Mechanistic Insights into Cathode Degradation During Startup‐Shutdown of PEM Water Electrolysis and Mitigation via Semi‐Embedded Pt/CeO _x

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for green H₂ production, yet cathode catalyst degradation during startup-shutdown cycles remains an important but overlooked challenge. Herein, we evaluate the durability of electrolyzers employing commercial Pt/C cathode catalysts under mimicking realistic startup-shutdown conditions, revealing severe performance degradation. By monitoring cathode potential evolution, we reveal that H₂ depletion during shutdown elevates the cathode potential to ∼1.0 V versus RHE. Post-mortem characterization identifies Pt agglomeration and carbon support corrosion as primary degradation mechanisms. To address these issues, we develop a Pt/CeO_x catalyst with a semi-embedded Pt structure. This unique architecture induces strong metal-support interaction between Pt and CeO_x, which enhances oxidation resistance and suppresses particle coalescence. Pt/CeO_x catalyst exhibits a H₂ evolution reaction overpotential of 27 mV at 100 mA cm^-2 and exceptional durability in a three-electrode configuration. In electrolyzer testing, at an ultralow loading of 0.05 mg_Pt cm^-2 (PGM loading of 0.25 mg cm^-2, half of the DOE 2026 target), the catalyst demonstrates a minimal degradation rate of 8.3 µV h^-1 over 600 h under aggressive startup-shutdown cycling, outperforming Pt/C (117 µV h^-1) by more than one order of magnitude. This work elucidates a previously overlooked cathode degradation pathway, validates the underlying mechanisms, and provides a practical catalyst design strategy for durable PEMWE.