Structural Evolution of Atomically Dispersed Fe Species in Fe–N/C Catalysts Probed by X-ray Absorption and ⁵⁷Fe Mössbauer Spectroscopies

Abstract

Iron and nitrogen codoped carbon (Fe–N/C) catalysts are considered the most promising nonprecious metal catalysts for the oxygen reduction reaction (ORR), with their activities approaching those of Pt-based catalysts. Recently, silica-based protective-layer or intermediate layer-assisted synthesis strategies have been developed to preferentially generate catalytically active Fe–Nx sites while suppressing inactive Fe clusters. However, the role of the silica layer in the formation of Fe–Nx sites remains elusive. In this study, we used X-ray absorption and 57Fe Mössbauer spectroscopies to determine the evolution of the structure of Fe-based species during the silica-coating-mediated synthesis. Through X-ray absorption near-edge structure and 57Fe Mössbauer spectroscopy analyses, the formation of iron silicide (Fe–Si) species after silica coating was identified. Peak parameter analyses of 57Fe Mössbauer spectroscopy data suggested that the density of active Fe–Nx species with the Fe–N/C catalyst prepared with silica coating was twice as high as that of the Fe–N/C without silica coating. Consequently, the Fe–N/C catalyst with silica coating exhibited a kinetic current density for the ORR (0.9 V vs reversible hydrogen electrode, RHE) twice as high as that without silica coating.