Hydrogen peroxide (H 2 O 2 ) generation and hydrogen oxidation reaction (HOR) properties of Ir(hkl) (hkl = 111, 100, and 110) were investigated in acidic media using scanning electrochemical microscopy. In the 0.06−0.3 V vs reversible hydrogen electrode potential region, H 2 O 2 generation properties were significantly suppressed for the Ir(hkl) surfaces in comparison with Pt(111): only the Ir(100) revealed slight evolution of H 2 O 2 at ca. 0.1 V. Conversely, the HOR activity trend of the Ir(hkl) was Ir(111) > Ir(100) > Ir(110), and the activity of Ir( 111) is approximately 70% of that of Pt(111). This study presents Ir-based nanosized catalysts whose surface comprises mainly ( 111) facets (such as octahedron-shape) as effective anode catalysts for polymer electrolyte fuel cells.

Oxygen reduction reaction (ORR) properties are investigated for the Pt/Zr/Pt(111) surfaces prepared through arc-plasma depositions of Zr and Pt on a Pt(111) substrate. The synthesized Pt(111)-shell surfaces on Pt–Zr(111) alloy layers exhibited 3- to 5-fold higher ORR activities than the clean Pt(111): the ligand effect induced by charge transfers between Pt and Zr was considered to be the dominant activity enhancement factor, because tensile strains in the Pt-shell were relieved by the stacking faults generated in the underlaid Pt−Zr alloy layers. Furthermore, the alloy surfaces are durable against an electrochemical potential cycling. The results demonstrate that Zr should be considered for alloying elements of Pt for developments of effective ORR catalysts.

Interlayer and surface Ir-modified Pt/Pd(111) model catalyst surfaces [Pt/Ir/Pd(111) and Ir/Pt/Pd(111)] were synthesized as surface structural models for third element-modified core−shell-type Pd@Pt catalysts by vacuum depositions of Ir and Pt on the Pd(111) substrate surface. The oxygen reduction reaction (ORR) properties (initial activity and electrochemical stabilities) were compared to non-Irmodified Pt/Pd(111) and discussed on the basis of atomic structural observations of the near surface regions. ORR activities for both the non-Ir-modified and Ir-modified Pt/ Pd(111) surfaces increased up to 500 potential cycles (PCs) of 0.6−1.0 V, which were likely caused by densification of the surface Pt(111) shell layers through dissolution of Pd atoms. The nonmodified Pt/Pd(111) surface deactivated monotonically from 500 to 5000 PCs. The interlayer Ir-modified Pt/Ir/Pd(111) surface exhibited improvements in ORR activity and durability. In fact, from over 500 to 5000 PCs, it outperformed the activity of surface Ir-modified Ir/Pt/Pd(111) in comparison. Furthermore, the Pt/Ir/ Pd(111) showed fivefold activity enhancement at 1000 PCs and fourfold after 5000 PCs vs clean Pt(111). In contrast, ORR activity of Ir/Pt/Pd(111) remained almost constant from 500 PCs, with an approximately 3.5 times enhancement at 5000 PCs. Considering atomically resolved observations by scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy and surface chemical state analysis by X-ray photoelectron spectroscopy, the ORR behavior suggests that Ir located in the Pt(111) shell layers contributed to ORR activity enhancement via charge transfer between Ir and Pt surface atoms, while surface Ir oxides generated by PC loadings are correlated with ORR durability improvement.This study demonstrates an effective way to enhance ORR performances of Pt-based core−shell-type catalysts, that is, the third element Ir addition, on the basis of the enhancement scenario deduced by the atomically resolved structural evaluations during the PC loading process.

High-entropy alloys (HEAs) have attracted considerable attention to improve performance of various electrocatalyst materials. A comprehensive understanding of the relationship between surface atomic-level structures and catalytic properties is essential to boost the development of novel catalysts. In this study, we propose an experimental study platform that enables the vacuum synthesis of atomic-level-controlled single-crystal high-entropy alloy surfaces and evaluates their catalytic properties. The platform provides essential information that is crucial for the microstructural fundamentals of electrocatalysis, i.e., the detailed relationship between multi-component alloy surface microstructures and their catalytic properties. Nanometre-thick epitaxially stacking layers of Pt and equi-atomic-ratio Cr-Mn-Fe-Co-Ni, the so-called Cantor alloy, were synthesised on low-index single-crystal Pt substrates (Pt/Cr-Mn-Fe-Co-Ni/Pt(hkl)) as a Pt-based single-crystal alloy surface model for oxygen reduction reaction (ORR) electrocatalysis. The usefulness of the platform was demonstrated by showing the outperforming oxygen reduction reaction properties of high-entropy alloy surfaces when compared to Pt-Co binary surfaces.

