Oxygen electroreduction on Ag(1 1 1): The pH effect

The rotating ring disk method (RRDE) is applied to investigate the pH effect on oxygen reduction reaction (ORR) on Ag(1 1 1) single crystal surface in 0.1 M KOH and 0.1 M HClO4. In 0.1 M KOH, the ORR proceeds through 4e− reaction pathway with a very small (0.5–2.5%) peroxide formation in the entire potential range. In 0.1 M HClO4 the onset potential for the ORR is shifted for ca. 400 mV toward the higher overpotentials compared to the 0.1 M KOH solution. At the low overpotentials, in 0.1 M HClO4 the ORR proceeds entirely as a 2e− process, i.e, 100% H2O2 formation. At higher overpotentials, the initial mixed a 2e− and 4e− reduction is followed by the potential region where the ORR proceeds entirely as a 4e− process, with H2O formation as a final product. The pH dependent shift in the onset of the ORR as well as the reaction pathway has been explained based on both: a thermodynamic analysis of pH independent rate determining step, and on the pH dependent change in availability of surface active sites and adsorption energies of molecular oxygen and reaction intermediates.