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

Abstract

The oxygen reduction reaction (ORR) has been studied on polycrystalline Pt 3 Ni and Pt 3 Co 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 Pt 3 Ni and Pt 3 Co 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 HClO 4 solution of the 75% Pt alloy surface indicated a decrease of H upd pseudocapacitance (ca. 30−40 μC/cm 2 ) consistent with the surface composition determined in UHV. With the exception of the “Pt-skin” surface on Pt 3 Ni, 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 H 2 SO 4 and 0.1 M HClO 4 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 H 2 SO 4 the activity increased in the order Pt 3 Ni > Pt 3 Co > Pt; in HClO 4, however, the order of activities at 333 K was “Pt-skin” > Pt 3 Co > Pt 3 Ni > Pt. The catalytic enhancement was greater in 0.1 M HClO 4 than in 0.5 M H 2 SO 4, with the maximum enhancement observed for the “Pt-skin” on Pt 3 Co in 0.1 M HClO 4 being 3−4 times that for pure Pt. Catalytic enhancement of the ORR on Pt 3 Ni and Pt 3 Co vs Pt was attributed to the inhibition of Pt−OH ad 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 H 2 O 2 on the Pt 3 Ni, Pt 3 Co, and “Pt-skin” alloys are the same as on pure Pt: reaction order, m = 1, two identical Tafel slopes in HClO 4, and a single Tafel slope in H 2 SO 4, 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 Pt 3 Ni and Pt 3 Co alloy surfaces is the same as one proposed for pure Pt, i.e., a “series” 4e - reduction pathway.