ORR Properties of Tensile-Strained Pt/Zr/Pt(111) Bimetallic Surfaces Prepared By Arc-Plasma Depositions of Pt and Zr on Pt (111)

Abstract

Introduction Pt-based alloys with group 9 – 11 elements M (M = Fe, Co, Ni, etc.) are widely studied as cathode catalyst materials for the polymer electrolyte fuel cell (PEFC), because such the Pt-M alloys improves the oxygen reduction reaction (ORR) activity per unit area (area-specific activity) with reduction of Pt usage. However, the drawback of the Pt-M catalysts is easy elution of the M under the operating conditions of PEFC, i.e., low pH and severe potential fluctuations. Hurken et al. [1] investigated the electrochemical stability of Zr in acidic environments and showed that the formation of passive layers of Zr stabilizes the Zr surface. Therefore, in this study, ORR activity and structural stability are investigated for the Pt/Zr/Pt(111) bimetallic surfaces prepared through alternating depositions of Zr and Pt on the Pt(111) substrate by an arc-plasma deposition (APD) method. Experimental The bimetallic surface synthesis procedure is described elsewhere [2]. After the surface cleaning of the Pt (111) substrate in ultra-high vacuum (UHV; ~10-8 Pa), the alternating APDs of Zr and Pt were carried out on the surface cleaned Pt(111) to synthesize Pt/Zr/Pt(111) bimetallic surfaces; the deposition sequences of Pt and Zr are schematically shown in Fig. 1(a). The total thicknesses of Pt and Zr and the top Pt layer thickness were fixed to be 6.0 and 1.6 nm respectively. The second-layer Zr thicknesses were changed from 0.4 to 3.2 nm. The Pt/Zr/Pt(111) bimetallic surfaces thus synthesized were designated hereafter by the second-layer Zr thicknesses (m nm) as “m-nm-Zr”. After the surface structural analysis, e.g., cross-sectional STEM, XPS and in-plane XRD, the Pt/Zr/Pt(111) bimetallic surfaces were transported to an electrochemical three-electrode-cell without air exposure to avoid surface contamination and oxidation. Then, CV and LSV measurements were conducted in N2–purged and O2–saturated 0.1 M HClO4. The ORR activity was evaluated from jk values at 0.9 V vs. RHE by using Koutecky–Levich equation. The surface electrochemical stability was discussed on the basis of ORR activity changes during applying the 5000 potential cycles (PCs) of 0.6-1.0 V vs. RHE in O2–saturated solution at a room temperature. Results and Discussion In-plane XRD results revealed that tensile strain less than 0.8 % relative to Pt(111) worked on the Pt(111)-shell layers for the synthesized Pt/Zr/Pt(111) bimetallic surfaces. Cross-sectional STEM image of the 3.2 nm-Zr is shown in Fig. 1(b). The atomically-resolved image reveals that the Pt(111)-shell layer is epitaxially grown on the Pt-Zr(111) alloy layers. One might notice that a twin boundary (black dashed line), where the stacking sequence of (111) planes is changed from ABC to CBA, is located at 10-stacking layers of (111) from the top surface. Therefore, estimated tensile strains of the Pt/Zr/Pt(111) bimetallic surfaces (less than 0.8 %) are probably caused by such the twin boundary generation that might release the surface strains of the Pt(111)-shell. CVs of the synthesized Pt/Zr/Pt(111) bimetallic surfaces (Fig. 1(c)) show decrease in hydrogen adsorption charges (QH; around 0.05 to 0.35 V) as well as lower shifts of onset potentials of the hydrogen adsorption for all the surfaces, though the features are insensitive to the underlying Zr deposition thicknesses. Furthermore, positive-shifts of the onset potentials due to hydroxyl-related (OH) species adsorption (ca. 0.6 V) can be seen on the CVs, while “so-called” butterfly peaks for each bimetallic surfaces shifted only slightly relative to that of the clean Pt(111) (0.8 V). The ORR activity trends during applying the PCs are summarized in Fig. 1(d). The pristine activities are ca. 3 to 5 fold higher than the clean Pt (111): the 0.8nm-Zr sample surface shows the maximum enhancement factor of 5. Furthermore, except for the changes in very early stage of the PCs (100 cycles), the activities are remained nearly unchanged by the PCs application, indicating high electrochemical stabilities of the Pt/Zr/Pt(111) bimetallic surfaces. The results obtained in this study suggest that alloys of Pt with 4 – 6 group early transition metal elements, e.g., Zr, should be considered for effective cathode ORR catalysts of PEFC. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan.