In fuel cell applications platinum is still the most used catalyst at the cathode for the oxygen reduction reaction (ORR). Nevertheless, it has been shown that both specific and mass activity can be enhanced by alloying Pt with one or more transition metals such as Ni, Fe, Co or Rh and by maximizing the exposure of (111) facets by the use of shape-controlled nanoparticles ^1-4. Due to the remarkably high activities of platinum-based octahedral shaped nanoparticles, research on them has become more important during recent years. Several approaches have been reported describing methods to synthesize octahedral nanoparticles. Solvothermal methods using long-chain surfactants turned out to be well controllable in terms of nanoparticle composition and size, although they require intensive cleaning procedures ^{5, 6}.

To combine an effective cleaning procedure with the benefit of an improved alloy structure of the nanoparticles, we investigated the influence of a thermal post-treatment in different gas atmospheres at different temperatures in terms of the surface structure and composition, the electrochemical ORR activity and the stability.

All catalysts in this study show a considerably high activity, but the one annealed at 300 °C in hydrogen exhibits an exceptionally high activity up to 2.7 A mg_Pt^-1. We also observe a correlation between the annealing temperatures, the initial activities and the electrochemical stability after 4000 potential cycles. The particles were characterized by electrochemical CO oxidation and corresponding in situ FTIR experiments to gain insights into the nanoparticle surface composition as well as by transmission electron microscopy (TEM) to provide information on the particle shape evolution. Furthermore aberration-corrected scanning TEM in combination with energy dispersive X-ray analysis offers insightful information on the atomic distribution. To obtain detailed understanding about the processes taking place during heat treatment, in situ heating XRD and in situ heating TEM measurements were performed. The combination of these methods allows detailed atomic understanding involving important conclusions for the influence of the annealing temperature.

Literature

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