Cobalt nanoparticles embedded in the N-doped carbon nanospheres as efficient oxygen catalysis for rechargeable flexible Zn-air batteries

Rechargeable Zn-air batteries are considered as a promising energy storage device due to their high energy density. Nevertheless, the sluggish kinetics of the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) involved in the Zn-air batteries seriously hinders their practical application. Hence, in this work, a novel and effective method is developed to synthesize ultrafine cobalt nanoparticles (about 35 nm in diameter) embedded in nitrogen-doped carbon spheres (labeled as Co/NDC). Benefiting from the uniform distribution of the ultrafine cobalt nanoparticles, the prepared Co/NDC catalyst owns a large specific surface area and abundant active sites. As shown in the electrochemical test, the Co/NDC bifunctional electrocatalyst prepared in this article exhibits advanced electrochemical performance. The Co/NDC electrocatalyst displays superior electrochemical activities with a half-wave potential of 0.76 V (vs. RHE) for ORR and a low overpotential of 460 mV at the current density of 10 mA·cm⁻² for OER. More significantly, the integrated rechargeable liquid-state Zn-air battery (ZAB) using Co/NDC electrocatalyst as air cathode shows a peak power density up to 149 mA·cm⁻², an open-circuit voltage (OCV) of 1.428 V and a long-term cycle stability for more than 50 h. In addition, the self-made flexible rechargeable ZABs also display excellent performance even under different bending and twisting conditions. Therefore, Co/NDC catalyst can effectively serve as a promising candidate for bifunctional electrocatalyst.