Surface Structures and Electrochemical Stabilities for Pt/Pd(111) Model Electrocatalysts

Abstract

Introduction Pd@Pt core-shell catalyst is one of the promising cathode electrode materials 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 of Pt [1]. Recently, some researchers have reported that ORR activities of the core-shell catalysts increase during applying potential cycles, as a result of structural changes at the near surface regions [2]. Their results suggest that comprehensive understandings of dynamic behavior of the surface atomic structures during potential cycles are crucial for developing highly-active and highly-durable Pd@Pt core-shell catalysts. In this study, we investigate the relation between surface structures and electrochemical stabilities for the Pt/Pd(111) model electrocatalysts prepared by molecular beam epitaxy (MBE). Experimental Sample fabrication processes of the Pt/Pd(111) were conducted in UHV. Pd(111) single crystal substrate was cleaned by repeating cycles of Ar+ sputtering and subsequent annealing at 800 °C. 4ML-thick Pt was deposited onto the cleaned Pd(111) (4ML-Pt/Pd(111)) by an electron-beam evaporation method at the substrate temperatures of 300 °C. The resulting surface structures were verified with reflection high-energy electron diffraction (RHEED), scanning tunneling microscope in UHV (UHV-STM), and low-energy ion scattering (LEIS). Then, the prepared samples were transferred without being exposed to air to an electrochemical system set in a N2-purged glove box. Cyclic voltammogram (CV) of the samples were recorded in N2-purged 0.1 M HClO4, and, then, linear sweep voltammetry (LSV) was conducted by using a rotating electrode (RDE) method after saturating the solution with O2. The ORR activities were estimated by kinetic-controlled current density (j k) at 0.9V vs. RHE by using Koutecky-Levich plots. The electrochemical stabilities were evaluated by applying potential cycles (PCs) between 0.6 V and xV (x = 0.8, 0.85, 0.9 and 1.0V) vs. RHE for each 3 seconds in O2-saturated 0.1 M HClO4 at 80 °C. The potential cycle conditions are hereafter denoted as “xV-PCs”. Results and Discussion Surface structures for the clean Pd(111) and 4ML-Pt/Pd(111) are summarized in Fig.1 (a) and (b). The UHV-STM image indicates that the clean Pd(111) surface has atomically flat terraces. On the other hand, UHV-STM image of the 4ML-Pt/Pd(111) shows islands-like structures having hexagonal-shaped terraces with ca. 20 nm width. However, the corresponding RHEED pattern indicates sharp streaks, suggesting that deposited Pt atoms have epitaxially grown (111) lattice on the substrate. Initial ORR activities for the 4ML-Pt/Pd(111), clean Pt(111) surfaces are shown in Fig.1 (c). The results indicates that the 4ML-Pt/Pd(111) exhibits ca. 4.5 times higher ORR activity than the clean Pt(111).Fig.1 (d) summarizes changes in ORR activities of the 4ML-Pt/Pd(111) during applying PCs at 80 °C. The activity of 1.0V-PCs applied sample drastically decreased with increasing PC numbers and dropped to less than that of the clean Pt(111) at 1000 PCs. However, it can be seen that decrease in upper limit potential of PCs suppress activity deterioration of the 4ML-Pt/Pd(111). In particular, the activities of 0.8V- and 0.85V-PCs applied samples after 5000 PCs were 120 % and 90 % of the initials, respectively. To investigate effects of surface structural changes in the ORR activity, the surface structures and compositions were evaluated by UHV-STM and LEIS after 5000 PCs. LEIS spectra (Fig.1 (e)) obtained after the PCs indicate that Pd surface concentration of 0.9V-PCs applied sample is higher by ca. 30 % than 0.8V-PCs applied sample, suggesting that activity deterioration during PCs is derived from increase in surface composition of Pd which show much lower ORR activity than Pt. Furthermore, judging from the corresponding UHV-STM images, although height roughness of the 0.8V-PCs applied surface is suppressed to lower than 0.7 nm, the 0.9V-PCs applied surface comprises agglomerated particle-like structures with 20 nm width and 3 nm heights. These results suggest that the structural and electrochemical stabilities for Pd@Pt core-shell catalysts strongly depend on operating conditions of PEFC. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan.