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, Pt 3 Sn ( 111 ) , and Pt 3 Sn ( 110 ) electrodes in 0.5 M H 2 SO 4 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 ( ≈ 40 % of saturation) of CO still remains on all surfaces. Only a very small fraction of consumed O 2 is used for CO oxidation, while the major fraction reacts via the competing ORR, producing H 2 O 2 . The H 2 O 2 yield is strongly dependent on the fractional coverage by adsorbed CO, the surface structure, and the composition of the electrode. While at Θ CO > 0.5 monolayer (ML) the H 2 O 2 yield is very high (depending on the structure/nature of the electrode ca. 60-100%), when Θ CO falls to its steady-state value of ≈ 0.4 ML , the H 2 O 2 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 H 2 O 2 production can be significant; e.g. , H 2 O 2 may cause an enhanced long-term degradation of the membrane. © 2004 The Electrochemical Society. All rights reserved.