Metal-Organic Framework-Induced Synthesis of Ultrasmall Encased NiFe Nanoparticles Coupling with Graphene as an Efficient Oxygen Electrode for a Rechargeable Zn-Air Battery

Rational design of electrocatalysts to replace the noble-metal-based materials for oxygen reactions is highly desirable but challenging for rechargeable metal-air batteries. Herein, we demonstrate a unique two stage encapsulation strategy to regulate the structure and performance of catalysts featured with thin graphene nanosheets coupling with full encapsulated ultrafine and high-loaded (similar to 25 wt %) transition metal nanoparticles (TMs@N(C)x) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By optimizing the electronic modulation effect from suitable metal cores, the best NiFe@NCx catalyst exhibits high stability and activity with an onset potential of 1.03 V for ORR and an overpotential of only 0.23 Vat 10 mA cm(-2) for OER, which is superior to commercial Pt/C and IrO2 catalysts. Rechargeable Zn-air battery using NiFe@NCx catalyst exhibited an unprecedented small charge-discharge overpotential of 0.78 V at 50 mA cm(-2), high reversibility, and stability, holding great promise for the practical implementation of rechargeable metal-air batteries.