## 3 Results and Discussion
### ORR Electrocatalytic Activities  
To assess the electrocataly's properties toward ORR of the N-doped (\(\mathrm{N}\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{3}\mathrm{2}\mathrm{5}}\)-4, \(\mathrm{N}\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{100}}\)-4, and \(\mathrm{N}\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{10}}\)-4) and non-doped (\(\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{3}\mathrm{2}\mathrm{5}}\)-3, \(\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{100}}\)-3, and \(\mathrm{r}\mathrm{G}\mathrm{O}_{\mathrm{10}}\)-3) graphenic materials, a cyclic voltammetry study was initially performed in a KOH electrolyte (0.1 mol dm\({}^{-3}\)) saturated with N\({}_{2}\) and O\({}_{2}\). Figure S6 (ESI) shows the CVs in the N\({}_{2}\) and O\({}_{2}\) electrolytes for the six samples tested. In the presence of N\({}_{2}\), no redox processes are detected, while in the electrolyte purged with O\({}_{2}\), the six graphene materials studied showed an irreversible ORR peak with 0.74 \(\geq E_{\mathrm{P}\mathrm{C}}\geq\) 0.72 V versus RHE. For comparison, the commercial Pt/C (20 wt %) was also measured in the same experimental conditions toward ORR (Figure S7a, ESI), presenting analogous behavior to the graphene materials with \(E_{\mathrm{P}\mathrm{C}}=\) 0.88 V versus RHE. Then, the electrocatalytic properties toward ORR were further studied by LSV with an RDE at several rotation speeds in KOH purged with O\({}_{2}\). The LSVs of all graphene materials are presented in Figure 7, while those corresponding to Pt/C can be seen in Figure S7b. All graphene materials presented a linear relationship between \(j_{\mathrm{L}}\) and rotation speed, suggesting that the electron transfer reaction is diffusion limited. The LSVs show three different regions: for \(E\) higher than \(\approx\) 0.80 V, the process is kinetically controlled; potential values between \(\approx\) 0.80 V and \(\approx\) 0.50 V indicate the mixed kinetic-diffusion region, and for \(E\leq\) 0.50 V, the process is controlled by O\({}_{2}\) diffusion. Table 3 shows comparisons of the obtained \(E_{\rm onset}\) and \(j_{\rm L}\), 0.26V, 1600 rpm values for the N-doped and non-doped graphene materials at 1600 rpm. All the ECs showed similar onset potentials (0.82 \(\geq E_{\rm onset}\)  
Figure 7: Oxygen reduction reaction (ORR) polarization curves of rGO\({}_{10}\)-3 (**a**), rGO\({}_{100}\)-3 (**b**), rGO\({}_{325}\)-3 (**c**), NrGO\({}_{10}\)-4 (**d**), NrGO\({}_{100}\)-4 (**e**), and NrGO\({}_{325}\)-4 (**f**) modified electrodes at different rotation rates in KOH purged with O\({}_{2}\) at 0.005 V s\({}^{-1}\). \(\geq\) 0.79 V versus RHE), but higher \(j_{L}\), 0.26V, 1600 ppm values were obtained for the N-doped materials. For the non-doped graphene materials, these values varied between \(-\)2.92 and \(-\)3.46 mA cm\({}^{-2}\), while for the N-doped materials, they varied between \(-\)3.03 and \(-\)4.05 mA cm\({}^{-2}\). The NrGO\({}_{325}\)-4 electrocatalyst showed the most promising result of all the prepared graphene materials with the more positive \(E_{\text{onset}}\) of 0.82 V versus RHE (0.12 V more negative than Pt/C) and the higher \(j_{L}\), 0.26V, 1600 ppm of \(-\)4.05 mA cm\({}^{-2}\). The introduction of nitrogen species led to an improvement in the ORR properties, which is in agreement with several works that have reported the enhancement of ORR electrocatalytic activity after the N-doping of carbon materials [56; 57]. Furthermore, comparing the results for the non-doped graphene materials, it seems that the particle size of the starting graphite does not have a clear direct relationship with their ORR properties, unlike is observed for the N-doped materials. For the latter, as the particle size of the starting graphite decreased, there was an improvement of the ORR features (more positive \(E_{\text{onset}}\) values and higher \(j_{L}\)), and this is a consequence of the higher degree of oxidation that favored the incorporation of increased amounts of nitrogen, leading to higher possible active sites for oxygen reduction.