Understandings on the Inhibition of Oxygen Reduction Reaction by Bromide Adsorption on Pt(111) Electrodes at Different pH Values

Oxygen reduction reaction (ORR) is studied on Pt(111) in the presence of different concentrations of bromide anions at different pH values ranging from very acidic to neutral solutions. While adsorbed bromide inhibits the ORR, the strength of the inhibition decreases when the pH is increased. This is a consequence of the lower relative adsorption energy of bromide at higher pH values, caused by the lower absolute applied potential. This is reflected in a shift of the onset of the ORR (as measured with the hanging meniscus rotating disc electrode, HMRDE) to higher values as the pH is increased. HMRDE measurements reveal that the limiting current density (jlim) coincides with the theoretical value for two electrons only at very acidic solutions. However, when pH is increased, jlim tends toward the value for a four electrons reaction. From pH > 3 jlim coincides both in the presence and in the absence of bromide despite the specific anion adsorption. Experiments in solutions with different ionic strength and hydrogen peroxide reduction measurements suggest that the formation of a reaction intermediate different from H2O2 is favored at neutral pH values. This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P). GABM thanks the post-doctorate fellowship from CNPq (grant no. PDE 233268/2014-6).