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 the acidic oxygen reduction reaction (ORR), yet their effectiveness is frequently limited by insufficient Fe/Co dual-site 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.