Construction of hollow urchin-like carbon frameworks with Co, P, and N doping and encapsulated Co/Co₂P heterojunction as bifunctional electrocatalysts for rechargeable Zn–air batteries

Controllable preparation of advanced electrocatalysts with well-designed architecture and desirable active sites for rechargeable Zn-air batteries (ZABs) is challenging. Herein, a hollow urchin-like bifunctional oxygen electrocatalyst (HCoPNC@Co/Co₂P-PNCNTs) was developed by assembling P, N-doped carbon nanotubes with confined Co/Co₂P heterojunctions on heteroatom-doped carbon hollow spheres <i>via in situ</i> polymerization/deposition, pyrolysis and topical phosphorization strategy. According to the analysis of experiments and theoretical calculations, the Co/Co₂P@NC core-shell structure at the top of each CNT was verified to facilitate the electron transfer and the conversion between the adsorbed intermediates, guaranteeing outstanding intrinsic catalytic activity. Meanwhile, the urchin-like hierarchical frameworks led to the formation of a desirable conductive and mass transport network, ensuring the efficient exposure of the electrocatalytic active sites and excellent structural stability. Therefore, the "framework-active sites" endowed HCoPNC@Co/Co₂P-PNCNTs with superior catalytic activities towards the oxygen reduction reaction (ORR, half-wave potential of 0.83 V) and oxygen evolution reaction (OER, overpotential of 350 mV@10 mA cm^-2) in alkaline electrolyte. Expectedly, the as-assembled ZABs based on HCoPNC@Co/Co₂P-PNCNTs delivered a large peak power density of 250 mW cm^-2 and a robust charge-discharge cycling stability with negligible voltage decay for 110 h at 2 mA cm^-2, illustrating its great practical application in advanced metal-air batteries.