Oxygen Reduction Reaction Activity and Durability for Model Pt Shell Layers on Ir(111) Prepared by Molecular Beam Epitaxy

Abstract

Introduction The core-shell type nanostructure, that is composed of ultra-thin Pt coating on other transition metal nanoparticles have attracted much attention from the viewpoints of cathode electrode catalysts in polymer electrolyte fuel cell (PEFC). Compressive strain of the Pt-shells induced by lattice mismatch and direct electronic interaction between the Pt shell and core elements influences electronic properties of the Pt shell layers and, thereby, contributing to enhanced oxygen reduction reaction (ORR) activity [1]. However, because of operating conditions of PEFC, i.e., low pH, high positive potentials, ORR activity of the core-shell catalysts decline through dissolution of core elements and rearrangements of surface Pt atoms. Considering standard electrode potentials, iridium is stable among the transition metals and Pt-shell/Ir-core nanoparticles might be stable under the PEFC operating conditions. However, at present, relationship between Pt-Ir bimetallic surface structures and ORR properties has been less reported [2]. In this study, we prepared well-defined Pt/Ir(111) bimetallic surfaces by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV) and studied the ORR activity and durability. Experimental Ir(111) single crystal substrate was cleaned by Ar+ sputtering and annealing at 1273 K in UHV. Various-monolayers(ML)-thick Pt was deposited onto the clean Ir(111) substrate by an electron beam evaporation method at the substrate temperature of 673K. The resulting surface structures were verified by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) in UHV. The UHV-fabricated samples were transferred to electrochemical evaluation systems set in a N2-purged glove box without air exposure. CV and LSV measurements were conducted in N2-purged and O2-saturated 0.1 M HClO4, respectively. The ORR activity was evaluated from j k values at 0.9 V vs. RHE by using the Koutecky-Levich equation. EC degradation of the Pt/Ir(111) surface was evaluated by applying potential cycles between 0.6 V (3s) and 1.0 V (3s) vs. RHE in the O2-saturated solution at room temperature. Results and Discussion Fig. 1 (a) summarizes CV curves of the MBE-prepared nML-thick Pt/Ir(111) (n=1-4: Pt n ML/Ir(111)) and clean Pt(111). All CV curves of the Pt nML/Ir(111) show symmetrical peaks similar to the clean Pt(111) (so-called butterfly peak) in the region of OH adsorption and desorption (0.6-1.0 V), accompanied with remarkable decrease in hydrogen adsorption and desorption charges (0.05-0.35 V). The CV features are typical for highly-ORR-active Pt-based bimetallic surfaces reported to date, suggesting that the topmost surfaces comprise pure Pt(111) lattice. Fig. 1 (b) presents changes in j k during potential cycles loading between 0.6 and 1.0 V. Initial ORR activity enhancement factors for the 2ML, 3ML and 4ML-thick Pt shell layers can be estimated to be ×24, ×12 and ×8, respectively compared with the clean Pt(111). Because Ir lattice constant is smaller than Pt, the Ir(111) substrate should induce compressive surface strain on the Pt-shell. As a result, the ORR activity is enhanced. Furthermore, because compressive strain of the Pt shell layers should be relaxed with increasing the Pt shell layer thickness, the initial activity enhancement factors decreased with increasing the thickness. After the 5000 potential cycles loading, the enhancement factors for the Pt2ML/Ir(111) decreased to ×2 vs. clean Pt(111) indicating that compressive surface strain is easily released by the potential cycle loading. In contrast, particularly, the initial activity of the 4ML-Pt-shell remains nearly unchanged even after the 5000 cycles loading: the activity is the highest among the other well-defined Pt-M(111) bimetallic systems, e.g., Pt/Pd(111) [3], Pt-skin surfaces of Pt-Ni(111) [4] and Pt-Co(111) [5], reported in our laboratory. The results can be explained by 3D migration of the surface Pt atoms during the potential cycle loading and by release in compressive strain in Pt shell. Therefore, the results suggest that Pt-Ir bimetallic system can be expected to be applied to ORR catalysts for practical use. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan and Grant- in Aid for young scientists (B) from the Japan society for the promotion of science (N. T.).