Coupled Fe₃O₄-Cluster Precipitates and Single Fe-Atom Catalysts Can Boost Oxygen Reduction via Concomitantly Accelerated Water Dissociation

Fe–N–C single-atom catalysts (SACs) can deliver high activity for catalyzing the oxygen reduction reaction (ORR) in alkaline conditions. However, the sluggish water dissociation upon Fe–N₄ active sites limits their proton-coupled electron transfer (PCET) capability, impeding their practical applications like anion-exchange membrane fuel cells (AEMFCs). Here, inspired by the known nanoprecipitation behavior in solid-solution alloys, a residual-oxygen-assisted precipitation strategy is undertaken to fabricate coupled Fe₃O₄-cluster precipitates along with single Fe-atom catalysts (Fe₃O₄/Fe_SA@NC), where Fe₃O₄ clusters are generated by slight oxidation and local enrichment of Fe atoms in the Fe_SA@NC matrix. Operando spectroscopy and theoretical calculations suggest that the Fe₃O₄-cluster precipitates not only induce asymmetric electronic structures of the Fe-N₄ active center to optimize the OH* adsorption, but also accelerate the water dissociation on Fe-N₄ sites to boost the PCET steps, thereby promoting the ORR. Notably, the coupled Fe₃O₄/Fe_SA@NC exhibits superb alkaline ORR performance with a high half-wave potential of 0.953 V versus RHE. When employed as cathode catalysts, the Fe₃O₄/Fe_SA@NC demonstrates a high peak power density of 909.3 mW cm⁻² and 219.4 mW cm⁻² in AEMFCs and Zn-air batteries, respectively, far exceeding that of the commercial Pt/C catalyst. The novel coupled metal-oxide cluster/SAC strategy can be exploited as a generic approach for improving electrocatalytic performance.