Self-Sacrificing Template Synthesis of Carbon Nanosheets Assembled Hollow Spheres with Abundant Active Fe-N O Moieties for Electrocatalytic Oxygen Reduction.

Single-atom Fe-N-C (Fe -N-C) materials represent the benchmarked electrocatalysts for oxygen reduction reaction (ORR). However, single Fe atoms in the carbon skeletons cannot be fully utilized due to the mass transfer limitation, severely restricting their intrinsic ORR properties. Herein, a self-sacrificing template strategy is developed to fabricate ultrathin nanosheets assembled Fe -N-C hollow microspheres (denoted as Fe /N-HCMs) by rational carbonization of Fe chelating polydopamine coated melamine cyanuric acid complex. The shell of Fe /N-HCMs is constructed by ultrathin nanosheets with thickness of only 2 nm, which is supposed to be an ideal platform to isolate and fully expose single metal atoms. Benefiting from unique hierarchical hollow architecture with highly open porous structure, 2 nm-thick ultrathin nanosheet subunits and abundant Fe-N O active sites revealed by X-ray absorption fine structure analysis, the Fe /N-HCMs exhibit high ORR performance with a positive half-wave potential of 0.88 V versus the reversible hydrogen electrode and robust stability. When served as air-cathode catalysts with ultralow loading mass of 0.25 mg cm , Fe /N-HCMs based Zn-air batteries present a maximum power density of 187 mW cm and discharge specific capacity of 806 mA h g in primary Zn-air batteries, all exceeding those of commercial Pt/C.