1. Classification of non-noble metal/nitrogen-doped carbon composite materials

2. Synthesis schemes of (a) Co@CNT/NC composites^[23], (b) Co-MOF//NF and Co-NCNTFs//NF^[25]; (c) OER polarization curves with 85% iR compensation in a 1 mol·L^-1 KOH solution at a scan rate of 1 mV·s^-1 and (d) related Tafel plots of Co-MOF//NF, Co-NCNTFs//NF, bare Ni foam, and RuO₂^[25]; (e) Stability tests of Co-NCNTFs//NF for OER and HER at a current density of 10 mA·cm^-2[25]; (f) Illustration of the Co@NC/CC composite fabrication^[27]

3. (a) “Confined recrystallization” self-template synthesis strategy for Fe-NC SAC^[28]; (b) Schematic illustration of the synthetic procedure of the SCoNC catalysts^[32]; (c) Schematic description of the synthetic process, (d, e) FESEM and (f) TEM images of the HCM@Ni-N^[35]; (g, h) FESEM and (i) TEM images for HCM@Ni-N after continuous operation for 10 h^[35]; (j) LSV curves in 1.0 mol·L^-1 KOH and (k) the related Tafel plots of HCM@Ni-N, HCM@Ni, HCM@N, and HCM^[35]

4. Schematic synthesis process of (a) FeCo@MNC^[42], (b) Ni_1.5Co_0.5 @ N-C NT/NFs^[44] and (c) FeNi@NC-x^[46]; (d) SEM image of FeNi@NC-2^[46]; (e) LSV curves and (f)Tafel plots of RuO₂, and FeNi@NC-x^[46]; (g) Illustration of the preparation of Ni₃Fe-GA_s^[47]; (h,i) SEM images and (j) TEM image for Ni₃Fe-GA₁^[47]; (k) OER polarization curves and (l) corresponding Tafel plots for Ni₃Fe-GA_s^[47]; (m) Chronopotentiometric curves of Ni₃Fe-GA₁ and IrO₂ catalysts^[47]

5. (a) Illustration of the synthesis process of N-Co₃O₄ and N-CoO/C^[59]; SEM images of (b, c) CoO and (d, e) N-CoO nanowire arrays^[60]; (f) Illustration of the synthesis process of CoO/Co@N-C^[61]; SEM images of (g) Co₃O₄ and (h) Co₃O₄@Glu microspheres, and (i) CoO/Co@N-C electrocatalyst^[61]; (j) LSV curves and (k) Tafel plots of CoO@N-C, CoO/Co@N-C, CoO/Co@N-C-x samples that etched in HCl with x representing etching time, Ir/C and Pt/C in 0.1 mol·L^-1 KOH solution; (l) LSV curves for the CoO/Co@N-C electrocatalyst before and after 2000 CV cycles in 0.1 mol·L^-1 KOH solution^[61]

Table 1. OER electrocatalytic performances of non-noble metal/nitrogen-doped carbon composites

Table 2. OER electrocatalytic performances of non-noble metal single atom/nitrogen-doped carbon composites

Table 3. OER electrocatalytic performances of non-noble alloy/nitrogen-doped carbon composites

Table 4. OER electrocatalytic performance of non-noble metal oxide/nitrogen-doped carbon composites