fig4

Figure 4. (A) The ORR overpotentials of FeN_x and the formation energies of FeN_x were calculated by DFT. Reproduced from Li et al.^[63] by permission from the Royal Society of Chemistry (RSC). (B) Schematic diagram of structural variations of S1 and S2 sites. The highly reactive and low-stability S1 site rapidly transforms to inactive iron oxide. On the contrary, the low-activity and high-stability S2 sites are critical for maintaining fuel cell performance late. Reproduced from Li et al.^[74] by permission from the Springer Nature. (C) The activity of both kinds of FeN₄ sites was calculated with DFT. Reproduced from Liu et al.^[77] by permission from the Springer Nature.