Recent advances of hierarchically porous bifunctional oxygen electrocatalysts derived from metal-organic frameworks for Zn-air batteries

On account of fossil energy depletion and the environmental crisis, Zn-air batteries with high energy density, eco-friendliness, and excellent safety have been considered as promising candidates for next-generation energy devices. The crucially important air cathode of the Zn-air battery undergoes gas-involving oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the discharging and charging steps, resulting in complicated reactions and sluggish kinetic processes at the solid-liquid-gas interface. Micropore-dominated metal-organic frameworks (MOFs) have been demonstrated as a promising precursor in the field of electrocatalysis. However, the microporosity of MOF derivatives severely impedes the mass and charge transfer for the ORR/OER. Hence, the rational design of a hierarchically porous structure for multiscale mass/electron transport channels is essential for bifunctional oxygen electrocatalysts derived from MOFs. In this review, the latest developments of hierarchically porous ORR/OER electrocatalysts derived from MOFs for Zn-air batteries are covered. First, we introduce the related fundamentals of rechargeable Zn-air batteries and focus on the electrochemical oxygen reactions occurring at the air cathode. Then, recent advances of hierarchically porous MOF derivatives for the ORR, OER, and Zn-air batteries are discussed in detail. Finally, the synthesis strategies of hierarchically porous MOFs and their derivatives are presented as well.