Recent Progress on Transition Metal Oxides as Bifunctional Catalysts for Lithium- Air and Zinc- Air Batteries

In recent years, rechargeable Li-air and Zn-air batteries have attracted wide attention due to their high theoretical specific energy densities. However, the high cost and poor stability of noble metal catalysts for the oxygen redox reactions limit their practical large-scale application. On contrast, low-cost transition metal oxide ( TMO)-based composite materials exhibit considerable bifunctional activity for oxygen reduction and oxygen evolution, and excellent stability, which holds great potential over the precious metal-based catalysts. This work briefly introduces the recent advances in TMO-based composites as bifunctional oxygen electrocatalysts for Li-air and Zn-air batteries. After a brief introduction into the research field of air batteries, metal oxides ( including Co-and Mn-based oxides) and polymetallic oxides ( mainly spinel and perovskite types) are reviewed. In consideration of the poor electronic conductivity of metal oxide catalysts, composites including additional conductive materials are also introduced. Finally, challenges and perspectives of bifunctional TMOs are emphasized to stimulate innovations for the future development of practical Li-air and Zn-air batteries.