Analysis of waste tire-based sulfur doped porous carbon from pyrolysis with potassium salts

Waste tires (WTs) are a major solid waste stream. Pyrolysis is a promising disposal method, generating ∼40 wt% char, which can be upgraded into sulfur doped porous carbon. This work systematically analyzes WT-based sulfur doped porous carbon from pyrolysis with potassium salts (KOH, K₂CO₃, and K₂FeO₄) activation for high specific capacitance. All three potassium salts effectively promote the formation of smaller pores, facilitate the transformation of aromatic rings and alkyl-aryl C-C bonds into ordered graphitic microcrystal. KOH and K₂CO₃ primarily promote the formation of sulfide bridge, whereas K₂FeO₄ significantly promotes sulfone bridge. K₂FeO₄ produces the largest specific capacitance (111.9 F/g at 1 A/g) with lowest activated char yield (16.6 wt%). K₂FeO₄ mainly increases the specific capacitance by significantly enhancing specific surface area by comparing with KOH and K₂CO₃. K atom and lattice oxygen facilitate the etching of char for mainly promoting mesopores and macropores, and Fe atom is conducive to micropores. K and Fe atoms both contribute to the transformation of disordered amorphous carbon into ordered graphitic microcrystal. Lattice oxygen destroys ordered graphitic microcrystals and promotes the conversion of sulfide bridges into sulfone bridges. While K and Fe atoms can react with lattice oxygen to inhibit this conversion pathway, they also react with sulfone bridges to form inorganic sulfur. The findings are beneficial for adjusting the structure of sulfur doped porous carbon from WTs.