Influence of different Fe doping strategies on modulating active sites and oxygen reduction reaction performance of Fe, N-doped carbonaceous catalysts

Fe/N/C catalysts, synthesized through the pyrolysis of Fe-doped metal–organic framework (MOF) precursors, have attracted extensive attention owing to their promising oxygen reduction reaction (ORR) catalytic activity in fuel cells and/or metal-air batteries. However, post-treatments (acid washing, second pyrolysis, and so on) are unavoidable to improve ORR catalytic activity and stability. The method for introducing Fe3+ sources (anhydrous FeCl3) into the MOF structure, in particular, is a critical step that can avoid time-consuming post-treatments and result in more exposed Fe-Nx active sites. Herein, three different Fe doping strategies were systematically investigated to explore their influence on the types of active sites formed and ORR performance. Fe-NC(Zn2+), synthesized by one-step pyrolysis of Fe doped ZIF-8 (Zn2+) precursor which was obtained by adding the anhydrous FeCl3 source into the Zn(NO3)2•6H2O/methanol solution before mixing, possessed the highest Fe-Nx active sites due to the high-efficiency substitution of Zn2+ ions with Fe3+ ions during ZIF-8 growth, the strong interaction between Fe3+ ions and N atoms of 2-Methylimidazole (2-MIm), and ZIF-8′s micropore confinement effect. As a result, Fe-NC(Zn2+) presented high ORR activity in the entire pH range (pH = 1, 7, and 13). At pH = 13, Fe-NC(Zn2+) exhibited a half-wave potential (E1/2) of 0.95 V (vs. reversible hydrogen electrode), which was 70 mV higher than that of commercial Pt/C. More importantly, Fe-NC(Zn2+) showed superior ORR stability in neutral media without performance loss after 5,000 cycles. A record-high open-circuit voltage (1.9 V) was obtained when Fe-NC(Zn2+) was used as a cathodic catalyst in assembled Mg-air batteries in neutral media. The assembled liquid and all-solid Mg-air batteries with high performance indicated that Fe-NC(Zn2+) has enormous potential for use in flexible and wearable Mg-air batteries.