Solution phase superoxide as an intermediate in the oxygen reduction reaction on glassy carbon in alkaline media

Abstract

A 3-mercapto-1-propanol (3M1P)-modified Au ring electrode has been employed to monitor solution phase superoxide, O2−(aq), generated at the surface of a glassy carbon (GC) disk of a ring-disk rotating electrode (RRDE) in 0.1 M NaOH aqueous solutions. Measurements performed at various rotation rates afforded unambiguous evidence that maximum yields for O2−(aq) is attained at potentials associated with a current minimum in the polarization curve at Edisk = 0.3 V vs RHE, with a ca. 18% faradaic efficiency. In stark contrast, no O2−(aq) could be detected in virtually identical experiments involving Au and Pt disk electrodes. This finding indicates that, if superoxide is involved in the mechanism of O2(aq) reduction on these metals, it remains adsorbed on the surface and is subsequently reduced, or, if desorbed, the rate of its reduction would be high enough for its concentration to decrease to undetectable levels at the functionalized Au-ring under the experimental conditions employed.