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

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.