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

ABSTRACT Proton exchange membrane water electrolysis (PEMWE) is a promising technology for green H 2 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 H 2 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 H 2 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.