Oxygen Reduction Reaction Activity for Surface-Magnetic-Anisotropy-Controlled Pt-Co/Pt(111) Bimetallic Surface

Abstract

Introduction Developments in effective oxygen reduction reaction (ORR) catalysts for the cathode of Polymer Electrolyte Fuel Cell is important to achieve “hydrogen society”. Because oxygen molecules (O2), the reactant of ORR, is paramagnetic having a triplet spin state, ORR that proceeds on the cathode catalyst surfaces might modify by magnetism of the catalyst materials. Indeed, ORR activity was improved by the influence of an external magnetic field applied to the catalyst [1]. The alloys of 3d transition magnetic elements M (M=Co, Fe, Ni) are known to exhibit ferromagnetism. Etz et al. reported that the direction of magnetization easy axis of the epitaxial Co/Pt(111) surface depends on the Co deposition thickness [2]. However, to our best knowledge, relation between the magnetic anisotropy of the cathode electrode surface and ORR activity is yet to be clarified. Therefore, to investigate influence of spontaneous magnetization of the material’s surface on the ORR should cast some light on the ORR activity enhancement mechanisms for such the Pt-M alloy surfaces with the magnetic. In this study, we fabricate Pt-Co/Pt(111) ferromagnetic bimetallic surfaces under ultra-high vacuum (UHV) through 1 monolayer(ML)-thick epitaxial growth of Co on the clean Pt(111) substrate at various temperatures and discuss the influence of magnetic anisotropy of the Pt-Co/Pt(111) bimetallic surface on corresponding ORR activity. Experimental The sample fabrication process of Pt-Co/Pt(111) bimetallic surface is as follows. After surface cleaning of the Pt(111) single crystal substrate through Ar+ sputtering and annealing at 1173 K in ultra-high vacuum (UHV: 10-8 Pa), 1ML-thick-Co was deposited on the clean Pt(111) by the molecular-beam epitaxy (MBE) method at elevated substrate temperature (723 K to 973 K) to generate the Pt(111)-skin through the surface segregation of substrate Pt atoms [3]. The Pt-Co/Pt(111) bimetallic surfaces are described hereafter by the Co deposition temperatures, for example Pt-Co773K/Pt(111), etc. The resulting surface structures were verified by the scanning tunneling microscopy (STM) and low-energy ion scattering spectroscopy (LE-ISS) in UHV. Then, polar magneto optical Kerr effect (polar MOKE), and in-plane XRD measurements are conducted in air to evaluate the surface magnetic anisotropies and compressive strains of the generated Pt(111)-skin surfaces. The MBE-prepared samples were transferred to electrochemical evaluation systems set in a N2–purged glove box without being exposed to air. Then, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were conducted in N2–purged and O2–saturated 0.1 M HClO4, respectively, in the glove box. ORR activities of the samples were evaluated based on j k values at 0.9 V vs. RHE by using Koutecky–Levich equation. Results and Discussion An LE-ISS spectrum of the Pt-Co773K/Pt(111) is shown in Fig. 1(a). Only the signal due to of the Pt atoms at the topmost surface is appeared on the spectrum, indicating that the Pt(111)-skin is generated for the Pt-Co773K/Pt(111) surface and the deposited Co should be located below the second atomic layer of the Pt-Co773K/Pt(111) as a result of surface segregation of the substrate Pt atoms at deposition temperature (of 773 K). Fig. 1(b) shows an STM image of the corresponding Pt-Co773K/Pt(111) surface. As shown in Fig. 1(b), the sample surface is atomically flat. One might notice that Moiré-pattern-like surface corrugations on the image (highlighted by white line), probably stemming from the lattice mismatch between Pt(111)-skin and underlying Co (Pt-Co alloy) [4]. The result also supports the Pt(111)-skin. Fig. 1(c) shows the polar MOKE curves of the Pt-Co773K/Pt(111) (red) and Pt-Co823K/Pt(111) (blue). Both the samples show the hysteresis loops of Kerr rotation angle vs. magnetic field H, indicating that perpendicular magnetic anisotropy (PMA) of the sample surfaces. Relation between the ORR activities and in-plane-XRD-estimated compressive strains of the Pt-Co773K/Pt(111) (red) and Pt-Co873K/Pt(111) (blue) surfaces are shown in Fig. 1(d). For comparison, the DFT-calculated ORR activities of the strained Pt(111) surfaces (green) are plotted in the figure [5]. The estimated ORR activities for both the Pt-Co773K/Pt(111) and Pt-Co873K/Pt(111) are 20 to 30 % higher than the DFT-calculated ones. The results suggest that PMA might affect ORR activity enhancements for the prepared PMA surfaces of the Pt-Co/Pt(111). At the meeting, effects of magnetic anisotropies (perpendicular and in-plane) are discussed more in detail.