Dual metal-organic frameworks-derived Fe-atomic sites bounded to fine Fe/FexC nanoparticles for enhanced oxygen electroreduction

Abstract

Maximizing metal utilization and increasing their activity are the main challenges to improve the oxygen reduction reaction (ORR) performance of iron and nitrogen co-doped carbons (Fe-N-Cs) catalysts. However, the activity contribution of the iron-containing nanoparticles (i.e. Fe/FexC NPs) inevitably formed during the preparation of Fe-N-C catalysts is often overlooked. Herein, we develop a new composite electrocatalyst (Fe/FexC@Fe-N-Cs) via a surfactant-assisted “MOF-on-MOF” oriented assembly and confined conversion strategy to enhance Fe utilization. The resultant Fe/FexC@Fe-N-C-900, featuring the combination of fine Fe/FexC NPs and Fe-N4 reactive sites, achieves excellent ORR activity with half-wave potentials (E1/2) of 0.91 V and 0.81 V in both alkaline and acidic electrolytes. When using as the cathode, Fe/FexC@Fe-N-C-900 exhibits promising performances in Zn-air battery and H2-O2 fuel cell, featuring the maximum power density of 150 mW cm−2 and 560 mW cm−2, respectively. The electronic migration from Fe/FexC NPs to neighboring Fe-N4 sites leads to moderate adsorption for reactants/intermediates on Fe atomic centers, subsequently elevating optimal ORR performance, which is revealed by density functional theory (DFT) calculations, confirming the crucial effect of Fe/FexC NPs in improving the activity and stability of Fe-N-Cs.