Oxygen electroreduction through a superoxide intermediate on Bi-modified Au surfaces

The mechanism of the electroreduction of oxygen on bare and Bi-submonolayer-modified Au(111) surfaces is examined using surface enhanced Raman scattering (SERS) measurements along with detailed density functional theory (DFT) calculations. The spectroscopy reveals the presence of superoxide-level species at potentials where oxygen is reduced. These species are not present in solutions absent either oxygen or Bi at these potentials. The spectroscopy also reveals the presence of Bi-OH species which are associated with peroxide reduction. Detailed calculations show oxygen associates much more strongly with Bi in the (2 x 2) configuration on Au(111) relative to the bare Au surface. Additionally, the O-O bond is elongated following O-2 association, which follows as a consequence of Bi-O bond formation and partial oxidation of the Bi adatom. These results show for the first time that the four-electron electroreduction of oxygen electroreduction occurs via a series pathway on the Bi-modified surface in acid solution.