Oxygen Reduction Reaction Mechanism of Connected Platinum-Iron Nanoparticle Catalysts Probed By EC-XPS

The widespread use of polymer-electrolyte fuel cells (PEFCs) requires more active, stable, and low cost electrocatalysts than platinum. Bimetallic electrocatalysts such as platinum-iron (PtFe) have been attracting a great deal of attention as an oxygen reduction reaction (ORR) catalyst since they have demonstrated both higher oxygen reduction activity and improved stability with much smaller amounts of platinum. PtFe-nanoparticle catalyst with a beaded network by connected PtFe nanoparticles and a superlattice structure (a chemically-ordered face-centered tetragonal (fct) structure) exhibit specific ORR activity that is higher than those of the commercial Pt catalyst supported on carbon black (Pt/C) and even fct-PtFe nanoparticles (without any beaded networks) supported on carbon black. [1] Thus, the unique structures of connected PtFe-nanoparticle catalysts, such as connected beaded network and hollow structure, enhance an ORR activity. Electrochemical x-ray photoelectron spectroscopy (EC-XPS) applied for the reaction mechanism analyses of PEFC catalysts. Electrochemical reactions occur at catalyst electrode surface, and then, the electrode is transferred to an UHV chamber immediately. Finally the electrode surface involving reaction intermediates is analyzed by using XPS without exposing to the air. Since structure of electrochemical-double layers is maintained during the measurement process, we can identify the intermediate species, and evaluate adsorption strength, etc., namely, we can analyze reaction mechanism. In this study, we performed electrochemical x-ray photoelectron spectroscopy (EC-XPS) measurements in order to understand the factors that influence the high ORR activity of the connected PtFe-nanoparticle catalyst. The EC-XPS results for connected PtFe-nanoparticle catalysts under ORR conditions showed the adsorption of oxygen species on the catalyst surfaces, indicating that ORR intermediates were detected. We could observe ORR processes of connected PtFe-nanoparticle catalysts. In this presentation, we will also show the EC-XPS measuring data for other platinum catalysts and discuss the factors that influence the high ORR activity of the connected PtFe-nanoparticle catalyst in detail. [1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Connected nanoparticle catalysts possessing a porous, hollow capsule structure as carbon-free electrocatalysts for oxygen reduction in polymer electrolyte fuel cells, Energy Environ. Sci., 2015, 8, 3545-3549.