Insights into the Structure - Composition - Activity Relationship of PtCo Alloy Nanoparticles towards Oxygen Reduction Reaction (ORR)

PtCo alloy nanoparticles (NPs) are widely used as highly active oxygen reduction reaction (ORR) catalysts for polymer electrolyte membrane fuel cells (PEMFCs). Despite large efforts, the critical relationships between structure, composition and ORR performance of catalyst materials are not fully understood to date. In this study, we prepared two PtCo alloy NP catalysts with an atomic ratio of 1:1 using wet-impregnation route by varying the annealing parameters under reductive conditions. The as-prepared PtCo alloy catalysts were structurally characterized using ex-situ HR-TEM, EDX, XRD, and EXAFS. We show that the annealing temperature and holding time affect the particle size, composition and homogeneity of the PtCo NPs. With higher annealing temperature and longer holding time, the particle size grows from 3.1 ± 0.7 nm (400 °C, 4 h) to 4.4 ± 0.6 nm (800 °C, 6 h) and simultaneously, the alloy formation within the NPs improves. After electrochemical activation in 0.1 M HClO 4 , the electrochemically active Pt surface area (ECSA) for activated PtCo T400 (65 ± 8 m 2 g Pt -1 ) is slightly lower than that for pure Pt/C (70 ± 11 m 2 g Pt -1 ), but significantly higher than that for the activated PtCo T800 (50 ± 4 m 2 g Pt -1 ). However, the activated PtCo T800 shows the highest ORR mass activity (0.56 ± 0.14 A mg Pt -1 at 0.9 V RHE, iR-free ) than the activated PtCo T400 (0.43 ± 0.03 A mg Pt -1 ) and Pt/C (0.24 ± 0.04 A mg Pt -1 ). Altogether, we provide deeper understanding of the structure - composition - ORR activity relationships for two differently annealed PtCo alloy catalyst materials.