Nitrified coke wastewater sludge flocs: an attractive precursor for N,S dual-doped graphene-like carbon with ultrahigh capacitance and oxygen reduction performance

The handling of a huge amount of sludge produced from industrial wastewater treatment plants is a critical issue. We report a facile and cost-effective pyrolysis approach to transform coke wastewater sludge flocs into valuable carbon materials that show the potential of being used in energy-storage devices and fuel cells. The nitrified sludge flocs are naturally rich in carbon, nitrogen, sulfur, and other inorganic particles and thus are attractive precursors for producing N,S dual-doped carbon with a hierarchical mesoporous graphene-like structure via the simple one-step pyrolysis method without the addition of external N- and/or S-containing organic compounds, chemical activation agents, or graphitization catalyst precursors. Owing to its unique features, the resulting nitrified sludge floc derived carbon (NSFC) exhibits outstanding capacitive performance. The specific capacitance determined in 1 M H2SO4 at a current density of 1 A g(-1) is 889 F g(-1), the highest among reported values for carbon-based materials in inorganic electrolytes, as far as we are aware. The NSFC also shows an excellent cycling stability with only 1.2% loss in capacitance after 10 000 cycles at a current density of 20 A g(-1). The NSFC also achieves superior activity towards the oxygen reduction reaction (ORR) and proves to be a promising metal-free ORR electrocatalyst showing comparable electrocatalytic performance, higher selectivity, and longer durability as compared to the commercial Pt/C benchmark.