Co nanoparticles encapsulated in N-doped carbon nanotube materials derived from new metal-organic frameworks for oxygen electrocatalysis

Bifunctional electrocatalysts used for oxygen reduction and evolution reactions (ORR/OER) are of great significance for metal-air batteries. Metal-organic frameworks (MOFs), after undergoing a high-temperature calcination process, emerge as promising precursors for preparing efficient metal-nitrogen-carbon (M-N-C) composites tailored specifically for electrocatalytic ORR/OER. Herein, we have successfully synthesized a novel rod-shaped MOF utilizing hydrazine hydrate and cobalt acetylacetonate as the building blocks, named Co-MOF. The Co-MOF has a tetrahedral Co-N4 coordination structure and has a high Co content. Upon undergoing the pyrolysis process, the corresponding M-N-C materials, featuring Co nanoparticles encapsulated within nitrogen-doped carbon nanotubes (Co@N-CNT), were obtained. These Co@N-CNT materials exhibit remarkable bifunctional ORR and OER performance with a half-wave potential of 0.88 V (versus reversible hydrogen electrode, vs. RHE) in 0.1 M KOH and an overpotential of 320 mV at 10 mA cm-2 in 1.0 M KOH, respectively. Moreover, when integrated into a Zn-air battery, the Co@N-CNT catalyst demonstrates unparalleled activities, boasting a peak power density of 235 mW cm-2 and a long-term cycling stability that extends beyond 160 hours. Theoretical calculations further revealed that active Co atoms within Co@N-CNT possess a high density of states (DOS) proximate to the Fermi level, coupled with a favorably low d-band center, both of which synergistically contribute to the enhanced ORR activity. This work offers profound insights into the fabrication of metal-encapsulated CNT materials based on novel frameworks enriched with Co, thereby creating a new path for the development of high-performance energy storage and conversion systems.