Oxygen reduction reaction on Pt(111): effects of bromide

Using the rotating ring-disk technique (RRDE) the oxygen reduction reaction (orr) was studied on the Pt(111) surface in the presence of Br^- anions. We found that the orr at the Pt(111) | Br_ad interface is always accompanied quantitatively by H₂O₂ oxidation currents on the ring electrode, implying that in the presence of Br^- anions the orr does not proceed entirely through the 4e^- reduction pathway, as in the Br^- -free solution. We propose that strongly adsorbed Br^- can simultaneously suppress both the adsorption of the O₂ molecule and the formation of pairs of platinum sites needed for the breaking of the O-O bond. Besides elucidating the effects of Br_ad, these studies also shed some light on the role of OH_ad on the kinetics of the orr on the Pt(111) surface in solution free of Br anions. We have developed a theoretical model and from simulations of I-E curves we propose two modes of action of the OH_ad state on the kinetics of the orr on the Pt(111) surface: (i) OH_ad can block the adsorption of O₂ on active platinum sites, i.e. they compete for the same sites, and (ii) OH_ad can alter the adsorption energy of intermediates which are formed during the orr on the bare Pt sites adjacent to the OH_ad.