Revealing Ionomer Loss From the Anode Catalyst Layer as a Critical Degradation Origin in Water Electrolysis

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for green hydrogen production, yet practical deployment is limited by insufficient durability. Herein, we identify ionomer loss as a critical physical degradation pathway in PEMWE. We demonstrate that ionomer loss under practical aqueous operating conditions induces severe structural deterioration of the anode catalyst layer by weakening its binding function, thereby causing electrode cracking and local delamination. When not promptly removed, the lost ionomers, bearing strongly acidic sulfonic side groups, can alter the local chemical environment and thereby promote corrosion and dissolution of catalysts and metallic components, leading to metal‐cation release and subsequent membrane electrode assembly (MEA) poisoning. These coupled effects severely hinder mass transport and cause pronounced performance degradation. Moreover, under start‐stop cycling, intensified chemical degradation of the catalyst synergistically accelerates ionomer loss, resulting in a 32.6% increase in ionomer loss and a 59.1% increase in performance degradation rate. In PEMWE stack systems, ionomer loss further accelerates corrosion of balance‐of‐plant components, exacerbating MEA poisoning and durability loss. Overall, this work identifies ionomer loss as a critical physical degradation pathway in PEMWE and underscores the necessity of mechanically robust catalyst layers and corrosion‐resistant interfaces to achieve long‐term stable operation.