High-Density Cobalt Nanoparticles Encapsulated with Nitrogen-Doped Carbon Nanoshells as a Bifunctional Catalyst for Rechargeable Zinc-Air Battery.

High efficient electrocatalytic activity and strong stability to both oxygen reduction reaction (ORR) and oxygen evolution (OER) are very critical to rechargeable Zn-air battery and other renewable energy technologies. As a class of promising catalysts, the nanocoposites of transition metal nanoparticles that are encapsulated with nitrogen-doped carbon nanoshells are considered as promising substitutes to expensive precious metal based catalysts. In this work, we demonstrate the successful preparation of high-density cobalt nanoparticles encapsulated in very thin N-doped carbon nanoshells by the pyrolysis of solid state cyclen-Co-dicyandiamide complex. The morphologies and properties of products can be conveniently tuned by adjusting the pyrolysis temperature. Owing to the synergetic effect of hybrid nanostructure, the optimized Co@N-C-800 sample possesses outstanding bifunctional activity for both ORR and OER in alkaline electrolyte. Meanwhile, the corresponding rechargeable zinc-air battery that is based on Co@N-C-800 air cathode also has excellent current density, low charge-discharge voltage gap, high power density, and strong cycle stability, making it a suitable alternative to take the place of precious metal catalysts for practical utilization.