A ZIF-triggered rapid polymerization of dopamine renders Co/N-codoped cage-in-cage porous carbon for highly efficient oxygen reduction and evolution

The ability to develop highly-efficient bifunctional electrocatalysts via facile and cost-effective routes is of practical interest for enabling concurrent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we report a simple yet robust strategy via zeolitic imidazolate framework (ZIF)-triggered rapid polymerization of dopamine for crafting cage-in-cage hollow porous carbon codoped with Co and N for highly-efficient ORR and OER electrocatalysts. First, dopamine (DA) is expeditiously polymerized on ZIF-67 within 10 min at room temperature, yielding yolk-shelled ZIF-67@polydopamine-x (denoted ZIF-67@PDA-x) particles with tunable PDA thickness. This contrast sharply to the copious past works where alkaline compounds are needed for the polymerization of DA. More importantly, the ZIF-67-triggered polymerization of DA is substantiated to undergo a coordination-dissociation-polymerization mechanism rendered by the synergistic effect of Co²⁺ and 2-methylamidazole. Subsequent one-step pyrolysis of ZIF-67@PDA yields Co and N codoped cage-in-cage porous carbon (denoted Co/N CCPC-x). The resulting Co/N CCPC pyrolyzed at 600 °C manifests a superior ORR with the onset and half-wave potentials comparable to Pt/C and limiting current density larger than Pt/C, OER with low overpotential and potential difference, and Zn-air performance with high open-circuit voltage, specific capacity, power density and remarkable charge-discharge reversibility. Such impressive ORR and OER performances of Co/N CCPC are a direct consequence of simultaneous compositional (i.e., Co and N codoping) and structural (i.e., cage-in-cage architecture with hierarchical porosity) tailoring enabled by judicious PDA coating.