Sulfate Ions Induced Concave Porous S-N Co-Doped Carbon Confined FeCx Nanoclusters with Fe-N4 Sites for Efficient Oxygen Reduction in Alkaline and Acid Media.

To improve the catalytic activity of the catalysts, it is key to intensifying the intrinsic activity of active sites or increasing the exposure of accessible active sites. In this work, an efficient oxygen reduction electrocatalyst is designed that confines plentiful FeCx nanoclusters with Fe-N4 sites in a concave porous S-N co-doped carbon matrix, readily accessible for the oxygen reduction reaction (ORR). Sulfate ions react with the carbon derived from ZIF-8 at high temperatures, leading to the shrinkage of the carbon framework and then forming a concave structure with abundant macropores and mesopores with S incorporation. Such an architecture promotes the exposure of active sites and accelerates remote mass transfer. As a result, the catalyst (Fe/S-NC) with a large number of C-S-C, Fe-N4 , and FeCx nanoclusters presents impressive ORR activity and stability. In alkaline media, the half-wave potential of the best catalyst (Fe/S2 -NC) is 0.91 V, which far exceeds that of commercial platinum carbon (0.85 V), while in acidic media the half-wave potential reaches 0.784 V, comparable to platinum carbon (0.812 V). Furthermore, for the zinc-air battery, the outstanding peak power density of Fe/S2 -NC (170 mW cm-2 ) superior to platinum carbon (108 mW cm-2 ) also highlights its great application potential.