SPring-8, the large synchrotron radiation facility

Release Date

22 Oct, 2014

The University of Electro-Communications
apan Synchrotron Radiation Research Institute (JASRI)

Key points
• The research group succeeded in mapping the degradation of fuel cell catalysts at the unique beamline BL36XU at SPring-8 with the best performance in the world for fuel cell catalyst analysis, which was constructed by the University of Electro-Communications under a New Energy and Industrial Technology Development Organization (NEDO) program.
• The research group succeeded in the two-dimensional mapping of platinum (Pt) species in electrode catalyst membranes and in the identification of dissolved species using the nano-X-ray absorption fine structure (XAFS)^*1 technique with the highest spatial resolution in the world.
• These achievements will promote the development of electrode catalysts for next-generation fuel cells, enabling the widespread use of fuel-cell-powered cars, and provide new scientific guidelines for the development and design of electrode catalysts.

1.Determination of the cause and mechanism of fuel cell degradation is desired because significantly improving the durability of electrode catalysts in polymer electrolyte fuel cells (PEFCs)*2is one of the key requirements for realizing the widespread use of fuel-cell-powered cars. However, direct observation of the catalysts has been very difficult. The research group has developed a technique to directly observe catalysts that are spatially distributed. 2.The research group has been the first to succeed in the two-dimensional mapping of Pt species in the membrane electrode assembly (MEA), the active site in practical PEFCs. The two-dimensional mapping of Pt species in PEFCs has been difficult but was enabled by nano-XAFS measurements (beam size, 570 × 540 nm2 or 228 × 225 nm2) performed at the unique beamline BL36XU at SPring-8, for XAFS measurements on fuel cells with the best performance in the world. This beamline was constructed by the University of Electro-Communications under a NEDO program. 3.The spatially heterogeneous processes of oxidation and dissolution of Pt nanoparticles on carbon carriers, namely, fuel cell electrode catalysts, in the MEA were directly observed in the XAFS measurements. Also, the selective oxidation and dissolution of the Pt nanoparticles as the four-coordinate Pt2+-O4 in the 2–3 µm region around the boundary between the cathode catalyst layer and the electrolyte membrane and around the micro-crack boundary were for the first time visualized as images. 4.The specific site where the dissolution and degradation of fuel cell catalysts begin and the underlying mechanism were partially but definitely clarified by the spatially resolved nano-XAFS technique. 5.The mapping information on the Pt species such as the amount of Pt and the oxidation state in MEA, the active site in fuel cells, is expected to provide scientific guidelines for the development of highly durable electrode catalysts for next-generation fuel cells required to promote the widespread use of fuel-cell-powered cars. Publication: Journal title: Angewandte Chemie International Edition（Angew. Chem. Int. Ed.） Article title: "Mapping Platinum Species in Polymer Electrolyte Fuel Cells by Spatially Resolved XAFS Techniques" Authors: Shinobu Takao, Oki Sekizawa, Shin-ichi Nagamatsu, Takuma Kaneko, Takashi Yamamoto, Gabor Samjeské, Kotaro Higashi, Kensaku Nagasawa, Takuya Tsuji, Motohiro Suzuki, Naomi Kawamura, Masaichiro Mizumaki, Tomoya Uruga, Yasuhiro Iwasawa＊（＊Correspondence Author） DOI：10.1002/anie.201408845

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<<Achievements>>

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《Glossary》
※1　X-ray absorption fine structure (XAFS)
Most solid catalysts such as fuel cell catalysts are fine nanoparticles not having a crystal-like periodic structure and being dispersed on the surface of carriers such as carbon. X-ray diffraction (XRD), used to analyze a crystal structure, cannot be applied to such catalysts. Also, the molecular structure including chemical bonding cannot be determined by electron microscopy because it is performed under high vacuum. The XAFS technique is very effective in analyzing the local structure of solid catalysts. When a substance is irradiated with X-rays, electrons are expelled from the target atom through X-ray absorption and scattered and interfered by neighboring atoms. A fine structure is observed in the X-ray absorption spectrum at that time, which is called XAFS.
As shown in Fig. 1, the X-ray absorption spectrum includes an X-ray absorption near-edge structure (XANES) and an extended X-ray absorption fine structure (EXAFS). XANES provides information on the oxidation number and symmetry of the target atom as well as the proportion of the substances in a mixture. EXAFS provides information on the local structure such as the type and number of atoms neighboring the target atom and the distance between the atoms.
Because of the use of hard X-rays with high penetrating power, the XAFS technique has the following characteristics: (1) it is hardly affected by the form and type of samples (crystals, amorphous materials, devices, liquids, gases, and biological materials); (2) it can be performed not only in a limited atmosphere but also under various conditions including an in situ environment and reaction conditions; (3) it is applicable to a mixture of multiple elements; (4) it is highly sensitive even to the order of a ppm concentration and a 0.1-nm-thick thin film. On the other hand, the disadvantage of XAFS is that this technique only provides information on the average structure. Despite this disadvantage, XAFS is the only technique that can analyze the molecular structure of many catalysts such as chemical process catalysts consisting of metallic nanoparticles heterogeneously dispersed and supported on the surface of a carrier, automobile catalysts, fuel cell catalysts, and environmental catalysts that cannot be analyzed by XRD. Therefore, the XAFS technique is particularly effective for the in situ analysis of fuel cell electrode catalysts under operating conditions that have hardly been studied using other analytical techniques. XAFS spectroscopy using synchrotron radiation has been established as a leading technique to analyze the structure of various solid catalysts.

※2　Polymer electrolyte fuel cell (PEFC)
PEFC is a fuel cell that uses an ion conductive polymer membrane (ion-exchange membrane) as its electrolyte. PEFC has a stacked structure in which a membrane electrode assembly (MEA) consisting of a fuel electrode (anode), a solid polymer electrolyte membrane, and an air electrode (cathode) bonded together is sandwiched between a pair of conductive plates each having a reaction gas supply path therein (Fig. 2). Anode and cathode catalysts are generally Pt nanoparticles supported on carbon carriers.