Electrochemical Stability Improvements By Ir for Ultra-High Vacuum Prepared Pt/Pd(111) Bimetallic Surfaces

Abstract

Introduction Pt-shell-Pd-core nanoparticles are effective cathode catalysts 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]. However, because of operating conditions of PEFC, e.g., low pH, and high positive potentials with severe fluctuations, the ORR activity of the core-shell type catalysts tend to decline through dissolution of the core elements, accompanying re-arrangements of shell layer’s Pt atoms. Considering standard electrode potentials, Ir is a stable element in the fore-mentioned PEFC conditions among the late 9-11 group transition metals. Therefore, Ir can be applied for surface-stabilizing elements of the core-shell type ORR catalysts for the PEFC. Indeed, to control dissolution behavior of the less-noble element, Li et al. investigated influence of Ir for the Pt–Ni nanostructures and showed that the surface modifications of Pt-Ni alloys by Ir improved electrochemical stability [2]. However, relation between locations of Ir in the core-shell nano-structures and ORR properties (activity and durability) has yet to be investigated. For developing the practical Pt/Pd core-shell type catalysts, to clarify a role of a modifying element Ir for the ORR properties are required. Therefore, in this study, we prepared well-defined, Ir-modified Pt/Pd(111) bimetallic surfaces by molecular beam epitaxy (MBE) of Pt and Arc-plasma deposition (APD) of Ir in ultra-high vacuum (UHV; ~10-8 Pa) and studied Ir-modified ORR activity and stability of Pt/Pd(111) bimetallic surfaces. Experimental Two Ir-modified Pt/Pd(111) bimetallic surfaces, (1) interlayer-modified Pt/Ir/Pd(111) and (2) surface-modified Ir/Pt/Pd(111) are fabricated as follows. The Pt/Pd(111) bimetallic surface was prepared through e-beam deposition of 4 monolayer (ML)-thick Pt on the UHV-cleaned Pd(111) substrate surface. As for the interlayer-modified sample (1), 1 monolayer (ML)-thick Ir was deposited by APD on the UHV-cleaned Pd(111), followed by 4ML-thick e-beam deposition of Pt at 573K. The surface-Ir-modified Pt/Pd(111) (2), sub-ML-thick Ir was deposited on the Pt/Pd(111) at room temperature. The UHV-prepared samples (Pt/Pd(111), Pt/Ir/Pd(111), Ir/Pt/Pd(111)) were transferred to the electrochemical evaluation system set in a N2-purged glove box without air exposure to avoid sample oxidation and contamination [3]. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed in N2-purged and O2-saturated 0.1 M HClO4, respectively in the glove-box. The ORR activity was evaluated from kinetic-controlled current density (j k) values at 0.9 V vs. RHE by using Koutecky–Levich equation. The electrochemical stabilities of the samples were discussed based on change in the j k values during applying 5000 square-wave potential cycles (PCs) between 0.6 (3 s) and 1.0 V (3 s) in O2-saturated 0.1 M HClO4 at a room temperature. Results and Discussion Fig. 1 (a) summarizes CV curves of the Ir0.1ML/Pt/Pd(111)(blue), Pt/Ir1.0ML/Pd(111)(red) and the Pt/Pd(111)(black) [4] sample surfaces. As shown in Fig. 1(a), in comparison with the Pt/Pd(111), hydrogen adsorption and desorption charges (QH; 0.15-0.35V) for the both Ir-modified samples clearly decreased. Particularly, the hydrogen storage current for the Pd(111) substrate (0.15V marked *) remarkably suppressed by Ir modifications. Furthermore, the Ir-modified samples show positive on-set potential shifts of the OH adsorption feature (0.6V-1.0V). The results clearly reveal that electrochemical properties of the Pt/Pd(111) bimetallic surface are modified by the third element, Ir, located not only at the surface (Ir/Pt/Pd(111)) but also at the interlayer (Pt/Ir/Pd(111)). Fig.1 (b) shows changes in ORR activity of the Ir-modified samples during applying the 5000 PCs. The initial, pristine ORR activities of the Pt/Ir/Pd(111) and Ir/Pt/Pd(111) are lower than non-Ir-modified Pt/Pd(111). In contrast, the ORR activities of the former interlayer-Ir-modified and the latter surface-Ir-modified samples become higher than the corresponding Pt/Pd(111) after the 3000 and 2000 PCs, respectively: the ORR activity enhancement factors vs. initial ORR activity of the clean Pt(111) for the interlayer-Ir- and surface-Ir-modified samples can be estimated to be ×3, ×4, respectively, after the 5000 PCs. The results obtained reveal that Ir can be applied for the modifying element for durability improvements of the Pt-shell/Pd-core type ORR catalyst. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan.