Investigating the effect of active site's coordination number on the oxygen reduction reaction activity of Fe-Co dual-atom catalysts: A theoretical study

Abstract

Dual-atom catalysts (DACs) have great potential to revolutionize electrocatalytic oxygen reduction reaction (ORR) by utilizing two adjacent active sites to cleave the O-O bond in O₂ molecules, but their full potential is limited by insufficient understanding of the coordination environment and electronic interactions between the active sites. Herein, we conducted a comprehensive density functional theory (DFT) analysis on DACs with two coordination configurations, FeN₃CoN₃ (3N-coordination) and FeN₄CoN₄ (4N-coordination), revealing how the coordination number around active sites significantly affects ORR performance. The results indicated that the dissociative O₂ adsorption configuration, facilitated by cis-bridged O₂ adduct formation on Fe and Co active sites of both DACs, was more thermodynamically and kinetically favourable than the associative mechanism on both DACs. The results of free energy calculations indicated that the ${\Delta G}_{{\ast O}_2}$ of FeN₃CoN₃ is lower than FeN₄CoN₄, demonstrating that the 3N-coordination environment endowed FeN₃CoN₃ with a higher adsorption ability as compared to FeN₄CoN₄ with a 4N-coordination environment. The elimination of ∗OH from the active site, required 0.28 eV and 0.36 eV as the limiting potentials for the FeN₄CoN₄ and FeN₃CoN₃ models, respectively, both of which are considerably lower than the 0.68 V (FeN₄CoN₄) and 0.86 V (FeN₃CoN₃) observed in single active sites during associative mechanism. Thus, the cis-bridge O₂ configuration across neighbouring sites in the DACs significantly reduces the overpotential, demonstrating FeN₄CoN₄ as a more effective catalytic model than FeN₃CoN₃. Further, the findings reveal that the 4N-coordination in FeN₄CoN₄ enhances orbital coupling and spin polarization, boosting its ORR activity compared to FeN₃CoN₃. The study emphasizes the importance of coordination number in DAC design, highlighting its role in optimizing electrocatalytic performance.