Electronic Structures of Non-Pt Carbon Alloy Catalysts for Polymer Electrolyte Membrane Fuel Cells Revealed by Synchrotr
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Electronic Structures of Non-Pt Carbon Alloy Catalysts for Polymer Electrolyte Membrane Fuel Cells Revealed by Synchrotron Radiation Analyses Masaharu Oshima1, 2, Hideharu Niwa1, Makoto Saito1, Masaki. Kobayashi1, 2, Koji Horiba1, 2, Yoshihisa Harada1, 2, Kiyoyuki Terakura3, Takashi Ikeda4 , Jun-ichi Ozaki5, Yuta Nabae6 and Seizo Miyata6 1 School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 1138656, Japan 3 Research Center for Integrated Science, Japan Advanced Institute of Science Technology (JAIST), 1-1 Asahidai, Nomi Ishikawa 923-1292, Japan 4 Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan 5 Department of Chemical & Environmental Engineering, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan 6 Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
ABSTRACT The oxygen reduction reaction (ORR) mechanism of non-Pt catalysts was elucidated by investigating electronic structures of carbon alloy catalysts (CACs) for polymer electrolyte membrane fuel cells (PEMFC). For metal phthalocyanine-based carbon alloy catalysts with 1-2% of nitrogen and less than 0.1% of Co or Fe, photoelectron spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray Absorption Fine Structure (XAFS) have revealed that Co or Fe metal sites are not the ORR active site, whereas the carbon atoms adjacent to graphitic nitrogens at a zigzag edge of graphite may act as the ORR active site, which is in good agreement with first principles calculations. These studies have also revealed that the zigzag edges, detected as a XAS shoulder component, are well evolved for catalysts pyrolyzed at 600°C, which show the maximum ORR activity. Based on these analyses, we have realized significantly improved carbon alloy based ORR performance, up to about 70 % of that with Pt catalysts. Further, to analyze the electronic structure of CACs during operation, we constructed a new in situ soft Xray emission spectroscopy system with very high energy resolution (ΔE ) of 120 meV at 640 eV at the University of Tokyo beamline BL07LSU in SPring-8. INTRODUCTION In the attempt to realize a low carbon emission society, various efforts have been devoted to the development of low energy consumption devices such as extremely low power large scale integration (LSI) devices and non-volatile memory storage, power generation devices such as solar cells and fuel cells, and energy storage devices such as Li ion rechargeable batteries. Among these devices fuel cells are regarded as the most promising for power generation because of their high energy-conversion efficiency. PEMFCs in particular have attracted great attention and have been implemented in vehicular and stationary applications for the conversion an
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