Oxygen reduction reaction at three-phase interfaces.

The kinetics of the oxygen reduction reaction (ORR) is studied at metal-supporting electrolyte-Nafion three-phase interfaces. We first demonstrate that the sulfonate anions of Nafion are specifically adsorbed on a wide range of surfaces ranging from Pt(hkl) single-crystal surfaces, Pt-poly, Pt-skin [produced on a Pt(3)Ni(111) surface by annealing in ultrahigh vacuum, UHV] to high-surface-area nanostructured thin-film (NSTF) catalysts. The surface coverage by sulfonate and the strength of the Pt-sulfonate interaction are strongly dependent on the geometry and the nature of the Pt surface atoms. Also, they are found to behave analogous to (bi)sulfate anion-specific adsorption on these surfaces, where for the Pt(hkl) surfaces, the trend is Pt(111)>Pt(110)>Pt(100) and for the Pt-skin surface on Pt(3)Ni(111), the interaction strength is found to be Pt-skin<Pt(111). We also found that irrespective of the surface orientation and/or the electronic properties of the surface atoms, the ORR is always inhibited by the presence of ionomers at the electrode surface, confirming that Nafion is not a non-adsorbing electrolyte. Finally, the knowledge gained from studying well-defined Pt(hkl) surfaces is applied to propose that deactivation of the ORR on Nafion-covered high-surface-area catalysts is also controlled by specific adsorption of sulfonate anions.