fig4

Figure 4. (A and B) DOS illustrations of the Fe-N-C/Fe₃C, Fe-N-C. The dashed line at 0 eV indicates the Fermi level (the inset is a charge density difference diagram). (C) The schematic illustration of Na⁺ storage mechanism of Fe-N-C/Fe₃C@HCNs, and structure changes during discharge/charge process. Reproduced with permission from Ref.^[60], © Tsinghua University Press 2021. (D) Fe-NPSA@NC and (E) Fe-ACSA@NC (the colors yellow and blue represent the increase and decrease of charge density, respectively). (F) OH^* adsorption energy as a function of Bader charge. Reproduced with permission from Ref.^[84], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2021. (G) The schematic illustration for the synthesis of Fe/Fe_xC@Fe-N-Cs electrocatalyst via “MOF-on-MOF” oriented assembly and the confinement conversion procedures. The free energy diagram for O₂ reduction on (H) Fe-N₄ and (I) Fe-N₄-Fe₁₀ under different potentials. Reproduced with permission from Ref.^[85], © Elsevier Ltd. 2023.