Enhancement of Oxygen Reduction Reaction Activity of Pd Core-Pt Shell Structured Catalyst on a Potential Cycling Accelerated Durability Test

Carbon-supported Pd core-Pt shell structured catalyst (Pt/Pd/C) was synthesized by a modified Cu under potential deposition/Pt displacement method without any precise potential control. The specific activity for oxygen reduction reaction (ORR) of the Pt/Pd/C catalyst increased by 5-fold after an accelerated durability test (ADT; rectangular wave potential cycling of 0.6 V (3 s)–1.0 V (3 s) performed in Ar-saturated 0.1 M HClO4 at 80 °C for 10,000 cycles), which enhanced ORR mass activity of the catalyst by 1.4-fold although its electrochemical surface area significantly decreased. TEM observation showed that the morphology of the catalyst particles changed into a spherical shape and the mean diameter decreased after the ADT. TEM-EDX compositional analysis revealed that the Pd core preferentially dissolved out and that the Pt shell was rearranged and thickened with the ADT. CV measurement using Bi3+ probe implied that the Pt(111) facet was partially formed on the Pt shell after the ADT. Furthermore, EXAFS analysis showed that the Pt–Pt bond distance of the Pt shell was shortened after the ADT. It was considered that the drastic enhancement in the ORR-specific activity of the catalyst after the ADT arises from the decrease in number of lowly coordinated surface Pt atoms, partially formed Pt(111) facet on the catalyst surface, and properly induced compressive strain in the Pt shell via the rearrangement of the Pt shell assisted by the preferential dissolution of the Pd core during the ADT.