Sulfur doped iron-nitrogen-hard carbon nanosheets as efficient and robust noble metal-free catalysts for oxygen reduction reaction in PEMFC

Transition metal-nitrogen-carbon(M-N-C) as a promising substitute for the conventional noble metalbased catalyst still suffers from low activity and durability for oxygen reduction reaction(ORR) in proton exchange membrane fuel cells(PEMFCs). To tackle the issue, herein, a new type of sulfur-doped ironnitrogen-hard carbon(S-Fe-N-HC) nanosheets with high activity and durability in acid media were developed by using a newly synthesized precursor of amide-based polymer with Fe ions based on copolymerizing two monomers of 2, 5-thiophene dicarboxylic acid(TDA) as S source and 1, 8-diaminonaphthalene(DAN) as N source via an amination reaction. The as-synthesized S-Fe-N-HC features highly dispersed atomic Fe Nxmoieties embedded into rich thiophene-S doped hard carbon nanosheets filled with highly twisted graphite-like microcrystals, which is distinguished from the majority of M-N-C with soft or graphitic carbon structures. These unique characteristics endow S-Fe-N-HC with high ORR activity and outstanding durability in 0.5 M H2SO4. Its initial half-wave potential is 0.80 V and the corresponding loss is only 21 m V after 30,000 cycles. Meanwhile, its practical PEMFC performance is a maximum power output of 628.0 mW cm-2and a slight power density loss is 83.0 m W cm-2after 200-cycle practical operation.Additionally, theoretical calculation shows that the activity of Fe Nxmoieties on ORR can be further enhanced by sulfur doping at meta-site near FeN4C. These results evidently demonstrate that the dual effect of hard carbon substrate and S doping derived from the precursor platform of amid-polymers can effectively enhance the activity and durability of Fe-N-C catalysts, providing a new guidance for developing advanced M-N-C catalysts for ORR.