Fig. 4

a Bader charge and chemical valence of Cu centers for different structures: Cu₃N bulk, Cu-N₂, Cu-N₃, Cu-N₄ and CuPc molecule. b Total density of states (DOS) (black) for the CuPc, Cu-N₄, Cu-N₃ and Cu-N₂, and projected DOS of Cu atoms (red) from them. c Total DOS (black) for one O adsorbed CuPc, Cu-N₄, Cu-N₃ and Cu-N₄, and projected DOS of Cu atoms (red) and O atoms (green) from them. d Volcano plot of the relationship between ORR activity and ΔE_O for Cu-N₂, Cu-N₃, Cu-N₄, CuPc and N-doped graphene. e Schematic models of W-N₃, W-N₄ and W-N₅, respectively. W (deep green), N (orange), O (red), and C (grey). f LSV curves of W-N₃, W-N₄, W-N₅, CN, and Pt/C catalysts in O₂-saturated 0.1 M KOH. (g) Volcano plot of the relationship between the limiting potential (U_L) and ΔG_OH. h Local density of the states of W and O for *OH absorbed on metallic W, W-N₃, W-N₄ and W-N₅. i Crystal orbital overlap population (COOP) analysis results for *OH absorbed on W-N₃, W-N₄ and W-N₅. a–d Reproduced with permission from Ref. [95]. Copyright 2016, Royal Society of Chemistry. (e-i) Reproduced with permission from Ref. [65]. Copyright 2019, Elsevier