In Situ Electrochemical Cell Enabling Multimodal X-ray Analyses for Tracking Pt/C Degradation in PEMFC Cathodes under AST Conditions

An accelerated stress test (AST) is widely used to shorten the time required to assess the long-term durability of fuel-cell catalysts; however, the degradation mechanism of Pt/C under AST conditions remains poorly understood. In this study, we introduce a three-electrode in situ electrochemical cell that combines electrochemical control with multimodal X-ray measurements to clarify Pt/C cathode degradation during AST. Using a commercial Pt/C model catalyst, we conduct in situ X-ray absorption spectroscopy, X-ray diffraction, and small-angle X-ray scattering under identical conditions to monitor electronic, crystallographic, and particle-size evolution. Aggregation occurs predominantly within the first ∼500 cycles, while coalescence and Ostwald ripening are minimal. The subsequent slowdown in structural evolution is consistent with progressive Pt oxidation and passivation, which suppresses further growth. Periodic cleaning during AST removes surface oxides and leads to more pronounced degradation. These findings highlight the importance of precision AST designs that explicitly consider early-stage aggregation and later-stage passivation while maintaining a sufficiently reducing environment to ensure meaningful acceleration.