Oxygen Reduction Reaction Activities for Various-Monolayer-Thick Pt Shells on PtxNi100-x(111)

Introduction

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 [1]. 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 Pt_xNi_100-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.

Experimental

First, Pt_xNi_100-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 Pt_xNi_100-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 j_k values at 0.9 V vs. RHE by using Koutecky-Levich equation.

Results and Discussion

STM images of as-prepared Pt_2ML/Pt_xNi_100-x (111) surfaces are summarized in Fig. 1(a).  Pt_2ML/Pt₅₀Ni₅₀ (111) and Pt_2ML/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 Pt_xNi_100-x (111) substrates. In contrast, such a height modulation is absent on the image of Pt_2ML/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 Pt_2ML/Pt_xNi_100-x (111) and Pt (111) are presented in Fig. 1(b). Depending upon the Pt/Ni alloy compositions of the substrates, Q_Hupd were markedly suppressed and OH_ads 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 [2] disappear for the Pt_2ML/Pt₅₀Ni₅₀ (111) and Pt_2ML/Pt₂₅Ni₇₅ (111). Initial ORR activities for the Pt_2ML/Pt_xNi_100-x (111) and Pt (111) are summarized in Fig. 1(c). The activities increased with increasing the substrate Ni compositions (100-x): the Pt_2ML/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 Pt_2ML/Pt₂₅Ni₇₅ (111) (× 25 vs. Pt (111)).

Acknowledgement

This study was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

[1] L. Gan, P. Strasser et al., Nano Lett., 12(2012), 5423-5430.

[2] M. T. M Koper et al., J. Electroanal. Chem., 485(2000), 161-165.