Degradation Mechanism of the Membrane Electrode Assembly Catalyst Layer in Proton Exchange Membrane Fuel Cells under High-Temperature Accelerated Stress

High-temperature (>100 °C) operation of proton exchange membrane fuel cells simplifies water and thermal management, accelerates electrode kinetics, and improves impurity tolerance. However, the degradation mechanism of the membrane electrode assembly (MEA) under such conditions remains unclear. Here, a high-temperature-resistant MEA based on a phosphoric–sulfonic acid copolymer membrane is investigated through an accelerated stress test at 105 °C for 50 h. The initiation and propagation of crack networks within the cathode catalyst layer are identified as a critical bottleneck in performance degradation. Mass transport loss is the most pronounced degradation mode. This loss arises from crack formation, ionomer degradation, and carbon corrosion. The MEA exhibits a voltage decay of 35.23% at 2 A cm–2 and an electrochemical surface area loss of 62.9%. Chemically, cracks initiate from the transformation of disordered carbon into ordered carbon in the carbon support (ID/IG decreasing from 1.1268 to 0.4735) and the detachment of sulfonic acid side chains from the ionomer (I1060/I1210 decreasing from 0.483 to 0.133). Physically, Ostwald ripening of platinum particles (mean size increasing from 3.48 to 4.29 nm) and stress concentration induced by temperature and humidity cycling drive crack growth. Finite-element simulations using a cohesive zone model reveal that cracks propagate steadily along the ionomer/catalyst interface─the path of least energy dissipation. This study establishes a direct link from microstructural evolution to macroscopic performance degradation. Crack network formation is highlighted as a key bottleneck. These findings provide guidance for material design and lifetime prediction of high-temperature-resistant MEAs.