Oxygen Reduction Reaction Activities for Various-Monolayer-Thick Pt Shells on PtxNi100-x(111)
Pt-M (M = Fe, Co, Ni, Pd and Cu) core-shell nanoparticles have been intensively studied as oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFCs). As for the core-shell catalysts, core elements M strongly modify the electronic properties of outermost Pt shell layers, leading to enhanced ORR activities . From electrocatalytic perspectives, however, discussion of ORR enhancement mechanisms is intricate, because the activity enhancements depend on many factors, such as particle sizes, electronic and/or strains of Pt shells, the shell thickness, etc. In this study, we fabricated various-monolayer-thick Pt (111) epitaxial layers on PtxNi100-x (111) (x = 75, 50, 25) substrates model core-shell catalysts by using molecular beam epitaxy (MBE) and discussed Pt/Ni-composition-dependent ORR activity enhancements of the surface Pt shells.
First, PtxNi100-x (111) single crystal substrates were cleaned by Ar+ sputtering and annealing at 1173 K in ultra-high vacuum (UHV). Then, 2 ML-thick Pt was deposited on the cleaned PtxNi100-x (111) substrates by using an electron-beam evaporation method at room temperature, followed by UHV-annealing at 673 K for 10 minutes to flatten the room-temperature-prepared surfaces. The resulting surface structures were verified by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The UHV-fabricated samples were transferred to electrochemical evaluation systems set in a N2-purged glove box without air exposure. CV and LSV measurements were conducted in N2-purged and O2-saturated 0.1 M HClO4, respectively. The ORR activities were evaluated from jk values at 0.9 V vs. RHE by using Koutecky-Levich equation.
Results and Discussion
STM images of as-prepared Pt2ML/PtxNi100-x (111) surfaces are summarized in Fig. 1(a). Pt2ML/Pt50Ni50 (111) and Pt2ML/Pt25Ni75 (111) surfaces exhibit Moire-like height modulations of the topmost surfaces that probably derived from large lattice mismatches between the 2 ML-Pt (111) shells and PtxNi100-x (111) substrates. In contrast, such a height modulation is absent on the image of Pt2ML/Pt25Ni75 (111). These results suggest that the 2 ML-thick Pt shell layers on the Pt50Ni50 (111) and Pt25Ni75 (111) substrates are subjected to larger compressive strain than that on the Pt75Ni25 (111). Corresponding CV curves of the Pt2ML/PtxNi100-x (111) and Pt (111) are presented in Fig. 1(b). Depending upon the Pt/Ni alloy compositions of the substrates, QHupd were markedly suppressed and OHads regions shifted positively in comparison to those for clean Pt (111). Especially, sharply-peaked “butterfly” features at 0.8 V that might stem from formation of an ordered overlayer of OH-related species on surface Pt (111) lattices  disappear for the Pt2ML/Pt50Ni50 (111) and Pt2ML/Pt25Ni75 (111). Initial ORR activities for the Pt2ML/PtxNi100-x (111) and Pt (111) are summarized in Fig. 1(c). The activities increased with increasing the substrate Ni compositions (100-x): the Pt2ML/Pt25Ni75 (111) exhibit 25 fold higher activity relative to that of Pt (111). The results obtained in this study suggest that, with increasing Ni compositions of the Pt-Ni substrates, compressive strains of the Pt shell layer become large, leading to remarkable ORR activity enhancements, in particular, for the Pt2ML/Pt25Ni75 (111) (× 25 vs. Pt (111)).
This study was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
 L. Gan, P. Strasser et al., Nano Lett., 12(2012), 5423-5430.