald ripening. The results reveal pronounced depth-dependent degradation, strongest near the catalyst/microporous transport layer interface, and identify Ostwald ripening as the dominant degradation mechanism.