Highly Enhanced Oxygen Reduction Reaction Activity and Electrochemical Stability of Pt/Ir(111) Bimetallic Surfaces

Abstract

We demonstrate highly enhanced ORR activity and electrochemical stability of Pt/Ir(111) model core-shell catalysts prepared by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns for the surfaces show that Pt grew epitaxially on the clean Ir(111) substrate and the corresponding scanning tunneling microscope images collected in UHV reveal atomically flat terraces with 50–80nm widths at a substrate temperature of 673K. In contrast, the corresponding surfaces prepared at a substrate temperature of 303K show island-like topmost surface structures. The two-monolayer (ML)-thick Pt grown on Ir(111) (Pt2ML/Ir(111)) surfaces, prepared at substrate temperatures of 303K and 673K, show ca. 6 and 24 times higher ORR activities than clean Pt(111), respectively. The anomalous activity enhancement for the latter surface prepared at 673K is probably caused by homogeneous surface strain acting on the Pt shells that is derived from the 2.2% lattice mismatch between the Pt and Ir. The preparation-temperature–dependent ORR activity suggests that the activity can be dominated by the topmost surface and interface structures of the Pt shell–Ir(111) bimetallic system. Furthermore, while the initial ORR activity of pristine surfaces decreases with increasing Pt shell thickness, the stability during room temperature potential cycling between 0.6 and 1.0V in a 0.1M HClO4 solution was greatly enhanced above three ML thickness; the Pt4ML/Ir(111) surface prepared at 673K retained 6.5 times higher ORR activity than Pt(111), even after 5000 potential cycles. The ORR activity and electrochemical stabilities for the Pt/Ir(111) bimetallic surfaces are the highest among the MBE-prepared Pt/M(111) (M=Ir, Pd, Au) systems reported to date. The results obtained in this study show that Pt/Ir core–shell nanostructures are potential candidates for highly active and durable ORR catalysts.