A 3D hierarchical porous Co₃O₄ nanotube network as an efficient cathode for rechargeable lithium-oxygen batteries

Herein, a three-dimensional (3D) hierarchical porous Co₃O₄ nanotube (Co₃O₄ HPNT) network was prepared using a polypyrrole nanofiber (PPyNF) as a sacrificial template. When employed as a cathode for lithium-oxygen batteries, the 3D Co₃O₄ HPNT network demonstrated superior bifunctional electrocatalytic activities towards both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), with a relatively low charge overpotential of 99 mV and a high discharge/charge capacity of 4164/4299 mA h g^-1. High-resolution scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements on Co₃O₄ HPNT-based cathode after discharge/recharge showed reversible formation and decomposition of Li₂O₂. This superior performance is ascribed to the 3D web-like porous tubular structure, which facilitates rapid oxygen flow, provides enough void volume for insoluble Li₂O₂ deposition, and increases the catalytic utilization of Co₃O₄. Moreover, the hierarchical porous structure with meso-/nanopores on the walls of the Co₃O₄ nanotubes facilitates O₂ diffusion, electrolyte penetration, and mass transport of all the reactants.