Electrochemical Properties of Pt Epitaxial Layers Formed on Pd(111) in Ultra-High Vacuum

Abstract

Electrocatalytic properties for oxygen reduction reaction (ORR) of Pt(111) epitaxial layers on a Pd(111) substrate are investigated. Scanning tunneling microscopy images of the as-prepared 0.6-nm-and 1.2-nm-thick Pt(111) epitaxial layers on Pd(111) (Pt 0.6nm /Pd(111) and Pt 1.2nm /Pd(111)) revealed that the topmost surface has atomically flat, 50-100-nm-wide terraces. Remarkable current features due to hydrogen storage and emission by the Pd(111) substrate dominated the cyclic voltammograms of Pt 0.6nm /Pd(111) in the potential range of 0.15-0.4 V vs. a reversible hydrogen electrode. In contrast, the curve of Pt 1.2nm /Pd(111) exhibited a shrinkage in the hydrogen charges (Q H ) in the potential range of 0.25-0.4 V, accompanied by the emergence of symmetrical redox features at 15 mV on the positive potential side, relative to Pt(111) "butterfly" feature. Both Pt 0.6nm /Pd(111) and Pt 1.2nm /Pd(111) showed ca. four times higher ORR activity than clean Pt(111), although their hydrogen-related behaviors and activity changes during applying potential cycles were much different. The results suggest that shrinkage in Q H and positive shift in the onset potential of hydroxyl species are common features of the CV of highly ORR-active well-defined Pt-M(111) bimetallic surfaces and that the activity enhancements and the structural stabilities fairly depend on the surface structure of the epitaxial Pt (111) High-performance low-cost polymer electrolyte membrane fuel cells (PEMFC) are required for wide-spread use in fuel cell vehicles. Developments in highly active oxygen reduction reaction (ORR) catalysts are one of the top priorities for developing PEMFC stacks. Since the pioneering work on Pt-M (M is a more affordable, lowcost material) alloys by Watanabe and co-workers, 1 numerous studies on the ORR properties of Pt-M bimetallic alloy with nanosized structures have been intensively performed. To date, several types of nanosized structures of Pt-M have been proposed for use as highperformance ORR catalysts, e.g., Pt-shell and M-core core-shell nanoparticles (NPs), 2-5 especially electrochemically de-alloyed M from Pt-M NPs, 6-9 nanostructured thin films of Pt-M, 10,11 shapecontrolled Pt-M NPs, [12][13][14][15] and nanoframe structures of Pt-Ni alloy. 16Irrespective of the proposed nature of the nanosized structures, the generation of electrochemically stable Pt-shell layers at the topmost surfaces is the consensus for synthesizing Pt-M ORR catalysts with high activity and durability. 17-20The topmost atomic-level structures of the Pt-shell layers for the Pt-M bimetallic alloys determine ORR performance. However, because ORR is governed by many factors, the discussions of ORR enhancement mechanisms are intricate. For example, atomic arrangements of the topmost Pt-shell, thickness of the Pt-shell layers, and interface structures between the surface Pt-shell and underlying M (Pt-M) core [21][22][23][24] are expected to have a serious influence on the ORR activities. When studying the topmost surface structures and the eval...

Oxygen Reduction Reaction of Third Element-Modified Pt/Pd(111): Effect of Atomically Controlled Ir Locations on the Activity and Durability

“…MLs,51,55 where similar monotonical deactivation behaviors against applied PCs are observed. On the other hand, the deactivation behaviors are markedly suppressed for the Ir-modified catalysts.…”

“…30 mV positive shifts of the so-called butterfly peak [appearing at 0.8 V for Pt(111)], and the value of the positive shift corresponds to our previous studies. 21,51,55 As for the CV curve of surface Ir-modified Pt/Pd(111) (green), the butterfly peak is too unclear to discuss an onset potential shift originating from the adsorption of OH-related species on the Ir surface. 52 Regardless, the cyclic voltammograms given in Figure 2 clearly show that the electrochemical properties of the Pt(111) shell surfaces were modified, markedly, by Ir, irrespective of its location.…”

“…Figure (b) reveals enlarged cyclic voltammograms in the potential region of 0.6–1.0 V; cyclic voltammograms of clean Pt(111) and Ir(111) are also shown by dotted and dash-dotted lines, respectively. The cyclic voltammogram of the clean Pd(111) substrate (blue) is characterized by marked electrochemical charges around 0.15–0.5 V, ascribed to hydrogen adsorption, storage, and emission reactions. , Reduced electrochemical charges for Pt/Pd(111) indicate that surface hydrogen atoms can access the Pd(111) substrate surface through the surface Pt(111) shell layers with a thickness less than 2 nm. Additionally, electrochemical charges due to hydrogen storage reaction for the Ir-modified Pt/Pd(111) apparently shrinks compared to the non-Ir-modified Pt/Pd(111), indicating that the Ir located both at the interface and topmost surface of the Pt/Pd(111) remarkably limit hydrogen storage and emission reactions of the Pd(111) substrate.…”

Anodic stripping voltammetry of silver(I) using unmodified GaN film and nanostructure electrodes

“…We choose GaN because of its excellent physical and chemical properties. It possesses unique electrical properties, low toxicity and excellent chemical stability [13,14]. In particular, wurtzite gallium nitride (GaN), with its remarkable wide direct band gap energy, extremely chemical stability in harsh environment, superior optical property and convenient controlling over its carrier concentration and conductivity type, has been recognized as an attractive candidate for light-emitting diode (LED) [15][16][17], optoelectronic devices [18], DNA biosensor [19], anion and pH selective potentiometric sensor [20,21], monitoring pH in aqueous solution and certain gases at relatively high temperatures.…”

Communication—Electrochemical Stability of Pt/Pd(111) Model Core-Shell Structure in 80°C Perchloric Acid

“…5,6 In this communication, we focus on the EC stabilities of Pt/Pd(111) core-shell model structures in 0.1 M HClO 4 at 80…”