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

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-N-x sites while suppressing inactive Fe clusters. However, the role of the silica layer in the formation of Fe-N-x sites remains elusive. In this study, we used X-ray absorption and Fe-57 Mossbauer 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 Fe-57 Mossbauer spectroscopy analyses, the formation of iron silicide (Fe-Si) species after silica coating was identified. Peak parameter analyses of Fe-57 Mossbauer spectroscopy data suggested that the density of active Fe-N-x 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