Fig. 5.

(a) H₂O₂ percentage (top); half-way potential for ORR (middle) and kinetic current ik (bottom) by Fe–4 phenantroline-based catalyst at a pH range of 1–13.7. (b) Schematic of N and Fe−Nx functionalities existing in Fe−N−C catalysts. Drawn of the possible reaction pathways and active sites during ORR: (c) 2x2e⁻ transfer on a dual site, (d) 2x2e⁻ transfer on a single site, and (e) direct 4e⁻ transfer mechanism on a single site. Identification of the active sites and the reaction pathways (f) with pyrrolic as S1 site, pyridinic as S2 site, and Fe-Nx as S or S* or S2 site. (Fig. 5a adapted from Ref. [322] with permission of Elsevier; Fig. 5b, c, d, e, f reprinted and adapted with permission from K. Artyushkova, A. Serov, S. Rojas-Carbonell, P. Atanassov, J. Phys. Chem. C 119 (2015) 25917−25928. Copyright (2015) American Chemical Society; Ref [339]).