Metal-organic frameworks and their derivatives for metal-air batteries

The challenges of energy depletion and environmental pollution raise the demands for the development of new energy technologies, such as metal-air batteries (MABs). The performance of a metal-air battery heavily relies on two fundamental electrocatalytic reactions, that is, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which take place at the air cathode during battery discharging and charging, respectively. Metal-organic frameworks (MOFs) and their derivatives stand out as promising candidate catalysts for the once sluggish redox reactions at the cathodes of MABs, due to their unique merits of crystalline porous structures and tunable chemistry. These distinct characteristics of MOFs offer an effective way to introduce bifunctionality (catalytic activity for ORR and OER in the case of MABs) through the design and synthesis of MOF-based catalysts with large surface area, well-developed porous networks, smooth electron and mass transfer pathways and active sites with different catalytic preferences. Herein, this review summarizes recent advances in the design and synthesis of MOF-based catalysts for a range of MABs, including Li-, Zn-, Al-, Fe- and Na-air batteries. By the demonstration of representative examples, this review also discusses the underlying mechanisms for the origin of bifunctional performance and enhanced catalytic activity with MOF-based catalysts. Future challenges and prospects for both MOFs and MABs are also proposed at the end of this review.