Tuning the Bifunctional Oxygen Electrocatalytic Properties of Core-Shell CoO@NiFe LDH Catalysts for Zn-Air Batteries: Effects of Interfacial Cation Valences.

The rational design of excellent electrocatalysts is significant for triggering the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable metal-air batteries. Hereby, we report a bifunctional catalytic material with core-shell structure constructed by CoO nanowire arrays as cores and ultrathin NiFe-layered double hydroxides (NiFe LDHs) as shells (CoO@NiFe LDHs). The introduction of CoO nanowires could provide abundant active sites for NiFe LDH nanosheets. Most importantly, the deposition of NiFe LDHs on the surface of CoO can modulate the surface chemical valences of Co, Ni, and Fe species via changing the electron donor and/or electron absorption effects, finally achieving the balance and optimization of ORR and OER properties. By this core-shell design, the maximum ORR current densities of CoO@NiFe LDHs increase to 3-7 mA cm, almost an order of magnitude increases compared to pure NiFe LDH (0.45 mA cm). Significantly, an OER overpotential as low as 226 mV (35 mA cm) is achieved in the designed core-shell catalyst, which is comparable to and/or even better than those of commercial Ir/C. Hence, the primary zinc-air battery employing CoO@NiFe LDH as an air electrode achieves a high specific capacity (667.5 mA h g) and first-class energy density (797.6 W h kg); the rechargeable battery can show superior reversibility, excellent stability, and voltage gaps of ∼0.8 V (∼60% of round-trip efficiency) in >1200 continuous cycles. Furthermore, the flexible quasi-solid-state zinc-air battery with bendable ability holds practical potential in portable and wearable electronic devices.