Impedance-based analysis of PEMFC degradation phenomena during accelerated stress tests

Degradation of the cathode catalyst layer (CCL) limits the durability of polymer electrolyte membrane fuel cells (PEMFCs) by reducing the electrochemically active surface area and impairing oxygen transport. These co-occurring effects are difficult to disentangle with standard electrochemical diagnostics. In this study, we used impedance-based analysis to quantify the individual contributions of catalyst and carbon support degradation in PEMFCs subjected to accelerated stress tests (ASTs): low-potential cycling (0.6–0.95 V, 55 000 cycles) and high-potential cycling (1.0–1.5 V, 50 000 cycles). In-operando electrochemical impedance spectroscopy under H 2 /air and impedance data analysis using the distribution of relaxation times and transmission line modeling were combined with complementary diagnostic techniques. This approach separated the ohmic, charge transfer, CCL ionomer, and mass transport resistances and tracked their evolution during ASTs. Low-potential cycling increased the charge transfer resistance by 29–56%, consistent with a loss of active surface area. High-potential cycling resulted in increased charge transfer, mass transport, and ohmic resistances, with a 77% reduction in CCL thickness, indicating severe carbon corrosion and collapse of the CCL structure. The resulting framework provides a practical tool to screen cathode materials and operating strategies by quantitatively linking specific degradation modes to electrochemical loss processes. • In-operando EIS with DRT separates kinetic, ionomer, and mass transport losses. • TLM quantifies charge transfer, ionomer and mass transport resistances during ASTs. • Low-potential cycling primarily raises kinetic losses consistent with ECSA loss. • High-potential cycling raises kinetic/transport losses due to structural degradation. • Ex-situ diagnostics validate impedance-derived resistance evolution and mechanisms.