fig4

Figure 4. (A) Schematic diagram of the formation of single-shell hollow Ni-N₄/GHSs/Fe-N₄. (B) TEM and (C) AC HAADF-STEM images of single-shell hollow Ni-N₄/GHSs/Fe-N₄. (D) Ni K-edge XANES spectra and (E) FT EXAFS of the Ni K-edge of Ni-N₄/GHSs, NiO and Ni foil. (F) Fe K-edge XANES spectra and (G) FT EXAFS of the Fe K-edge of Fe-N₄/GHSs, Fe₂O₃ and Fe foil. (H) ORR and (J) OER polarization curves in O₂-saturated 0.1 M KOH. (I, K) Tafel plots of Ni-N₄/GHSs/Fe-N₄ and reference catalyst. (L, M) Schematic diagram and (N) discharge curves of a Zn-air battery assembled with Ni-N₄/GHSs/Fe-N₄^[111]. TEM: Transmission electron microscopy; AC-HAADF-STEM: aberration-corrected high angle annular dark field scanning transmission electron microscopy; FT: fourier transforms; EXAFS: extended X-ray absorption fine structure; XANES: X-ray absorption near edge structure; ORR: oxygen reduction reaction; OER: oxygen evolution reaction.