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 metal-based 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 iron-nitrogen-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 FeN_x moieties 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 mV after 30,000 cycles. Meanwhile, its practical PEMFC performance is a maximum power output of 628.0 mW cm⁻² and a slight power density loss is 83.0 mW cm⁻² after 200-cycle practical operation. Additionally, theoretical calculation shows that the activity of FeN_x moieties 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.