Correlating the electrocatalytic stability of platinum monolayer catalysts with their structural evolution in the oxygen reduction reaction

Platinum monolayer (Pt-ML) core-shell electrocatalysts for the oxygen reduction reaction (ORR) have attracted great attention because of their exceptional activity and stability for promising practical applications in fuel cells. Here, we describe our in-depth investigation of the relationship between the ORR activity and structure of the Pt-ML/Pd/C catalyst during the stability test. By virtue of the rotating disk electrode technique, an accelerated degradation test with the potential window of 0.65 to 1.05 V was applied to the Pt-ML/Pd/C to interrogate its long-term reliability in the ORR, the change of its electrochemical surface area, and its surface composition and components. The Pt-ML/Pd/C catalyst displayed a volcano-like mass/dollar activity profile in the stability test up to 100k cycles. The overall loss of the activity was recorded to be as low as 17% of the initial value. The ORR activity increased in the initial 20k cycles because the freshly prepared Pt-ML did not entirely encompass the whole Pd core, but it was integrated to a full coverage with a more stable configuration during the potential cycling owing to its self-healing property. Then, the activity decreased at a much slower rate than the standard Pt/C because the Pd-Pt core-shell structure due to its structural self-retaining property remained intact and impeded the electrochemical Ostwald ripening of the entire particles. Changes in the morphology and configuration of Pt-ML were mapped by combining our experimental investigation with model analyses. The proposed self-healing and self-retaining mechanisms account for the structure-dependent stability in the ORR and play cornerstone roles in formulating ORR core-shell electrocatalysts.