fig3

Figure 3. (A) DFT-calculated FeN_x (x = 1-6) formation energies and FeN_x (x = 1-5) ORR overpotentials. This figure is quoted with permission from Li et al.^[103]; Temperature-dependent inverse susceptibility and electron configurations of (B) Fe-N₅-LS, (C) Fe-N₄-HS and (D) Fe-N₃-HS. This figure is quoted with permission from Xue et al.^[104]; (E) The Fe 3d orbital energy levels and the electron configurations of PFePc and PFePc-L were calculated; (F) Molecular orbitals of HO-PFePc-L. This figure is quoted with permission from Zhao et al.^[109]; (G) ORR active volcano curve through the 4e^- path (onset potential plotted as a function of ΔG_OH). This figure is quoted with permission from Sabhapathy et al.^[110]. (H) Schematic representation of the formation of Fe/Co/Ni-SAs/NS in different coordination environments. This figure is quoted with permission from Zhang et al.^[111]. DFT: Density functional theory; ORR: oxygen reduction reaction; LS: low spin state; HS: high spin state.