Controlled synthesis of porous nitrogen-doped carbon nanoshells for highly efficient oxygen reduction

Abstract

Hollow nanostructures, owing to their unique structural features, have attracted tremendous attention in electrochemical energy storage and conversion. Herein, we develop a facile and controllable, but rational and effective, method to prepare a highly active hollow nitrogen-doped carbon nanoshell (termed as HNS-800) oxygen reduction electrocatalyst. The hollow nano-shelled catalyst was synthesized by using 9,11,20,22-tetraaza-tetrapyridopentacene (tatpp) as the building block and coated with silica nano-spheres as the template, followed by carbonization at 800 °C and finally chemical etching. The average thickness of the nanoshell is ∼10 nm and about three times improvement in the specific surface area (from 253 to 647 m² g⁻¹) was obtained owing to the formation of the nanoshell structure. Compared with the traditional solid nanorod structure prepared without the controlled template (termed as NR-800), the newly formed hollow nanoshell structure contributed to the huge promotion of ORR in both alkaline and acidic media. The as-obtained HNS-800 exhibited very efficient activity with a high kinetic current density (J_k) of 18.02 mA cm⁻² at 0.8 V vs. RHE and a small Tafel slope of 69 mV dec⁻¹. In particular, the half-wave potential of HNS-800 (0.87 V vs. RHE) was higher by 25 mV than that of the benchmark Pt/C in 0.1 M KOH. Unlike the commercial Pt/C electrode, the HNS-800 electrocatalyst was almost free from the methanol crossover effect and showed a better stability.