Introduction Nanoparticles of Pt as well as Pt-based alloys are widely used as cathode catalyst materials for proton exchange membrane fuel cells (PEMFC). However, electrochemical stability of the materials is rather low under practical operating conditions of PEMFC cathode, resulting in severe deactivation of oxygen reduction reaction (ORR). Therefore, further material’s developments are required for next-generation PEMFC cathode catalysts, i.e., more enhanced ORR durability with low platinum group metal (PGM) usage. High entropy alloys (HEAs), defined as single phase solid solutions of five or more elements in equal composition ratios, are known as thermodynamically stable, in comparison to conventional binary alloys. Furthermore, complex atomic-level local structures bring about unique electronic as well as (electro-)chemical properties that originating from lattice strains induced by specific local structures and/or so-called sluggish diffusion of the constituent elements. [1] However, to our best knowledge, no study has been made for ORR properties of Pt alloying with non-PGM Cantor alloy (fcc structure HEA with equi-atomic ratio of Cr-Mn-Fe-Co-Ni [2]) elements in strong acid condition. In this study, we synthesized lattice stacking structures of Pt/HEA(hkl) (hkl = (111), (110), (100)) through arc-plasma deposition (APD) of the Cantor alloy layer on Pt(hkl) substrate surfaces, followed by deposition of the surface Pt layer in ~10-7 Pa. Then, we performed cross-sectional STEM-EDS observations for Pt/HEA/Pt(hkl) stacking structures with atomic-level resolution and evaluated the ORR properties (initial activity and structural stability). Experimental An APD target of Cr-Mn-Fe-Co-Ni (Cantor alloy) was fabricated by sintering of equal quantity corresponding elements. 10 ML(monolayer)-thick (1 ML = ca. 0.3 nm) Cantor alloy layer (as HEA) was vacuum-deposited by APD on surface cleaned Pt(hkl) substrate surfaces at 300 K, and subsequently annealed in vacuum at 773 K for 30 minutes. Then, 4ML-thick Pt layer was deposited on the pre-deposited Cantor alloy layer at 300 K and annealed at 623 K. The samples thus prepared are designated as Pt/HEA/Pt(hkl). The atomic-level micro-structures and chemical bonding states of Pt/HEA/Pt(hkl) surfaces were characterized by cross-sectional STEM-EDS, RHEED, XPS etc. CV and LSV with the RDE method were conducted in N2-purged and O2-saturated 0.1 M HClO4. ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon activity transitions during applying the potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion Atomically-resolved, cross-sectional HAADF-STEM images for Pt/HEA/Pt(hkl) (a) and EDS line profiles of elemental distributions at corresponding yellow arrows (b) are presented in Figure 1. As clearly shown in (a), irrespective of the Pt surface indices, (hkl), HEA (Cantor alloy) and surface Pt layers are epitaxially grown on the substrates. By contrast, elemental distributions in each surface normal (b) seriously depend on the substrate Pt lattice indices. Notably, severe thermal diffusion of the constituent elements including Pt is confirmed by (a) and (b) for both Pt/HEA/Pt(110) and (100). Figure 2 summarizes electrochemical results. 4ML-thick-Pt/10ML-thick-Co/Pt(hkl) that prepared under the same preparation condition of Pt/HEA/Pt(hkl), and clean Pt(hkl) (light blue and gray, respectively) are also shown as references. As shown in the figure, the Pt/HEA and Pt/Co fabricated on Pt(111) substrate (top) show quite similar CV characteristics (shrink in hydrogen adsorption charges (0 – 0.3 V) and higher potential shifts in oxygen-related species adsorption (0.6 – 1.0 V)), in comparison to clean Pt(111), and almost the same initial ORR activity. Meanwhile, the Pt/HEA fabricated on Pt(110) and (100) substrates show more reduced hydrogen absorption charges, compared with corresponding Pt/Co samples. Particularly, distinctive redox features for clean Pt(110) at 0.12 V and for Pt(100) at 0.35 V are absent for corresponding CVs, suggesting specific topmost surface atomic-structures might be formed in the electrolyte, that probably resulting from significant electronic interactions between surface Pt and HEA constituent elements (Cr, Mn, Fe, Co, Ni) and/or local strain of the surface Pt layer induced by distorted Pt-HEA lattices located nearby. One might notice that Pt/HEA/Pt(110) reveals remarkable ORR activity enhancement compared with corresponding Pt/Co/Pt(110), while the activities for Pt/HEA and corresponding Pt/Co surfaces fabricated on Pt(100) are almost the same value. At the meeting, correlations between surface atomic-level micro-structures of Pt/HEA/Pt(hkl) and ORR properties will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan and JST SPRING, Grant Number JPMJJSP2114. Reference [1] J. Yeh, JOM, 65, 1759 (2013). [2] B. Cantor et al., Mater. Sci. Eng. A, 375, 213 (2004). Figure 1