Effects of Core-Shell Interface Structures on ORR Activities: a Model Catalyst Study of Pt/Pd(111)

Introduction

 Pd@Pt core-shell catalyst is one of the promising cathode electrode materials for polymer electrolyte fuel cell (PEFC) from the view-points of a great potential to reduce Pt usage and to improve the oxygen reduction reaction (ORR) activity [1]. Recently, Daimon found that the activity of the Pd@Pt core-shell catalyst was markedly enhanced with dissolution of the core Pd atoms during applying the specific potential cycles [2]. Their results clearly indicate that comprehensive understandings not only of the Pt-shell surface but also of the interface between Pt-shell and Pd-core are crucial for developing highly-active and durable Pd@Pt core-shell catalysts. We have previously reported that electrocatalytic properties for various-thick Pt monolayers on Pd(111) model catalysts prepared by molecular beam epitaxy (MBE) [3]. In this study, we focus on the relation between ORR activities and atomic structures of near surface vicinity for the MBE-prepared Pt/Pd(111) bimetallic system.

Experimental

 Sample fabrication processes of the Pt/Pd(111) were conducted in UHV. Pd(111) single crystal substrate was cleaned by Ar⁺　sputtering and subsequent annealing at 1073 K. 0.6 nm-thick Pt was deposited onto the cleaned Pd(111) by an electron-beam evaporation method at the substrate temperatures of 573 K and 673 K. The resulting surface structures were verified with reflection high-energy electron diffraction (RHEED), scanning tunneling microscope in UHV (UHV-STM), and low-energy ion scattering (LEIS). Then, the prepared Pt_{x nm}/Pd(111) surfaces were transferred without being exposed to air to the EC system set in an N₂-purged glove box. Cyclic voltammogram (CV) of the samples were recorded in N₂-purged 0.1M HClO₄, and, then, linear sweep voltammetry (LSV) was conducted by using a rotating electrode (RDE) method at 1600 rpm after saturating the solution with O₂. The ORR activities were estimated by kinetic-controlled current density (j_k) at 0.9V vs. RHE by using Koutecky-Levich equation.

Results and Discussion

 Surface structures for the 573 K- and 673 K-fabricated Pt_{0.6 nm}/Pd(111) are summarized in Figs.1 (a) and (b). The RHEED patterns (insets) and UHV-STM images clearly show that the Pt grow epitaxially on Pd(111) substrate (RHEED) and topmost surfaces are comprised of atomically flat terraces with ca. 50 nm width (UHV-STM), irrespective of the deposition temperatures. In contrast, the interface structures of the samples evaluated by LEIS (Fig. 1(c)) are clearly different: Pt compositions decays more rapidly with supporting time for the 573 K-prepared samples in comparison to that for the 673 K-sample. The results suggest that the interface between topmost Pt(111) epi-layers and substrate Pd(111) is sharper for the 573 K-prepared sample.

 Fig.1 (d) shows the CV curves for the 573 K- and 673 K-Pt_0.6nm/Pd(111) (red and blue lines) and clean Pt(111) (black line). For both the Pt_{0.6 nm}/Pd(111) surfaces, the hydrogen adsorption and desorption behaviors are almost same. In contrast, for the latter 673 K-surface, so-called butterfly peaks at 0.8 V that stem from OH adsorption and desorption shift to higher potentials by ca. 30 mV relative to clean Pt(111), although the positive shift of the former 573 K-surface is not clean. Fig.1 (e) presents the evaluated ORR activities for the 573 K- and 673 K-prepared samples, showing 6.2 and 3.6 fold higher activities than that of clean Pt(111). The results reveal that a steep interface between the Pt-shell and Pd-core is an essential for highly active Pd@Pt core-shell catalysts.

Acknowledgement

This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.        

References

[1] R. Adzic, et al., Top. Catal., 46, 249 (2007).

[2] H. Daimon et al., 226^thECS Meeting, Abstract #1063, Cancun, Mexico (2014).

[3] Y. Bando, et al., J. Electrochem. Soc., 162, F463 (2015).

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