Structural Effects on Oxygen Reduction Activity of Carbon-Free Connected Platinum-Iron Nanoparticle Catalysts

We developed connected carbon-free platinum-iron (PtFe) nanoparticle catalysts with porous hollow capsule structure (Fig. 1) as oxygen-reduction-reaction (ORR) electrocatalysts for polymer electrolyte fuel cells (PEFCs). [1] This catalysts consist of a beaded network by connected PtFe nanoparticles with a crystallite size of 6~7 nm and a chemically-ordered (face centered tetragonal) superlattice structure. The beaded network is electrically conductive; thus, carbon-supports can be removed from catalyst layers in PEFCs. We have demonstrated that an MEA prepared using a carbon-free cathode of connected PtFe-nanoparticle catalysts was highly durable against start-up/shut-down operations because of the elimination of carbon-corrosion problems. Moreover, an ORR specific activity of a connected PtFe catalyst is about 9 times higher than that of a commercial Pt-nanoparticle catalyst supported on carbon black (Pt/C). In this study, the structural effects of a connected PtFe catalyst on an ORR activity were investigated in order to elucidate the factors of its high ORR activity. Note that the connected PtFe catalyst exhibited about twice higher ORR specific activity than the PtFe-nanoparticle catalyst (just nanoparticles without any connection between them) on carbon black. In addition, the ORR specific activity of the connected PtFe nanoparticles with hollow structure (with no supports) was 1.5 times higher than that of the connected PtFe nanoparticles on the electroconductive carbon support. These results indicate that the unique structures of connected nanoparticle catalysts, such as connected beaded network and hollow structure, enhance an ORR activity. We also have performed the refined structural analyses of a connected PtFe catalyst by using in-situ X-ray absorption spectroscopy (XAS) combined with Rietveld structure refinement with powder X-ray diffraction data, and spherical aberration corrected scanning transmission electron microscope ( Cs -corrected STEM). The in-situ XAS analysis disclosed shorter Pt‒Pt bond distance and more Pt 5d vacancy of the connected PtFe catalyst compared with the commercial Pt/C. In addition, the Cs -corrected STEM observations revealed that the beaded network formed by connected PtFe nanoparticles possessed polycrystalline structure. In this presentation, the effects of the geometric and electronic structures of a connected PtFe catalyst on an ORR activity will be discussed in details. The knowledge obtained in this study provides guidelines for the design of ORR catalysts with enhanced ORR activities to achieve a high performance PEFC. [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. Figure 1