Structural Evolution of Fe Center in Fe–N/C Oxygen Reduction Reaction Electrocatalysts during Silica Coating-Mediated Synthesis

Abstract

Iron and nitrogen-doped carbon (Fe–N/C) catalysts are considered the most promising alternative to Pt-based catalysts due to their excellent oxygen reduction reaction (ORR) activity. Recently, we have developed a silica coating-mediated synthesis strategy that can preferentially generate ORR-active Fe–Nx sites while suppressing inactive Fe clusters. However, in previous studies, the origin of preferential formation of Fe–Nx active sites was not clearly revealed. In this work, we investigated how the active Fe center evolves during the steps of silica coating-mediated synthesis by X-ray absorption spectroscopy and 57Fe Mӧssbauer spectroscopy. We confirmed that the bond between the Fe center and Si of the silica is formed when the Fe–N–C composite is coated with silica. As a result, the catalyst prepared with silica coating comprised twice higher density of Fe–Nx sites than the catalyst without the silica coating, leading to twice higher current density.