Humidity-Induced Degradation Mapping of Pt3Co ORR Catalyst for PEFC by In-Operando Electrochemistry and Ex Situ SAXS.

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 3Co 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 3Co 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 3Co catalysts, indicating significant performance degradation in high humidity conditions, with Pt 3Co 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 3Co MEAs' cathode charge transfer resistance and mass transport resistance during accelerated stress tests. Local current distribution analysis highlights differences between Pt catalyst and Pt 3Co, 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 3Co. Under humidity gradients, Ostwald ripening plays a significant role in altering the catalyst's Pt 3Co structure and subsequently impacting its performance.