An efficient carbon-based ORR catalyst from low-temperature etching of ZIF-67 with ultra-small cobalt nanoparticles and high yield

Abstract

A facile and controllable low-temperature (450 °C) route is reported to produce ultra-small Co₃O₄/Co nanoparticles in nitrogen-doped hyperporous graphenic networks (Co₃O₄/Co@N-G-450). Firstly, a monolayer of ZIF-67 nanocrystals is directly grown in thermal-shock exfoliated graphene networks (EGO) of suitable porosity and pore-widths. Later, the ZIF-67 is etched by targeting the small concentrations of residual oxygen functionalities on EGO (≈13 atom%) under a nitrogen atmosphere at 450 °C. Therefore, the partial gasification of ZIF-67 followed by oxidation of the resultant open cobalt metal centres produces a highly active nanophase of Co₃O₄/Co@N-G in a mass yield of >65 wt%. The as-synthesised Co₃O₄/Co@N-G-450 catalyst, without any further acid washing or oxidation process, exhibits an outstanding ORR performance with a high onset (0.962 V vs. RHE) and half-wave (0.808 V vs. RHE) potential as well as limiting current density (5.2 mA cm⁻²) in 0.1 M KOH solution. These merits are comparable to those of commercial Pt/C and many ZIF-derived catalysts, synthesised under extended and complex chemical treatment. Moreover, the catalyst also exhibits fast reaction kinetics with a dominant 4-electron reaction pathway and high durability.