Proton exchange membrane fuel cells (PEMFCs) are promising technology for the decarbonization of future heavy-duty transportation. However, their widespread deployment is hindered by durability challenges, particularly accelerated degradation under dynamic and high-load operating conditions. Multiple degradation mechanisms may occur within PEMFCs, necessitating the use of a range of electrochemical characterization techniques to identify the underlying processes. Among these, electrochemical impedance spectroscopy (EIS) is a powerful diagnostic tool that enables the separation and analysis of membrane resistance, activation losses, and mass transport phenomena. These impedance spectra and their associated frequencies can be interpreted using equivalent circuit models, in which various processes within the fuel cell are represented by discrete electrical components [1]. As degradation progresses, the values of these components are adjusted, allowing changes in specific physicochemical processes to be identified. In this study, EIS-based equivalent circuit modeling was applied to a PEMFC subjected to an accelerated stress test designed to emulate heavy-duty load cycling conditions. The most pronounced changes were observed in cathode activation processes. Analysis of the equivalent circuit parameters revealed that the greatest changes occurred in the capacitance associated with the cathode catalyst layer, while the corresponding ohmic resistance remained largely unchanged throughout the stress test. This decrease in capacitance is consistent with known degradation mechanisms such as catalyst particle agglomeration and dissolution. These findings were supported by additional electrochemical diagnostics and microscopic analyses. REFERENCES [1] L. Zhao, H. Dai, F. Pei, P. Ming, X. Wei, and J. Zhou, “A Comparative Study of Equivalent Circuit Models for Electro-Chemical Impedance Spectroscopy Analysis of Proton Exchange Membrane Fuel Cells,” Energies, vol. 15, no. 1, Jan. 2022, doi: 10.3390/en15010386.