Observing the Structural Evolution of Quasi-Monolayer Pt Shell on Pd Core in the Electrocatalytic Oxygen-Reduction Reaction.

The use of a quasi-monolayer Pt shell (Pt qms ) on a Pd core (Pd c ) can reach cost and activity targets for the electrocatalytic oxygen-reduction reaction (ORR). The structure of Pd c Pt qms in the ORR will vary; however, direct observation of this issue is scarce. Here, during cyclic staircase voltammetry (ranging from 0.5 to 1.15 V RHE ) in 0.1 M O 2 -saturated HClO 4 , the structure of Pd c Pt qms was monitored by in situ X-ray absorption spectroscopy. The qualitative and quantitative structural information clearly exhibits a complete picture that Pt qms will directly restructure to form Pt clusters and holes, while Pd c almost remains stable. These findings identify the initial structural evolution of Pd c Pt qms in the ORR, highlighting the importance of protecting Pd c in the development of high-performance Pd c Pt qms electrocatalysts.