Surface Composition Effects in Electrocatalysis:  Kinetics of Oxygen Reduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces

The oxygen reduction reaction (ORR) has been studied on polycrystalline Pt3Ni and Pt3Co alloys in acid electrolytes using the rotating ring disk electrode (RRDE) method. Preparation and characterization of alloy surfaces were performed in ultrahigh vacuum (UHV). Clearly defined surface composition was determined via low-energy ion-scattering (LEIS) spectroscopy. Polycrystalline bulk alloys of Pt3Ni and Pt3Co were prepared in UHV having two different surface compositions: one with 75% Pt and the other with 100% Pt. The latter we call a “Pt-skin” structure and is produced by an exchange of Pt and Co in the first two layers. The base voltammetry in 0.1 M HClO4 solution of the 75% Pt alloy surface indicated a decrease of Hupd pseudocapacitance (ca. 30−40 μC/cm2) consistent with the surface composition determined in UHV. With the exception of the “Pt-skin” surface on Pt3Ni, all the alloy electrodes exhibited stable i−E curves with repeated cycling between 0.05 and 1.0 V at all temperatures. Activities of Pt-alloys for the ORR were compared to the polycrystalline Pt in 0.5 M H2SO4 and 0.1 M HClO4 electrolytes in the temperature range of 293 < T < 333 K. It was found that the order of activity is dependent on the nature of anions of supporting electrolytes: in H2SO4 the activity increased in the order Pt3Ni > Pt3Co > Pt; in HClO4, however, the order of activities at 333 K was “Pt-skin” > Pt3Co > Pt3Ni > Pt. The catalytic enhancement was greater in 0.1 M HClO4 than in 0.5 M H2SO4, with the maximum enhancement observed for the “Pt-skin” on Pt3Co in 0.1 M HClO4 being 3−4 times that for pure Pt. Catalytic enhancement of the ORR on Pt3Ni and Pt3Co vs Pt was attributed to the inhibition of Pt−OHad formation on Pt sites surrounded by “oxide” covered Ni and Co atoms beyond 0.8 V. Kinetic analyses of RRDE data revealed that kinetic parameters for the ORR and the production of H2O2 on the Pt3Ni, Pt3Co, and “Pt-skin” alloys are the same as on pure Pt: reaction order, m = 1, two identical Tafel slopes in HClO4, and a single Tafel slope in H2SO4, apparent activation energy ≈ 21−25 kJ/mol. The fact that essentially the same kinetic parameters are assessed from the analysis of the ORR data on all three surfaces implies that the reaction mechanism on Pt3Ni and Pt3Co alloy surfaces is the same as one proposed for pure Pt, i.e., a “series” 4e- reduction pathway.