Hierarchical Carbon Nanocages Embedding High-loading Sulfur for Catalyzing Oxygen Reduction Reactions

The carbon-based nanomaterials have attracted intensive interests as promising metal-free electrocatalysts to reduce or even eliminate the use of precious-metal catalysts on oxygen reduction reactions (ORR). In this regard, we developed 3D hollow carbon nanocages confining high-loading sulfur fabricated by chemical vapor deposition (CVD) method employing self-sacrificial templates (basic magnesium carbonate). Under different fabrication temperatures, the as-designed samples obtained different morphologies, compositions and structural properties. With the largest specific surface area (1306 m² g⁻¹), highest heteroatom doping content (6.04 at.% of S) and well-balanced pore distribution, the electrocatalysts synthesized under 800 °C could catalyze the ORR process through the ideally efficient four-electron transfer pathway. The remarkable catalytic selectivity and stability of the optimal material in alkaline electrolyte resulted from the synergistic effects of structural and chemical characteristics. Moreover, primary Zn-air batteries built with the sulfur loaded carbon nanocage air electrodes revealed high open-circuit voltage (1.42 V) and good stability in comparison with the counterpart using commercial Pt/C catalyst. These findings provide a new avenue for designing the metal-free electrocatalysts for various renewable energy storage and conversion technologies.