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

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/Fe_xC NPs) inevitably formed during the preparation of Fe-N-C catalysts is often overlooked. Herein, we develop a new composite electrocatalyst (Fe/Fe_xC@Fe-N-Cs) via a surfactant-assisted “MOF-on-MOF” oriented assembly and confined conversion strategy to enhance Fe utilization. The resultant Fe/Fe_xC@Fe-N-C-900, featuring the combination of fine Fe/Fe_xC NPs and Fe-N₄ reactive sites, achieves excellent ORR activity with half-wave potentials (E_1/2) of 0.91 V and 0.81 V in both alkaline and acidic electrolytes. When using as the cathode, Fe/Fe_xC@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/Fe_xC 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/Fe_xC NPs in improving the activity and stability of Fe-N-Cs.