ORR on Platinum Single Crystal Electrodes. pH Effects

Oxygen reduction reaction is a structure sensitive reaction, as the studies with single crystal electrodes demonstrates. Additionally, anion adsorption and pH also affects significantly the activity of the platinum electrodes. In this communication, we will describe recent results on the effects of pH and anion adsorption on the activity of platinum single crystal electrodes for the ORR. In acidic solution, the highest activity is observed for surfaces with a high step density both in perchloric and sulfuric acid media. This fact implies that the step is an active site for the ORR in these acidic media. However, a completely different scenario is found in alkaline solutions. In 0.1 M NaOH, the Pt(111) electrode has the highest activity and the presence of steps significantly diminishes the activity of the surface. Moreover, the extrapolation of the results suggests that the activity of the step is very small in comparison with that of the (111) terrace. Initially, it could be expected that the activity of the electrode in 0.1 M NaOH and 0.1 M HClO₄ was the same, because there is no specific adsorption of anions in both media, and the only possible adsorbed species at the onset of the ORR is OH. However, the observed differences between both media clearly indicates that significant changes in the interphase occurs associated with the pH change. These results will be analyzed in view of the experimental and DFT calculations to determine the role of the possible adsorbed species on the surface in the activity of the surface for the oxidation.