Sulfur doped FeNC catalysts derived from Dual-Ligand zeolitic imidazolate framework for the oxygen reduction reaction

Iron-nitrogen-carbon (FeNC) catalysts derived from zeolitic-imidazolate frameworks (ZIFs) are worldwide accepted to be the most promising candidates for the oxygen reduction reaction (ORR), but the insufficient stability, the low FeN_x exposure and poor density restrict their ORR activity. Here, we demonstrate a strategy to synthesize FeN_x sites embedded in a micro/mesoporous N, S co-doped graphitic carbon (FeNC/MUS) by tuning the ligand linkers via the addition of 2-undecylimidazole as a co-ligand in ZIF precursors, and optimizing the electronic structure of Fe center by an in-situ addition of thiourea molecules as sulfur (S) source. 2-undecylimidazole offered an open porous structure to incorporate more FeN_x, while the S-doping increased the density of FeN_x. Besides, 2-undeclyimidazole cooperatively with S-doping caused favorable changes into the catalyst structure, particularly improved the exposure and density of FeN_x sites and doubled the Brunauer-Emmetter-Teller surface area to 1132 m² g^-1 contrasted to the pristine FeNC/M (544 m² g^-1). FeNC/MUS displayed an accelerated ORR activity with a higher half-wave potential of 0.86 V (vs. reversible hydrogen electrode (RHE)) than that of Pt/C (0.84 V) in addition of a longer durability with a 11 % of activity decay after 30000 s in alkaline media. This work offers a new insight to design optimal ZIFs precursor and a facile electron withdrawing S-doping strategy for efficient electrocatalysis.