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 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 center dot 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.