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.