Oxygen reduction reaction on Pt and Pt-bimetallic electrodes covered by CO - Mechanism of the air bleed effect with reformate

The oxygen reduction reaction (ORR) has been studied on CO-covered Pt(111), Pt(110), Pt-poly, Pt-Ru, Pt3Sn(111), and Pt3Sn( 110) electrodes in 0.5 M H2SO4 solution at temperatures varying between 298 and 333 K using the rotating ring-disk electrode (RRDE) method. The RRDE measurements showed that even during O-2 reduction a substantial amount (approximate to40% of saturation) of CO still remains on all surfaces. Only a very small fraction of consumed 02 is used for CO oxidation, while the major fraction reacts via the competing ORR, producing H2O2. The H2O2 yield is strongly dependent on the fractional coverage by adsorbed CO, the surface structure, and the composition of the electrode. While at Theta(CO) > 0.5 monolayer (ML) the H2O2 yield is very high (depending on the structure/nature of the electrode ca. 60-100%), when Theta(CO) falls to its steady-state value of approximate to0.4 ML, the H2O2 yield is almost the same as on the respective CO-free surfaces. Although the production of peroxide on some electrodes is rather low at 333 K (e.g., 5% on PtRu and 10% on Pt-poly), the practical consequences of the H2O2 production can be significant; e.g., H2O2 may cause an enhanced long-term degradation of the membrane. (C) 2004 The Electrochemical Society. All rights reserved.