A Porous Nano-Micro-Composite as a High-Performance Bi-Functional Air Electrode with Remarkable Stability for Rechargeable Zinc-Air Batteries

The development of bi-functional electrocatalyst with high catalytic activity and stable performance for both oxygen evolution/reduction reactions(OER/ORR) in aqueous alkaline solution is key to realize practical application of zinc-air batteries(ZABs).In this study,we reported a new porous nano-micro-composite as a bifunctional electrocatalyst for ZABs,devised by the in situ growth of metal-organic framework(MOF) nanocrystals onto the micrometersized BaSrCoFeO(BSCF) perovskite oxide.Upon carbonization,MOF was converted to porous nitrogen-doped carbon nanocages and ultrafine cobalt oxides and CoNnanoparticles dispersing inside the carbon nanocages,which further anchored on the surface of BSCF oxide.We homogeneously dispersed BSCF perovskite particles in the surfactant;subsequently,ZIF-67 nanocrystals were grown onto the BSCF particles.In this way,leaching of metallic or organic species in MOFs and the aggregation of BSCF were effectively suppressed,thus maximizing the number of active sites for improving OER.The BSCF in turn acted as catalyst to promote the graphitization of carbon during pyrolysis,as well as to optimize the transition metal-tocarbon ratio,thus enhancing the ORR catalytic activity.A ZAB fabricated from such air electrode showed outstanding performance with a potential gap of only 0.83 V at 5 mA cmfor OER/ORR.Notably,no obvious performance degradation was observed for the continuous charge-discharge operation for 1800 cycles over an extended period of 300 h.