Transition metal ions-chelated COFs derived bifunctional oxygen catalysts for rechargeable Zn-air batteries

Improving energy conversion efficiency and battery cycle stability is an essential goal in energy conversion and storage, while a critical factor in achieving this goal is the design of effective bifunctional catalysts. The covalent organic framework is a new type of high molecular material that can be employed as an ideal template for quantitative chelate metal ions to synthesize highly efficient bifunctional catalysts with high dispersion metal active sites. In this work, the Fe₂Ni₁/NiFe₂O₄@NCG bifunctional catalysts are constructed by employing metal-chelated COFs and MA/GO mixture as primary precursors combined with a high-temperature pyrolysis strategy. COFs and MA serve as chelators and spacers to improve the dispersion of metal nanoparticles. The Fe₂Ni₁/NiFe₂O₄@NCG composite exhibits a large BET surface area and hierarchical structure with plentiful nanoparticles on the carbon layers. HRTEM proves the coexistence of FeNi and NiFe₂O₄. The optimal Fe₂Ni₁/NiFe₂O₄@NCG-800 composite shows satisfactory catalytic O₂ performance, providing a half-wave potential of 0.857 V for ORR and an overpotential of 244 mV for OER. Meanwhile, DFT calculations prove that electron redistribution occurs at the interface between FeNi and NiFe₂O₄ after combination. The Fe₂Ni₁/NiFe₂O₄@NCG-based liquid and solid-state ZABs perform very well, exhibiting large specific capacities (796 mAh·g⁻¹ for aqueous ZAB; 742 mAh·g⁻¹ for solid-state ZAB) and stable charge-discharge cycle performance (300 h for aqueous ZAB; 180 h for solid-state ZAB).