Host-guest interactions promoted formation of Fe-N₄ active site toward efficient oxygen reduction reaction catalysis

FeNC is the most promising material to replace the noble metal catalyst for cathodic oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, the practical performance of FeNC catalyst is significantly limited by its low active site (Fe-N₄) density. Herein, we propose to promote the formation of Fe-N₄ active sites in FeNC catalyst by strengthening the interaction of N precursors and Fe precursors during the carbonization synthesis. In our approach, ionic liquid (IL, [EMIM][NTf₂]) with high nitrogen content and good thermal stability is caged in the pores of Fe-ZIF-8 through the host-guest interactions. These interactions are critical for the preservation of Fe and N species and formation of active sites during the synthesis. The optimal catalyst developed with this approach (Fe_0.05NC/10) has a high density of accessible Fe-N₄ sites (1.88*10¹⁹ sites g^-1). Therefore, in both acidic and alkaline media, Fe_0.05NC/10 showed excellent ORR activity comparable to commercial Pt/C catalyst. Moreover, PEMFC performance with a peak power density of 300 mW cm^-2 was demonstrated with Fe_0.05NC/10 under H₂/O₂ conditions. The synthetic approach reported herein may be used for tailoring of advanced catalyst with high intrinsic activity.