X-ray absorption spectroscopic and electrochemical analyses of Pt-Cu-Fe ternary alloy electrocatalysts supported on carbon

Carbon-supported Pt-Cu-Fe ternary alloy electrocatalysts have been prepared with two atomic compositions of Pt : Cu : Fe = 6 : 1 : 1 (denoted as Pt6CuFe/C) and 2 : 1 :1 (denoted as Pt2CuFe/C). The metal alloy structures are investigated by means of powder X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) at the Pt L-m-edge. Powder XRD analysis shows that alloy particles in the Pt6CuFe/C catalyst form a face-centred cubic structure (AuCu3 type), while those in Pt2CuFe/C catalyst form a face-centred tetragonal structure (AuCu type). EXAFS analyses of the first-neighbour coordination at the Pt L-m-edge for both catalysts show the contraction of Pt-Pt bond distances compared to bulk Pt metal, and also that the ratio of the coordination numbers for Pt and Cu/Fe atoms are statistically consistent with the catalyst composition. Some alloy catalysts exhibit ca. 50% higher catalytic activity towards the oxygen reduction reaction than Pt-only catalysts. The enhanced catalytic activity may be due to the formation of an ordered alloy. The catalysts which display higher ordering show higher mass activity. Cyclic voltammetry reveals the influence of the adsorptive strength of oxygen molecules on the kinetics of oxygen reduction. Unexpectedly, as the adsorptive strength becomes weaker, the specific activity of the alloy catalysts continuously increases, while the mass activity shows a maximum value.