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Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal–organic frameworks (MOFs). These robust, porous materials have garnered significant interest because of their outstanding properties as catalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, the synthesis of Co‐ and Ni‐containing ZIF‐67 embedded in carbon nanotubes (CNTs) to be used as a cathode material for rechargeable zinc–air batteries is described. Carbon‐nanotube‐encapsulated metallic Co and Ni form via calcination of the MOF at 700 °C for 4 h and are characterized by X‐ray diffraction and transmission electron microscopy (TEM). Co and Ni graphitization within the CNTs is analyzed by Raman spectroscopy, and morphologies are assessed by scanning electron microscopy (SEM) and TEM. Excellent electrochemical performance of the prepared catalysts during charging and discharging reveals that the catalyst is stable for well over 500 h. Moreover, superior OER performance is observed when CNT@Co or CNT@Ni is used as a bifunctional catalyst in the zinc–air battery. [ABSTRACT FROM AUTHOR]

A metal–organic framework (MOF) was synthesized using 4,4′‐stilbenedicarboxylic acid (H2SDA) in dimethylformamide (DMF). Carbon‐coated Co (C@Co) catalysts were prepared by the direct calcination of Co3(SDA)3(DMF)2 MOF crystals at various temperatures. The highest surface area of the C@Co catalyst is obtained at 600 °C which can further be increased by the ball mill operation. The X‐ray photon spectroscopy and X‐ray diffraction analysis proved the formation of metallic Co in the carbon matrix at temperatures over 500 °C, while Co3O4 formation was observed at 500 °C. Furthermore, the graphitic properties of the Co particles were analyzed by Raman spectroscopy. The durability of the carbon‐coated Co catalysts derived from MOFs (C@Co catalysts) was analyzed by the cycle test (charge/discharge at 20 mA/cm2 per 2 h) and was retained over 350 h. These catalysts showed remarkable stability for both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction), comparable to that of Co3O4 and Co metal powder in alkaline solution. [ABSTRACT FROM AUTHOR]

• Synthesis and characterization of Co 3 O 4 -NFs by the calcination of Co(OH) 2 @SiO 2. • Fabrication of Ni@Co 3 O 4 -NFs by chemical deposition of nickel nanoparticles. • Higher catalytic activity for the OER and ORR. • Higher stability and longer life cycles. Co 3 O 4 nanoflowers (NFs) were obtained from cobalt hydroxide coated on silica, Co(OH) 2 @SiO 2 , prepared from a homogenous mixture of urea and cobalt nitrate in an aqueous solution. The Co 3 O 4 @SiO 2 was obtained by calcining Co(OH) 2 @SiO 2 under atmospheric air for 3 h and then immersing in sodium hydroxide to yield Co 3 O 4 -NFs. The Ni-deposited Co 3 O 4 NFs (Ni@Co 3 O 4 -NFs) were synthesized by chemical deposition of nickel nanoparticles onto Co 3 O 4 -NF by a reducing agent, NaBH 4. The physical characterization of Co 3 O 4 -NFs and Ni@Co 3 O 4 -NFs was performed by Brunauer-Emmett-Teller analysis, scanning electron microscopy and X-ray diffraction. The electrochemical properties of Co 3 O 4 -NFs and Ni@Co 3 O 4 -NFs for the oxygen reduction and evolution reaction were determined in an 8 M KOH solution. Ni@Co 3 O 4 -NF showed a better cell performance than that of Co 3 O 4 -NF. Moreover, Ni@Co 3 O 4 -NF exhibited a higher stability and longer life cycles at 20 mA cm−2 at a 2 h charging and discharging rate as compared to Co 3 O 4 -NF. Therefore, Co 3 O 4 -NF and Ni@Co 3 O 4 -NF could be used as a cathode material for Zn-air rechargeable batteries. [ABSTRACT FROM AUTHOR]