Platinum-enriched Ni/Pt(111) surfaces prepared by molecular beam epitaxy: Oxygen reduction reaction activity and stability

Oxygen reduction reaction (ORR) activities and electrochemical stabilities were evaluated for Ni/Pt(111) model electrode-catalysts fabricated by molecular beam epitaxy (MBE). Exposure of clean Pt(111) to 1.0-Langmuir (i.e., fractional coverage, θ = 1.0 in the Langmuir isotherm) CO at 300K produced linear-bonded and bridge-bonded COPt IR bands at 2093 and 1858 cm^-1. In contrast, 3.0-nm-thick Ni deposition onto Pt(111) at 823K (823 K-Ni _3.0nm/Pt(111)) showed broad IR bands for adsorbed CO at around 2064 cm^-1; the separation of reflection high-energy electron diffraction (RHEED) streaks of the 823 K-Ni_3.0nm/Pt(111) is greater than for clean Pt(111). In contrast, 923 KNi_3.0nm/ Pt(111) yielded a single sharp IR band because of linear-bonded CO at 2080 cm^-1, and the separation of the RHEED streaks is almost the same as that for the Pt(111). The results suggest that a Pt-enriched topmost surface is generated above 923K through surface segregation of the substrate Pt atoms during the Ni deposition. After transferring the sample from ultra-high vacuum to an electrochemical system, without being exposed to air, changes in ORR activities of the MBE-prepared 923 K-Ni_3.0nm/Pt(111) were evaluated in a 0.1-mol L ^-1 HClO₄ aqueous solution under applied potential cycles of between 0.6 and 1.0V vs. RHE. The ORR activity enhancement factor vs. clean Pt(111) for the asprepared 923 K-Ni_3.0nm/Pt(111) was estimated to be twelve, and this factor was reduced to four after the application of 1000 potential cycles. However, a 943 K-Ni_3.0nm/Pt(111) sample, which showed a relatively low ORR activity (an enhancement factor of eight), had the activity enhancement factor of six even after the application of 1000 potential cycles. These results reveal that the sub-surface structures and composition of Pt-based alloys determine not only initial ORR activity but also electrochemical durability of the ORR catalysts.