Noncompetitive Sequential Co-Fe Coordination for High-Density Fe/Co Dual-Atom Catalysts toward Efficient Acidic Oxygen Reduction

Abstract

Fe/Co dual-atom catalysts (DACs) offer a promising route to circumvent the *OH-*OOH linear scaling relation in acidic oxygen reduction reaction (ORR), yet their effectiveness is frequently limited by insufficient Fe/Co dual-sites formation caused by competitive Fe-Co coordination during synthesis. Here, we report a noncompetitive, sequential Co-Fe coordination strategy to construct high-density Fe/Co dual-atom sites. Unlike conventional ionic Fe precursors, which strongly coordinate with imidazolate ligands during ZIF assembly, solid Fe2O3 remains coordination-inactive, enabling Co2+ to anchor with ligands preferentially. During pyrolysis, the pre-generated Co-ligand motifs convert into Co-N4 sites, while thermally activated Fe2O3 gradually releases Fe species to form Fe-N4 sites, resulting in an ~80% increase in active-site density. Meanwhile, the spatial confinement of Fe2O3 within the ZIF matrix induces a more graphitized and hierarchically porous carbon framework, further improving site accessibility and utilization. In situ characterization and density functional theory (DFT) reveal that the sequentially coordinated S-Fe/Co-N-C catalyst, enriched with proximal Fe/Co dual-atom sites, favors a dissociative ORR pathway with bridge-mode O2 adsorption, circumventing the *OH-*OOH linear scaling relation, and accelerating ORR kinetics. In contrast, conventional C-Fe/Co-N-C and physically mixed Fe-N-C/Co-N-C catalysts follow an associative pathway. Benefiting from the synergy between proximal Fe/Co dual-atom sites and the graphitized porous carbon framework, S-Fe/Co-N-C delivers a half-wave potential of 0.842 V in acidic media and a peak power density of 382 mW cm-2 in a 25 cm2 H2-air membrane electrode assembly. This work establishes a general strategy for constructing high-density dual-atom active sites for efficient ORR.