Tris(4-isocyanatophenyl)methane (TIPM) and N, N ′-dimethylformamide react at room temperature with no externally added catalyst to yield polyisocyanurate (PIR) gels. The obtained PIR gels were converted to N- and S-doped porous carbon monoliths by thermal treatment at 1000 °C with elemental sulfur under inert conditions. The PIR linkage acts as precursor for carbon and nitrogen, and %S doping was varied by changing the concentrations of elemental sulfur during pyrolysis. The optimized concentration of sulfur (5.6%) into the carbon matrix displayed excellent oxygen reduction activity with direct four-electron transfer relative to its pristine counterparts by (1) introducing micro- and mesopores in addition to the already existing macropores by etching the carbon surface (confirmed by N 2 sorption isotherms and microscopic images) with the increase in the external surface area providing more active centers and efficient diffusion of electrolyte ions, (2) providing more – C–S–C– active species than oxidized sulfur species (confirmed by XPS and FT-IR) with more oxygen adsorption sites, and (3) filling the micropores of the carbon as a monolayer, affording increased electronic conductivity to the amorphous carbon. This simple and facile method of incorporating N- and S- together into the porous carbon matrix can be considered as an alternate for nonprecious metal catalysts for oxygen reduction reaction.
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Chandrasekaran et al. (2018) studied this question.