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

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*O2 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.