Oxygen and Hydrogen Peroxide Reduction on Polycrystalline Platinum in Acid Electrolytes: Effects of Bromide Adsorption

Changes in the electrocatalytic activity of Pt for the oxygen and hydrogen peroxide reduction reactions (ORR and HPRR, respectively) in an aqueous acidic electrolyte induced by the adsorption of bromide, as a model impurity, have been investigated using chronoamperometric techniques under forced convection. Experiments were carried out using a polycrystalline Pt|Pt rotating ring-disk electrode in O₂-saturated 0.1 M HClO₄ containing 10 μM KBr (800 ppb). Potential steps were applied to the Pt disk from E_o, at which Br^- is fully desorbed, to more positive values, E_step, at which Br^- undergoes adsorption. During this period, the currents both at the disk, i_disk, and at the ring, i_ring, were monitored, with the ring polarized at a potential at which H₂O₂(aq) oxidation proceeds under diffusion-limited conditions. The results obtained, assuming the ORR does not interfere with Br^- adsorption, made it possible to correlate i_disk and i_ring with the coverage of adsorbed Br^-, θ_Br ^-, and E_step. As evidenced from the data collected, significant drops in the diffusion-limited currents for O₂ reduction induced by the presence of 10 μM KBr could only be observed for θ_Br ^- > 0.25 regardless of E_step. Similar measurements involving the same Pt(poly) rotating disk electrode performed in 1 mM H₂O₂ in deaerated 0.1 M HClO₄ devoid of O₂ displayed a similar trend. This behavior was found to be consistent with a simple blockage of surface-active sites by adsorbed bromide, as predicted by the attenuation model proposed by Levart.