Sulfur self-doped char with high specific capacitance derived from waste tire: Effects of pyrolysis temperature

Abstract Preparation of sulfur self-doped char derived from waste tire (WT) was realized via two successively processes of pyrolysis and activation treatment. WT was firstly pyrolyzed at 400 °C, 600 °C, 800 °C, and 1000 °C to collect waste tire chars (WTCs) and they were subsequently activated at 800 °C with potassium ferrate (K2FeO4). The specific capacitance of activated waste tire chars at different pyrolysis temperatures (AWTCs-x-800) decreased from 92.60 F/g to 54.05 F/g at 1 A/g with pyrolysis temperature rising from 400 °C to 1000 °C. As for AWTCs-x-800, higher pyrolysis temperature promoted pore-forming process before 800 °C, and higher pyrolysis temperature enlarged pores after 800 °C. Increase of pyrolysis temperature promoted decomposition of alkyl–aryl C C bonds, transformation of relative small to large aromatic ring system, ordered arrangement of carbon atoms. Besides, it was found that sulfur doping content dominated in specific capacitance performance before 800 °C while surface area dominated after 800 °C. The large surface area and high S 2p3/2 (-C-S-C-, sulphide bridge) content were beneficial for the larger specific capacitance while more S 2p5/2 (-C-SOx-C- (x = 2–4, sulphone bridge) had the negative effect. Pyrolysis mainly affected sulfur doping properties, lower pyrolysis temperature promoted sulfur enrichment and S 2p3/2 generation. Activation promoted surface area improvement and sulfur conversion, higher pyrolysis temperature promoted surface area improvement and sulfur release before 800 °C while the promotion effects weakened after 800 °C, and sulfur transformation of S 2p3/2 converting to S 2p5/2 strengthened at higher pyrolysis temperature.