Optimizing diffusion kinetics of two-dimensional structures via nano-assembling towards rapid oxygen reduction electrocatalysis

In the realm of metal-air batteries and fuel cells, managing the thickly catalytic layer of metal-nitrogen-carbon (M-N-C) catalysts is pivotal, where the design of mass transport pathways in meso- and macro-scale is essential around the active metal sites. Such arrangements are crucial to achieving adequate three-phase boundaries and timely kinetic responses during oxygen reduction reactions. Yet, when it comes to materials with low-dimensional morphologies, such as nanolayers and nanosheets, the high aspect ratios render new challenges to structure maintenance during the pore formation, usually involving templating or etching, and against the pyrolysis collapse. Herein, we have developed an in-situ nano-assembling methodology to design hierarchical porosity in M-N-C catalysts pyrolytically derived from 2D materials. By controlling the solvothermal synthesis, the 2D nonporous precursors, fusiform ZIF-L nanolayers are taken as particular experimental model, can scale down into secondary monomers and restack meso- and macro-porously. After pyrolysis, the derived Fe-N-C catalysts well inherit the hierarchical morphology and thus showcase calcined micro-pores along with a spectrum of meso- and macro-pores. Advanced characterization techniques such as the spherical aberration correction electron microscopy and X-ray absorption spectrum allow us to pinpoint atomically dispersed FeN4 motifs as the primary active sites. Notably, their upgraded accessibility exhibits a direct correlation to the performance parameters in half-cells and prototype zinc-air batteries. This investigation heralds new pathways for the optimization of low-dimensional nanocatalysts, aiming to exploit their activity within catalytic layers to the fullest.