fig2

Figure 2. (A) Current density and half-wave potential of M-Pani /C-Mela (M = Co, Fe, Mn, Cu and Ni) and metal-free catalysts in 0.1M KOH; (B) N content (at%) of oxidized, Py-, Pyr-, and Gr-N; (C) BET surface areas of M-Pani /C-Mela; (D) Pore-size distribution of transition metal-codoped carbon catalysts. This figure is quoted with permission from Peng et al.^[97]; (E) Projected density of states of Fe-3d orbitals of FeN₄ and FeMoN₆OH. This figure is quoted with permission from Zhu et al.^[99]; (F) Atomic structures of electrocatalysts (blue: N, orange: Fe, purple: Co, and green: Ni). This figure is quoted with permission from Zhai et al.^[101]. BET: Brunauer–Emmett–Teller.