Single iron site catalysts with increased metal-site loading via a high-temperature imprinting approach for proton exchange membrane fuel cells

Abstract

Fe–N–C materials have been widely accepted as the most promising catalysts to replace Pt in future fuel cells. However, the loading of active atomic Fe sites in catalysts remains insufficient (<1.0 wt%) due to Fe agglomeration and carbothermal reduction during the synthesis at elevated heating temperatures (>900 °C). Here, we explored an active-site imprinting approach to convert less active ZnN_x or nitrogen vacancies (V-N_x) into FeN₄. We demonstrated that the reaction barrier of ZnN₄ to FeN₄ (trans-metalation) pathways is significantly lower than that of V-N₄ to FeN₄ (metalation) ones, indicating the importance of forming high-loading ZnN₄ sites first. FeCl₂ precursors are preferable over FeCl₃ during active-site imprinting despite their relatively high boiling point. Eventually, the high-temperature active-site imprinting strategy based on a vacuum-sealed reaction system enables an Fe–N–C catalyst containing exceptionally high atomic Fe site loading up to 5.65 wt%. The resulting catalyst exhibited encouraging ORR activity and stability in challenging acidic media.