Molten Salt-Assisted Synthesis of Ferric Oxide/M-N-C Nanosheet Electrocatalysts for Efficient Oxygen Reduction Reaction

Efficient, stable, and low-cost oxygen reduction catalysts are the key to the large-scale application of metal-air batteries. Herein, high-dispersive Fe₂O₃ nanoparticles (NPs) with abundant oxygen vacancies uniformly are anchored on lignin-derived metal-nitrogen-carbon (M-N-C) hierarchical porous nanosheets as efficient oxygen reduction reaction (ORR) catalysts (Fe₂O₃/M-N-C, M═Cu, Mn, W, Mo) based on a general and economical KCl molten salt-assisted method. The combination of Fe with the highly electronegative O induces charge redistribution through the Fe-O-M structure, thereby reducing the adsorption energy of oxygen-containing substances. The coupling effect of Fe₂O₃ NPs with M-N-C expedites the catalytic activity toward ORR by promoting proton generation on Fe₂O₃ and transfer to M-N-C. Experimental and theoretical calculation further revealed the remarkable electronic structure evolution of the metal site during the ORR process, where the emission density and local magnetic moment of the metal atoms change continuously throughout their reaction. The unique layered porous structure and highly active M-N₄ sites resulted in the excellent ORR activity of Fe₂O₃/Cu-N-C with the onset potential of 0.977 V, which is superior to Pt/C. This study offers a feasible strategy for the preparation of non-noble metal catalysts and provides a new comprehension of the catalytic mechanism of M-N-C catalysts.