Effect of palladium core size on the activity and durability of Pt-Monolayer electrocatalysts for oxygen reduction reaction

Pd-based core-shell catalysts coated with a Pt monolayer (ML) are promising catalysts in polymer electrolyte membrane fuel cells. However, the effect of Pd core-size on the performance of these electrocatalysts remains underexplored. Therefore, this study systematically investigated the effect of Pd core-size on the electrochemical activity and durability of Pt ML electrocatalysts. We synthesized Pd@Pt/C catalysts using a CO-assisted reduction method, and Pd core sizes were precisely controlled at 3.8 nm and 4.9 nm by adjusting the pH levels. Subsequently, a Pt ML was deposited through copper underpotential deposition, yielding conformal Pt layers on the Pd cores. Pd@Pt/C with the smaller Pd core (PdS@Pt/C) demonstrated superior initial oxygen reduction reaction activity. However, its electrochemical active surface area (ECSA) and mass activity (MA) (41.1 % and 48.5 %, respectively) substantially decreased after an accelerated stress test (AST) with 50 k cycles. This can be attributed to Pt-shell thickening and Pd leaching. In contrast, Pd@Pt/C with the larger Pd core (PdL@Pt/C) demonstrated superior durability, with minimal ECSA (33.8 %) and MA (25.6 %) losses and stable Pt-shell thickness. Based on the ab-initio approaches regarding oxygen adsorption energy and Pt dissolution, the activity and durability are enhanced as (i) the overall particle size increases, (ii) Pd core size increases, and (iii) the number of Pt layers on the Pd surface decreases. These findings highlight the pivotal role of the core size in optimizing the performance of core-shell catalysts, wherein larger cores enhance durability of the shell material by mitigating core leaching and maintaining shell integrity.