Humidity‐Induced Degradation Mapping of Pt ₃ Co ORR Catalyst for PEFC by In‐Operando Electrochemistry and Ex Situ SAXS

Abstract Understanding the degradation mechanisms of Pt‐alloy catalysts is crucial for enhancing their durability. This study investigates the impact of relative humidity on Pt and Pt ₃ Co catalysts using potential‐cycling‐based accelerated stress tests. Two conditions are investigated: 100% relative humidity on both sides, and a gradient with 30% at the anode and over 100% at the cathode. Pt ₃ Co demonstrates sensitivity, with 77% performance loss and reductions in electrocatalyst surface area. Results demonstrate a 30% decrease in potential loss for Pt catalysts and a 77% increase for Pt ₃ Co catalysts, indicating significant performance degradation in high humidity conditions, with Pt ₃ Co exhibiting greater sensitivity. Measurements of electrochemically active surface area reinforce these findings. Resistance analysis using electrochemical impedance spectroscopy using equivalent circuit modeling reveals a threefold increase in Pt ₃ Co MEAs' cathode charge transfer resistance and mass transport resistance during accelerated stress tests. Local current distribution analysis highlights differences between Pt catalyst and Pt ₃ Co, with the latter displaying dealloying effects. Small‐angle X‐ray scattering reveals changes in particle and cluster sizes, indicating structural changes. Scanning electron microscopy highlights catalyst and membrane thickness variations, suggesting heterogeneity in Pt ₃ Co. Under humidity gradients, Ostwald ripening plays a significant role in altering the catalyst's Pt ₃ Co structure and subsequently impacting its performance.