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Iron pyrite (FeS2) has attracted significant attention as a promising inorganic material in various applications, such as electrode materials for high-energy batteries, medical diagnostics, semiconductor materials, and photovoltaic solar cells. In this study, we characterized the crystalline structure and magnetic properties of FeS2 using X-ray diffraction (XRD), vibrating sample magnetometry, and Mössbauer spectroscopy. The refined XRD patterns confirmed that the crystalline structure of FeS2 was cubic (Pa-3 space group) with lattice constant a0 = 5.417 Å. The temperature dependence of the zero-field-cooled and field-cooled curves and the hysteresis loops were measured at various temperatures between 4.2 and 295 K. The Mössbauer spectra collected in the temperature range of 4.2–500 K were fitted with one doublet. The ΔEQ values increased slightly with decreasing temperature owing to changes in the Fe–S distance. The charge state was determined to be Fe2+ based on the isomer shift (δ).

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, silicabased 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 57 Fe 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 57 Fe Mossbauer spectroscopy analyses, the formation of iron silicide (Fe−Si) species after silica coating was identified. Peak parameter analyses of 57 Fe 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.

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