Metal-air batteries, such as zinc-air batteries and magnesium-air batteries, can potentially enable a widespread implementation of clean renewable energy. The primary limitations of metal-air battery are the sluggish electrocatalytic kinetics on the air side and poor stability of electrode performance. Hence, highly efficient and stable Fe/Ni-based electrode can simultaneously address these challenges. In this work, a novel ORR and OER catalyst, NiFe₂O₄ quantum dots (QDs) is synthesized, which are self-assembled on carbon nanotubes (CNTs). A composite of NiFe₂O_4(QDs)/CNTs exhibits a superior bifunctional oxygen electrocatalytic activity with a low the potential gap between ORR and OER (ΔE: 0.9 V). The NiFe₂O_4(QDs)/CNTs electrode displays a high power density and a high specific capacity in both zinc-air batteries and magnesium-air batteries at ambient conditions. Particularly, the rechargeable zinc-air batteries show a low charge-discharge voltage gap of only 0.62 V and long-term rechargeability for more than 800 hours. The remarkable performance of NiFe₂O_4(QDs)/CNTs can potentially facilitate the largescale implementation of renewable energy storage by rechargeable metal-air batteries.

Keywords: NiFe₂O₄ quantum dots, ORR, OER, Low voltage gap, Metal-air battery