Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO@MnO_x Core-Shell Structures for Rechargeable Zn-Air Batteries

Rechargeable Zn-air battery is limited by the sluggish kinetics and poor durability of the oxygen catalysts. In this Research Article, a new bifunctional oxygen catalyst has been developed through embedding the ultrafine NiFeO nanoparticles (NPs) in a porous amorphous MnO_x layer, in which the NiFeO-core contributes to the high activity for the oxygen evolution reaction (OER) and the amorphous MnO_x-shell functions as active phase for the oxygen reduction reaction (ORR), promoted by the synergistic effect between the NiFeO core and MnO_x shell. The synergistic effect is related to the electron drawing of NiFeO core from MnO_x shell, which decreases the affinity and adsorption energy of oxygen on MnO_x shell and significantly increases the kinetics of ORR. The electrocatalytic activity and durability of NiFeO@MnO_x depends strongly on the NiFeO:MnO_x ratio. NiFeO@MnO_x with NiFeO:MnO_x weight ratio of 1:0.8 shows the best performance for reversible ORR and OER, with a potential gap (ΔE) of 0.792 V to achieve a current density of 3 mA cm^-2 for ORR (E_ORR=3) and 5 mA cm^-2 for OER (E_OER=5) in 0.1 M KOH solution. The high activity of the NiFeO@MnO_x(1:0.8) has been demonstrated in a Zn-air battery. Zn-air battery fabricated using the NiFeO@MnO_x(1:0.8) oxygen electrode shows similar initial performance with that of Pt-Ir/C oxygen electrode but a much better durability under charge and discharge cycles as the result of the structure confinement effect of amorphous MnO_x. The results demonstrate NiFeO@MnO_x as an effective bifunctional oxygen catalysts for rechargeable metal-air batteries.