Popcorn-like Co3O4 nanoparticles confined in a three-dimensional hierarchical N-doped carbon nanotube network as a highly-efficient trifunctional electrocatalyst for zinc-air batteries and water splitting devices

A novel unique popcorn-like three-dimensional (3D) hierarchical structural electrocatalyst is synthesized by the pyrolysis of ZIF-8/ZIF-67 and polyacrylonitrile fiber composites, where popcorn-like Co3O4 nanoparticles coated with nitrogen-doped amorphous carbon anchor onto the tips of N-doped carbon nanotubes (NCNTs), and the NCNTs grow on carbon nanofiber (NAC@Co3O4/NCNTs/CNF). Owing to the positive synergistic effects of 3D hierarchical NCNT networks and popcorn-like Co3O4 species, NAC@Co3O4/NCNTs/CNF shows a record outstanding hydrogen evolution reaction performance (HER; the overpotential at 10 mA cm(-2) is just 76 mV) and wonderful stability for the oxygen reduction reaction (ORR; 73% retention of initial ORR activity after 70 h). Density functional theory (DFT) calculations demonstrate that the chemical interaction between the popcorn-like Co3O4 species and NCNTs benefit the chemisorption of hydrogen and oxygen-containing intermediates, thus accelerating the performance of ORR, OER, and HER. Furthermore, we assembled NAC@Co3O4/NCNTs/CNF as a rechargeable Zn-air battery and an overall water splitting device, displaying a maximum power density of 267.58 mW cm(-2) and almost 100% faradaic efficiency, which provides a promising prospect for surpassing traditional trifunctional electrocatalysts. It is expected that this finding will offer a new concept for synthesizing highly efficient trifunctional materials and applications.