Improvement of Oxygen Reduction Reaction Performance of Fe-N-C Catalyst

Abstract

Fuel cells are gaining worldwide attention as an acceptable technology for achieving carbon neutrality. However, existing fuel cells use large amounts of precious metals, which limits their widespread adoption. Precious metal-free fuel cells have been studied in our laboratory. The oxidation-reduction reaction (ORR) at the cathode of the fuel cell must start at a higher potential and proceed smoothly in a four-electron reduction reaction without producing hydrogen peroxide1,2. We have synthesized iron-based complex catalysts using the method reported by Plamen Atanassov3 et al. Iron nitrate dihydrate is mixed in aqueous solution with glucose, 2-methylimidazole, and zinc nitrate hexahydrate, followed by hydrothermal synthesis at 200°C for 24 hours. It is then acid-treated with nitric acid to dissolve excess metallic iron, and the process is repeated twice at 950°C to complete the Fe-N-C catalyst. However, SEM and XRD analysis confirmed the presence of large amounts of metallic iron. In response to these results, the synthetic process of the Fe-N-C catalyst was repeatedly improved, and the prepared catalyst was subjected to in-situ XAFS (X-ray absorption fine structure) analysis using synchrotron radiation at SPring-8 (Fig. 1). The peak around 7110 eV is significantly different from that of metallic iron (Fe foil), indicating that iron complex, iron phthalocyanine (II), and it is like the peak of iron phthalocyanine (II), confirming that the prepared Fe-N-C catalyst forms a good complex. The performance of the Fe-N-C catalyst with the confirmed complexation was then investigated, and a cathode that exhibited ORR performance comparable to that of the Pt catalyst was successfully synthesized (Fig. 2). In the future, further improvement of the catalyst preparation process will be discussed to develop a catalyst with much better performance than Pt.Reference 1N. 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 2S. Kusano, D. Matsumura, K. Ishii, H. Tanaka, J. Mizuki, “Electrochemical Absorption on Pt Nanoparticles in alkaline Solution Observed Using in-situ High Energy Resolution X-ray Absorption Spectroscopy”, Nanomaterials, 9, 4, (2019), 642 3 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 Figure 1