Effect of Carbon Surface Features on the Formation and Stabilization of γ-Sulfur Within the Pore Structure of Sulfur-Tuned Carbons.

Surface chemistry of porous carbon black was modified by oxidation either with hydrogen peroxide or nitric acid, and also by a thermal treatment with urea. The latter reduced the surface and introduced nitrogen groups to the carbon matrix. Sulfur was inserted into carbon pores using a steam-assisted sulfur insertion method. It resulted in a gradual and controllable pore filling, from ultramicropores to mesopores. Orthorhombic α-sulfur and monoclinic γ-sulfur were detected. In small pores, only Sx linear fragments could be formed. The fraction of γ-sulfur increased with an increased electrical conductivity and the amount of intrinsic defects of the initial carbon hosts. While an increase in the former could be directly linked to thermal conductivity, the defectous carbon likely binds to sulfur, helping to form and stabilize monoclinic crystals. Confinement effects further contributed to stabilizing metastable sulfur allotropes by physically limiting mobility and retarding phase transitions. Although amorphous sulfur was also formed during fast cooling, higher fractions of γ-sulfur were detected in the crystalline phase, especially in most conductive carbons. These findings highlight a complex interplay among carbon chemistry, microstructure, and electronic properties affecting the formation and stabilization of γ-sulfur and provide insights into sulfur-carbon interactions for designing advanced sulfur-based carbon materials.