To further elucidate the catalytic activity and reaction mechanism of FeNi DACs for ORR and OER, first-principles calculations were performed using density functional theory (DFT) methods. Different types of models for both heteronuclear and homonuclear sites FeNiN6, FeN4, NiN4 were constructed with coordination environments based on experimental XANES/EXAFS results (SI, Figure ). Also, a model for nonadjacent FeN4-NiN4 sites was constructed as a reference to assess the critical role of adjacent hetero-bimetallic active sites. According to the adsorption energy of OH* (ΔGOH*) on active sites in different models (SI, Table ), the pristine FeNiN6 site exhibits a negative adsorption energy of -0.24 V for OH*, suggesting the strong adsorption and difficult desorption of OH* on the metal sites. Thus, the OH* would be spontaneously adsorbed on the metal sites during catalysis in alkaline media, and it has been reported that OH* serves as modifying ligand to optimize the electronic configuration of metal sites and influence their catalytic ORR/OER activity. As depicted in Figure (SI), the FeNi-NC model is the favorable configuration with OH* pre-adsorbed on the Fe site of FeNiN6, denoted as FeNi-OH-NC. Based on EXAFS fitting, we proposed that FeNiN6 (FeNi-OH-NC) could be the main contributor to the enhanced catalytic performance of FeNi-NC@MWCNT. Therefore, the FeNi-OH-NC configuration was chosen as the key model for the subsequent ORR/OER computational investigations, and Fe-NC, Ni-NC and nonadjacent Fe-Ni-NC were also simulated for comparative investigation. Particularly considering the possible reactive sites for the nonadjacent Fe-Ni-NC, two models of Fe@Fe-Ni-NC (on Fe site) and Ni@Fe-Ni-NC (on Ni site) were both calculated for the ORR/OER. Note that the optimized results for the adjacent FeNi-OH-NC model suggest that Fe is the reactive site. Figure illustrates the proposed 4e -pathway for the ORR mechanism on optimized FeNiN6 sites in the FeNi-OH-NC model (other possible models and their oxygen-containing intermediates are depicted in Figure -S28, SI). The corresponding ORR calculated Gibbs free energy diagrams of these constructed models are evaluated at different electrode potentials. At the potential U of 0 V, the four elementary steps are energetically downhill for all models, indicating all the involved reaction steps are exothermic in nature (SI, Figure ). When the potential is at 0.86 V (SI, Figure ), the fourth step (OH* + OH -→ O* + H2O + e -) is recognized as the rate-determining step (RDS) for the FeNi-OH-NC model with a free energy difference of only 0.04 eV. In comparison, the RDS for both Fe@Fe-Ni-NC and Fe-NC is also the fourth step with corresponding variation in free energy of 0.67 eV and 0.66 eV, respectively. The RDS for Ni-NC and Ni@Fe-Ni-NC is the second step (OOH* + e -→ O* + OH -) with corresponding variation in free energy of 0.52 eV and 0.51 eV, respectively. When the potential U increases up to 1.23 V, the fourth step is still recognized as the RDS for the FeNi-OH-NC model and the calculated free energy difference is determined as 0.41 eV, which is lower than those of Fe@Fe-Ni-NC (1.04 eV) and Fe-NC (1.03 eV) with the same RDS, and also lower than those of Ni-NC (0.89 eV) and Ni@Fe-Ni-NC (0.88 eV) with RDS in the second step, suggesting the FeNi-OH-NC has the lowest energy barrier for ORR. Notably, the free energies of each step for the Fe-NC and Fe@Fe-Ni-NC model are close to each other at the three investigated potentials, as are those for the Ni-NC and Ni@Fe-Ni-NC models, indicating that the nonadjacent metal sites can't leverage the synergistic effects. Figure displays the proposed reaction mechanism for OER process, the corresponding OER free energy diagrams of these constructed models are shown in Figure , Figure and S29d (SI). At the potential of 0 V (SI, Figure ), all the interaction steps are uphill, implying that the OER is energetically unfavorable at this potential. When U = 1.23 V, the RDS of FeNi-OH-NC is the second step (OH* + OH -→ O* + H2O + e -) with a free energy difference of 0.59 eV, which is the lowest compared with those of the other four models as shown in Figure . When U = 1.57 V (SI, Figure ), the free energy difference for the RDS of FeNi-OH-NC towards OER is evaluated as 0.25 eV, which is also lower than those of the other four models. Similarly, the Fe-NC model exhibits comparably overlapped free energies diagrams with the nonadjacent Fe@Fe-Ni-NC model, as are those for the Ni-NC and Ni@Fe-Ni-NC models. All the above results indicate that the adjacent FeNi heteroatom configuration as active site effectively promote ORR/OER catalytic performance. The ORR/OER catalytic activity can also be evaluated by considering the adsorption energy (∆GOH*) and ∆G*O -∆G*OH as indicator to establish a volcano-shaped curve with ORR/OER overpotentials. Figure (SI) shows the ORR overpotential η versus ∆GOH*, the FeNi-OH-NC model is located at the top of the volcano curve with smallest overpotential of 0.41 V, which is significantly lower than the monometallic models Fe-NC (1.03 V), Ni-NC (0.89 V) and the nonadjacent bimetallic models Fe@Fe-Ni-NC (1.04 V) and Ni@Fe-Ni-NC (0.88 V). Thus, the theoretically calculated trends of the ORR reactivity are consistent with the experimental results. Similarly, FeNi-OH-NC also exhibits the best OER catalytic activity with an overpotential of 0.59 V (SI, Figure ), followed by Fe@Fe-Ni-NC (0.61 V), Fe-NC (0.63 V), Ni@Fe-Ni-NC (1.23 V) and Ni-NC (1.24 V). Here also, theory supports the experimental findings. Furthermore, as shown in Figure (SI), the charge distribution analysis reveals that N atoms in FeNi-OH-NC model are more negatively charged compared to Fe-NC, Ni-NC and nonadjacent Fe@Fe-Ni-NC models, suggesting the formation of metal-metal interactions of adjacent Fe-Ni sites in FeNi-OH-NC. This feature is expected to enhance the charge transfer and electron redistribution at the active sites. This modifies the electronic configuration of the Fe and Ni atoms, and facilitates the adsorption of oxygen intermediates, resulting in improved catalytic activity. Furthermore, analysis of the total density of states (TDOS) shown in Figure suggest that FeNi-OH-NC feature higher density of states cross the Fermi level (followed by Ni-NC, Fe@Fe-Ni-NC and Fe-NC), indicating the highest electronic conductivity of the FeNi-OH-NC model. This calculated finding is in agreement with the EIS experimental results described above. Figure presents the detailed projected density of states (PDOS) for Fe 3d, Ni 3d, N 2p and C 2p orbitals of FeNi-OH-NC model. Compared with monometallic models Fe-NC, Ni-NC and nonadjacent bimetallic Fe@Fe-Ni-NC models (SI, Figure -Figure ), the adjacent bimetallic FeNi-OH-NC model demonstrates more hybrid electrons near the Fermi level, implying the existence of electronic interaction of Fe, Ni and N on the active sites. Therefore, the DFT analysis demonstrates the adjacent diatomic FeNi-OH-NC configuration induces subtle modulation of the asymmetric electronic configuration of FeNiN6 active sites, which facilitates the kinetics of electrocatalytic ORR/OER process.