Ionic Liquid Derived Active Atomic Iron Sites Anchored on Hollow Carbon Nanospheres for Bifunctional Oxygen Electrocatalysis

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-N-x-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-N-4 and Fe-3 species) have a homogeneous dispersion throughout the HCS framework, resulting in high catalytic activity. The bifunctional Fe-N-x-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 PVC catalyst. Density functional theory (DFT) calculations verified that the Fe-3 active species enhanced Fe-N-4 catalytic activity and promoted both ORR and OER. This work provides a new insight into the design and fabrication of highly active bifunctional electrocatalysts.