Synchrotron Radiation Study of Fe-N-C Catalysts for Dissemination of Fuel Cells
Authors: Nakamura Genki , Tanaka Hirohisa 
Published: December 2024

Fuel cells are attracting worldwide attention as a technology for achieving carbon neutrality. However, existing fuel cells use large amounts of precious metals, which limits their widespread use. In our laboratory, we are studying fuel cells that do not use precious metals. We have synthesized iron-based complex catalysts using the method reported by Plamen Atanassov1 et al. Iron nitrate nonahydrate is mixed in aqueous solution with glucose, 2-methylimidazole, and zinc nitrate hexahydrate, followed by hydrothermal synthesis at 200°C for 24 hours. Hydrogen heat treatment is performed at 950 °C, followed by acid treatment with nitric acid to dissolve excess metallic iron. Then, ammonia heat treatment is performed at 950 °C to complete the Fe-N-C catalyst. In our laboratory, we have been working on various process improvements to reduce the amount of metallic iron to make the four-electron oxygen reduction reaction proceed more seamlessly. What is particularly noteworthy is that we have dramatically reduced the amount of iron nitrate dihydrate, a catalyst raw material, by 1/40.

The results of the XAFS measurement of the reduced catalyst at SPring-8 BL14B1 showed that the prepared Fe-N-C catalyst formed a good complex, since the peak was like that of iron phthalocyanine (II), an iron complex, which was very different from the peak of metallic iron (Fe foil) (Figure 1). Through these process improvements, we succeeded in creating a catalyst that exceeds the performance of current platinum catalysts. The structure of the catalyst changes the ease of oxygen adsorption. The prepared catalysts were used to determine the catalyst fine structure by High-Energy-Resolution-Fluorescence-Detected (HERFD-XAFS)2 and Kβ X-ray emission spectroscopy (XES) measurements using synchrotron radiation at BL-11XU in SPring-8, where the spectral broadening due to the inner-shell lifetime width is suppressed.

Using a half-cell with only a cathode, the prepared Fe-N-C catalyst was sprayed onto carbon paper, and a potentiostat was manipulated to measure the adsorbed species on the Fe surface at a specific potential.We used N2 and O2 gases and discussed the structure and valence of the catalyst by the peak transitions, focusing on Fe in the Fe-N-C catalyst, The structure of Fe was discussed based on the energy position transition for each potential.

The structure of Fe-N-C oxide was discussed based on the energy of the Fe K-edge (7115eV) in high-resolution XAFS (Fig. 2).The catalysts were measured on Fe-N-C catalysts prepared in the laboratory and were a mixture of planar 4- and 6-coordinated catalysts. From these results, it was considered that the highly active iron complex catalyst we prepared was a hybrid of multiple complex structures. The structure and activity will be discussed in an oral presentation.

References

1 R. Gokhale, L-K. Tsui, K. Roach, Y. Chen, M. M. Hossen, K. Artyushkova, F. Garzon, P. Atanassov, “Hydrothermal synthesis of platinum-Group-Metal-Free Catalysts: Structural Elucidation and Oxygen Reduction Catalysis”, ChemElectroChem, 5, 14, (2017), 1848-1853

2N. Yamamoto, D.Matsumura, Y. Hagihara, K.Tanaka, Y. Hasegawa, K. Ishii, H. Tanaka, “Investigation of hydrogen superoxide adsorption during ORR on Pt/C catalyst in acidic solution for PEFC by in-situ high energy resolution XAFS”, Jounal of Power Sources, 557, 15, (2023), 232508

Acknowledgments

The authors would like to express their sincere appreciation and celebration to Professor Dr. P. Atanassov of University of California, Irvine for his contribution to the fuel cell research. The authors would like to acknowledge Dr. S. Kusano and Emeritus Professor Dr. J. Mizuki of Kwansei Gakuin University for their cooperation in establishing the in-situ XAFS measurement method. And they also thank Dr. H. Kishi, Dr. T. Sakamoto and Dr. K. Asazawa of Daihatsu Motor Co., Ltd. for development of electrochemical cells.

The synchrotron radiation experiments were performed at the BL11XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Numbers of 2022B-H08,2023A-H06,2023B-H08,2024A-H10)

The SPring-8 experiment was also supported by the Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM Japan) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Grant No. JPMXP1222QSO108, JPMXP1223QS0006, JPMXP1223QS0108, JPMXP1224QS0010)

This work was supported by JSPS KAKENHI Grant Number JP22H02188)

Figure 1