Proton exchange membrane fuel cells (PEMFCs) constitute a highly promising pathway for decarbonising heavy-duty transport, a sector distinguished by strongly fluctuating load profiles and a substantial demand for on-board energy storage. Beyond their globally essential role in enabling emission-free propulsion, PEMFCs combine high energy density with excellent efficiency and short refuelling times. Yet, the harsh operating conditions inherent to heavy-duty applications accelerate degradation and ageing processes [1], thereby constraining the broader deployment of this technology. Consequently, the development of operating strategies optimised for long service life is imperative. The extent of degradation within individual fuel cell layers is strongly dependent on operating conditions, as varying load profiles trigger distinct ageing mechanisms. To elucidate the influence of operating parameters on PEMFC materials, an accelerated stress test (AST) with a single-cell setup was conducted, in which the PEMFC materials were exposed to pronounced transient load changes over several hundred hours. Degradation was continuously monitored throughout the AST using a suite of electrochemical characterisation techniques, such as cyclic voltammetry or electrochemical impedance spectroscopy. Following the test, microscopic investigations employing transmission electron microscopy (TEM) were performed to assess ageing phenomena and identify degradation pathways across the different cell layers. The integration of experimental findings with microscopic analyses enabled the validation of existing degradation models for simulations, revealing a strong concordance. In summary, microscopy demonstrated changes in catalyst particle size during operation that corresponded to a reduction in electrochemically active surface area observed in the electrochemical measurements. These trends were consistently captured by ageing models, which predicted shifts in the catalyst particle size distribution. In addition, the initial formation of a platinum band within the membrane was examined using TEM, which was faithfully reproduced in the simulations. Taken together, the close agreement between simulations, electrochemical data, and microscopic analyses substantiates the robustness of the degradation models. REFERENCES [1] M. Cong, K. Wang, N. Yao, and T. Ma, “Study on the degradation of proton exchange membrane fuel cell under load cycling conditions,” International Journal of Hydrogen En