A direct H₂O₂ production based on hollow porous carbon sphere-sulfur nanocrystal composites by confinement effect as oxygen reduction electrocatalysts

Carbon-sulfur composites have drawn increasing interest in various fields including electrocatalysis because of their unique structures. However, carbon-sulfur composite with tiny sulfur nanocrystals has still received little attention. Herein, hollow porous carbon sphere-sulfur composite (HPCS-S) which possesses excellent electrochemical performance towards H₂O₂ has been prepared for the first time via a simple silica template method. The 2–5 nm sulfur nanocrystals being restricted in the channel of the hollow porous carbon spheres are under a strong compressive stress, which was further confirmed by high-resolution transmission electron microscopy (HRTEM) and GPA. The HPCS-S nanocrystals show better conductivity than amorphous sulfur clusters because of the reduction of the steric hindrance which efficiently promotes the electron transportation. Consequently, the higher activity and selectivity towards the 2e^- oxygen reduction reaction (ORR) to H₂O₂ in alkaline solution was obtained. The H₂O₂ selectivity rises from 20% to over 70% after the sulfur addition and the H₂O₂ production rate achieves 183.99 mmol·g_catalyst^-1 with the Faradaic efficiency of 70%. Furthermore, performance enhancement mechanism was also investigated using the density functional theory (DFT) calculations. After the introducing of sulfur nanocrystals, the appearance of S–S bond greatly decreases the overpotential compared with S-doping, which results in significant enhancement of the electrocatalytic property. Consequently, the HPCS-S can be an efficient H₂O₂ production electrocatalyst in alkaline solution.