Advances in Performance Degradation Mechanisms and Improvement Measures of Perfluorosulfonic Acid Ionomers in Catalyst Layers of Proton Exchange Membrane Fuel Cells Under Moderate‐Temperature and Low Relative Humidity Operation: A Review

ABSTRACT Proton exchange membrane fuel cells (PEMFCs) demonstrate significant advantages when operating at moderate temperatures (>100°C), including optimized kinetics of the oxygen reduction reaction, simplified water‐heat management system, and enhanced resistance to CO poisoning. Given these advantages, PEMFCs demonstrate excellent applicability in heavy‐duty vehicles and are expected to significantly advance the commercialization of this technology. Ionomers, which serve as electrolytes, binders, and dispersants for the catalyst layer (CL), play a crucial role in PEMFCs. However, commercially available perfluorosulfonic acid ionomers (PFSAs) suffer from core issues such as decreased proton conductivity, increased oxygen transmission resistance, and accelerated chemical degradation at moderate temperature. Currently, significant research efforts have been invested in this field, leading to the development of numerous innovative material systems. However, the preparation of moderate temperature PFSAs remains an extremely challenging task. A comprehensive and in‐depth analysis of the performance failure mechanisms of PFSAs in CL and a summary of relevant mitigation measures are of critical importance for understanding and addressing these issues. Therefore, this review systematically summarizes the challenges faced by PFSAs at moderate temperatures and their underlying mechanisms, introduces relevant optimization strategies, and finally proposes constructive insights into the existing challenges and future development directions for moderate temperature PFSAs.