Oxygen reduction on high surface area Pt-based alloy catalysts in comparison to well defined smooth bulk alloy electrodes

The oxygen reduction reaction (orr) has been studied on carbon supported multimetallic catalysts in aqueous acidic electrolyte. The bimetallic catalysts had the nominal compositions of 50 and 75 at.% Pt with the alloying elements being Ni and Co. A ternary catalyst consisted of Pt, Rh and Fe in the ratio of 1:0.3:0.7. Comparison is made to carbon supported Pt. The particle size distribution of the metallic phases was obtained by high resolution transmission electron microscopy (HRTEM). An estimation of the surface composition was obtained from the hydrogen adsorption charge and the particle size. Electrochemical measurements were performed using the thin-film rotating ring-disk electrode (RRDE) method in 0.1 M HClO4 at 20-60 degreesC. Kinetic analysis in comparison to pure Pt revealed an activity enhancement (per Pt surface atom) of a factor of 2 to 3 for the 50 at.% Co-catalyst. The 25 at.% Co(Ni) and the ternary catalyst revealed similar activity compared with pure Pt. The 50 at.% Ni catalyst was actually less active than the Pt standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide indicated no significant differences between the alloy catalysts and pure Pt with the exception of the 50 at.% Ni catalyst which had a higher peroxide yield. Comparison was also made to results obtained under equal experimental conditions on polycrystalline (pc) Pt3Ni and Pt3Co alloy electrodes. These bulk alloys were prepared in UHV to obtain a well defined surface composition of 75 at.% Pt and 25 at.% Co and Ni, respectively. A Pt(pc) electrode served for comparison. The bulk alloys also have an enhanced activity for the orr by about a factor of 2 versus the identically prepared Pt(pc) electrode. (C) 2002 Elsevier Science Ltd. All rights reserved.