Hierarchically porous two-dimensional Fe/N-codoped carbon nanoleaves for enhanced mass transfer and electrocatalytic oxygen reduction reaction

Zinc-air batteries are promising energy conversion devices with high theoretical energy density, but their practical performance is limited by the kinetically sluggish oxygen reduction reaction kinetics at the air electrode. This kinetic bottleneck stems from the inefficient mass transport and insufficient accessible active sites. In order to solve this problem, constructing porous structure at the air electrode could be an efficient strategy to improve mass transfer and expose more active sites. Herein, we successfully constructed hierarchical porous structure with mesopores and micropores in two-dimensional (2D) Fe/N-codoped carbon nanoleaves. F127 micelles on the surface were introduced for the formation of mesopores, while microporous structure came from 2D Fe-doped Zeolitic Imidazolate Framework-L (ZIF-L) precursors. After pyrolysis in Ar, the derived 2D meso/microporous Fe/N-codoped carbon nanoleaves possess atomically dispersed Fe-Nx sites. Kinetic experiments demonstrate that the hierarchical porous structure reduces the mass transfer resistance. Furthermore, density functional theory calculations reveal that the Fe-Nx active sites with concave curvature within the hierarchical porous structure can lower the *OH binding energy, thereby enhancing the oxygen reduction reaction activity. The nanostructure-engineered fabrication of this hierarchical porous structure is critical for accelerating mass transfer, ultimately maximizing the efficiency of active sites.