Fig. 5

Electronic control effect. a Free energy diagram of the oxygen reduction reaction (ORR) on single-atom iron (Fe-SAs)/nitrogen-doped carbon (N-C), Fe-SAs/nitrogen, phosphorus co-doped carbon (NP-C) and Fe-SAs/nitrogen, phosphorus, sulfur co-doped carbon (NPS-C) (U ^RHE = U ^NHE + 0.0591 pH; U ^RHE = 0 V, U ^NHE = −0.77 V; U ^RHE = 1 V, U ^NHE = 0.23 V). b−d Calculated charge density differences of Fe-SAs/N-C (b), Fe-SAs/NP-C (c), and Fe-SAs/NPS-C (d). Yellow and blue areas represent charge density increase and reduction, respectively. The cutoff of the density-difference isosurfaces is equal to 0.01 electrons Å⁻³. (C: gray, N: blue, Fe: orange, S: yellow, P: green). e Linear relationship between OH* binding energy and Bader charge of single-atom iron in Fe-SAs/N-C, Fe-SAs/NP-C and Fe-SAs/NPS-C, respectively. Insets: The corresponding schematic models of samples, Fe (orange), N (blue), P (green), S (yellow) and C (gray). f, g Comparison of J _k at 0.85 V and E _1/2 (f) and the Tafel plots (g) for Fe-SAs/N-C, Fe-SAs/NP-C, Fe-SAs/NPS-C, and single iron atomic sites supported on a nitrogen, phosphorus and sulfur co-doped hollow carbon polyhedron (Fe-SAs/NPS-HC)