Confined growth of porous nitrogen-doped cobalt oxide nanoarrays as bifunctional oxygen electrocatalysts for rechargeable zinc–air batteries

Cobalt oxides are promising bifunctional oxygen electrocatalysts due to their intrinsic activities toward both dissociation and formation of oxygen molecules. Unfortunately, their practical utilization is seriously hindered by the limited accessible active sites and inferior activity. Herein, vertically-aligned porous nanoarrays composed of ultrafine nitrogen-doped cobalt oxide (NP-Co₃O₄) nanoparticles (4-5 ​nm) are in-situ fabricated on carbon cloth (CC) via mild oxidation of Co-based zeolitic-imidazolate-framework (Co-ZIF) nanoarrays. The oxidation decomposition of 2-methylimidazolate ligands between Co nodes not only confines the growth of cobalt oxides, but also provides the nitrogen sources for realizing low-temperature (200 ​°C) nitrogen doping. In alkaline electrolyte, the as-built NP-Co₃O₄/CC manifests an extremely high oxygen reduction reaction (ORR) half-wave potential of ~0.9 ​V and a decreased oxygen evolution reaction (OER) overpotential of 330 ​mV ​at 10 ​mA ​cm⁻², displaying a record low potential gap of 0.66 ​V. As bifunctional oxygen electrodes in rechargeable aqueous and all-solid-state flexible zinc-air batteries, the NP-Co₃O₄/CC-based batteries exhibit ultrahigh power densities up to 200 ​mW ​cm⁻² and 99.8 ​mW ​cm⁻³, respectively, which are much higher than those of reported bifunctional oxygen electrocatalysts. This work paves a new avenue for the fabrication of porous nitrogen doped metal oxides towards high-performance zinc-air batteries.