J. Electrochem. Soc. 2013, volume 160, issue 6. F591-F596.
Naoto Todoroki, Yuki Iijima, Ryota Takahashi, Yu Asakimori, Toshimasa Wadayama.
Environmental Materials Surface Science Laboratory, Graduate School of Environmental Studies, Tohoku University, Aoba-ku, Aramaki Aoba 6-6-2, Sendai 980-8579, Japan
We investigated the atomic-scale structure and electrochemical stability of a Pt-enriched topmost surface (Pt-enriched Ni/Pt(111)) prepared through monolayer Ni deposition on Pt(111) at 823 K using molecular beam epitaxy. Reflection high-energy electron diffraction patterns and an ultra-high vacuum scanning tunneling microscopic (UHV-STM) image of the Pt-enriched Ni/Pt(111) surface showed that the surface has long-range-ordered six-fold symmetry with atomic-scale corrugations. Although the oxygen reduction reaction (ORR) activity of the as-prepared Pt-enriched surface was eight times higher than that of clean Pt(111), the ORR activity degraded steeply during potential cycles between 0.6 and 1.0 V in O2-saturated 0.1 M HClO4. After 1000 potential cycles, the enhancement factor was estimated to be 2. An STM image collected after the potential cycles showed island-like structures (ca. 5–9 nm in diameter and 0.4–0.8 nm in height). Furthermore, the intensity of the Ni 2p core-level band for the as-prepared sample decreased after the 1000 potential cycles, revealing dissolution of the Ni atoms located at the subsurface layer. This experimental study clearly demonstrates that the underlying-Ni-induced specific surface strains and electronic state of the Pt-enriched topmost surface sorely contribute to the remarkable ORR activity of the Ni/Pt(111) surface.
In recent years, several Pt–base metal (Pt–M) alloys have been found to exhibit higher oxygen reduction reaction (ORR) activity than pure Pt. In particular, intermetallic compounds of Pt (Pt–M) with 3d transition metals (such as Pt3M, where M = Fe, Co, Ni, etc.) show significant enhancement of ORR activity. Therefore, Pt-based alloys have been regarded as some of the most promising materials for cathode electrode catalysts. Although the ORR activityof Pt–M alloyscan depend on the electronic states and the geometrical surface symmetry, the effect of the atomic-scale topmost surface structures of Pt–M alloy catalysts on the ORR activity are still unclear. In addition, in view of the highly acidic environments of PEFCs in practice, not only the initial ORR activity but also durability is essential for developing highly efficient cathode electrode catalysts. However, the durability mechanisms of Pt-based alloy catalysts are yet to be discussed.In this study, we focused on the relationship between Pt-enriched topmost surface structures and electrochemical stability during potential cycles in an O2-saturated solution. We fabricated a Pt-enriched topmost layer (Pt-enriched Ni/Pt(111)) by depositing 0.3-nm-thick Ni onto a clean Pt(111) substrate at an elevated deposition temperature using molecular beam epitaxy (MBE).The topmost surface of the Pt-enriched Ni/Pt(111) surface that showed the most remarkable specific ORR activities (an enhancement factor of 8) comprised a Pt(111) lattice having a sub-atomic-scale corrugation, whose Pt 4f CL bands of the XPS spectra shift to a higher binding energy relative to that for clean Pt(111). The results clearly indicated that both strain and electronic effects contribute to the ORR activity enhancement of the Pt–M alloy catalysts. In addition, EC stability of the Pt-enriched Ni/Pt(111) surface was investigated. Although the as-prepared Pt-enriched Ni/Pt(111) surface showed eight-times higher ORR activity than clean Pt(111), the activity decreased steeply during the potential cycles between 0.6 and 1.0 V. The enhancement factor was estimated to be 2 after 1000 potential cycles. The STM and XPS analysis conducted for the potential-cycled sample revealed that the degradation behavior of the topmost surface can be explained through dissolution of the underlying Ni atoms in the subsurface layer. The results obtained in this study indicate that the ORR activity of the Pt-enriched Ni/Pt(111) surface is determined by a morphologically and/or electronically modified topmost Pt(111) lattice by the underlying alloying element of Ni.