Ionic liquid derived active atomic iron sites anchored on hollow carbon nanospheres for bifunctional oxygen electrocatalysis

[Display omitted] •Synthesis of a new Fe-containing ionic liquid (Fe-IL).•Construction of the core–shell precursor structure.•One-step pyrolysis for the controlled synthesis of Fe-Nx sites.•Abundant Fe-N4 and Fe3 species with high catalytic activity.•Excellent electrochemical performance of a rechargeable Zn-air battery. Rational design of highly active bifunctional catalysts with satisfactory cost-performance as alternatives to precious metals for oxygen reduction and evolution reaction (ORR/OER) still poses challenges. Herein, highly catalytic active atomic Fe sites anchored to hollow carbon nanospheres (Fe-Nx-HCS) have been designed with optimized geometric features by introducing a new Fe-containing ionic liquid followed by a self-sacrificing, template-assisted, and controlled pyrolysis. Morphological and structural characterizations revealed abundant atomic Fe catalytic sites (Fe-N4 and Fe3 species) have a homogeneous dispersion throughout the HCS framework, resulting in high catalytic activity. The bifunctional Fe-Nx-HCS electrocatalyst had a more positive half-wave potential, higher diffusion-limiting ORR current density, as well as a lower overpotential for OER in both 0.1 M KOH and 0.05 M H2SO4. The stability of active sites and structural integrity of Fe-Nx-HCS significantly contributed to long-term cycling stability even after 10,000 potential cycles under alkaline conditions. Moreover, a rechargeable zinc-air battery device fabricated with Fe-Nx-HCS showed superior performance compared to state-of-the-art Pt/C catalyst. Density functional theory (DFT) calculations verified that the Fe3 active species enhanced Fe-N4 catalytic activity and promoted both ORR and OER. This work provides a new insight into the design and fabrication of highly active bifunctional electrocatalysts.