fig3

Figure 3. (A) Coordination or structural changes of the S1 and S2 under in situ conditions. This figure is quoted with permission from Li et al.^[43]. (B) The ⁵⁷Fe Mössbauer spectra on Fe-N-C after degradation at different potentials for 150 h and relative contents of different types of FeN₄. (C) The in situ Mössbauer spectra of the ⁵⁷Fe-N-C and relative contents of different types of FeN₄. (Red: D1; Purple: D2; Blue: D3; Green: D4). (D) The relationship between structural evolution and binding energy or bond length of Fe-N illustrates the bond-breaking process. These figures are quoted with permission from Xu et al.^[44].