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 O-2 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 OHad 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 OHad state on the kinetics of the orr on the Pt(111) surface: (i) OHad can block the adsorption of O-2 on active platinum sites, i.e. they compete for the same sites, and (ii) OHad can alter the adsorption energy of intermediates which are formed during the orr on the bare Pt sites adjacent to the OHad. (C) 1999 Published by Elsevier Science S.A. All rights reserved.