fig4

Figure 4. (A) The free energy diagram of typical ORR pathway of various FeN₄ sites; (B) Fe-N-C in-situ Mössbauer spectra and the relative content of different types of FeN₄. This figure is quoted with permission from Xu et al.^[113]; (C) Optimized structure of a perfect FeN₄ site and Divacancy defect models 6, 6a, 6I, and 6II. This figure is quoted with permission from Liu et al.^[115]; (D) Molecular orbital distributions showing the HOMO and LUMO for c-ND-Fe and e-ND-Fe configurations. This figure is quoted with permission from Wang et al.^[116]; (E) Calculated free energy diagrams for the ORR on in-plane-type FeN₄P₂ and edge-type FeN₄P₂ at zero electrode potential. This figure is quoted with permission from Yin et al.^[117]; (F) The optimized structures of basal-plane Co-N₄/G and edge-hosted Co-N₄ configurations (brown: C; grey: N, blue: Co, and pink balls: H); (G) Computationally derived volcano plot for the 2e^- ORR, illustrating the relationship between limiting potential (UL) and Gibbs free energy change for OOH^* formation (ΔG_OOH*). This figure is quoted with permission from Tian et al.^[118]. ORR: Oxygen reduction reaction.