Controllable construction of FeCo nanoparticles embedded 3D porous N-doped carbon nanonetworks as high efficiency bifunctional electrocatalysts for Zn-air batteries

Construction of efficient and low-cost bifunctional electrocatalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for rechargeable Zn-air batteries is still a great challenge but highly desirable. Herein, a series of low-cost bifunctional electrocatalysts including Fe/C, Fe-NPCNs, FeCo-NPCNs, and FeCo-CNTs are prepared by simply carbonizing the mixture of Fe-based metal-organic frameworks (Fe-MIL-53), melamine, and CoCl2.6 H2O with different contents. Structure and electrocatalytic performance of the as-fabricated four samples are systematically investigated and the FeCo-NPCNs catalyst with a structure of FeCo nanoparticles uniformly embedded 3D porous N-doped carbon nanonetwork exhibits the highest electrochemical activity with a positive half-wave potential of 0.87 V vs. RHE for ORR and a low overpotential of 240 mV vs. RHE for OER with a ΔE of 0.60 V at 10 mA cm-2, together with excellent stability and methanol tolerance, even outperforming the noble-metal-based Pt/C-RuO2 catalysts. Besides, the results of density functional theory (DFT) calculations reveal the boosted intrinsic activity due to the reduced energy barrier both for ORR and OER. Furthermore, the assembled aqueous rechargeable Zn-air battery with FeCo-NPCNs catalyst manifests a large power density of 161.2 mW cm-2, a low charge-discharge gap of 0.78 V, and long-term ability. Likewise, the flexible solid-state Zn-air battery with FeCo-NPCNs catalyst also demonstrates high round-trip efficiency and excellent durability under various bending angles. This work may serve as a reference to designing advanced bifunctional electrocatalysts with 3D porous carbon nanonetworks for rechargeable Zn-air batteries.