We investigated the oxygen reduction reaction (ORR) activity of several monolayer-thick Co-deposited Pt(111) model catalyst surfaces (n-monolayer(ML)-Co/Pt(111): n = 0.13-2.0) in 0.1M KOH. Cobalt layer thicknesses of less than 0.25 ML enhance the ORR activity in contrast to clean Pt(111) surfaces. The maximum activity enhancement factor of 0.25 ML-Co/Pt(111) was ca. 1.7. The rotating ring-disk electrode measurements of n MLCo/Pt(111) surfaces show an increase in the generation rates of HO2 - at ∼0.8V vs. reversible hydrogen electrode (RHE). This suggests that the deposited Co, in form of (hydro)oxide in 0.1M KOH, functions as oxidation site and activates the Pt sites for the ORR. The results demonstrate that tuning the Pt and Co alloy compositions at the topmost surface is the key in developing highly active Pt-based alloy catalysts for the ORR, not only in acid but also in alkaline electrolytes.
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