Integrating Co Clusters and Atomically Dispersed Fe–N ₄ Sites on N-Doped Carbon Nanotubes for Efficient Oxygen Reduction Reaction

As leading nonprecious metal electrocatalysts for ORR, Fe–N–C systems still face critical limitations. The insufficient density of accessible metal active sites and the intrinsic balance conflict between activity and durability largely constrain their practical application. In this study, we employed a coupled strategy of electrostatic assembly and defect confinement, followed by two-step high-temperature pyrolysis under distinct activation atmospheres. Cobalt clusters were in situ grown on the surface of carbon nanotubes to form a micro–nano anchoring structure with edge defects, enabling the simultaneous immobilization of Fe–N 4 moieties and uniform dispersion of Co clusters on nitrogen-doped carbon nanotube substrates. The resulting nanotube-shaped Co@Fe/NC-a catalyst (prepared at 1100 °C with hydrogen activation) exhibited markedly improved ORR activity and stability in 0.1 M KOH compared with Fe/NC-a. X-ray absorption spectroscopy revealed that the Co clusters modulate the electron distribution at the Fe–N 4 sites. Further, density functional theory (DFT) calculations confirmed that the Co clusters alter the electronic structure of Fe–N 4 sites, reducing the reaction energy barrier and improving catalytic performance. Overall, this work offers a viable strategy for improving ORR catalysts by electronically tuning Fe–N 4 sites through adjacent metal clusters.