## 3 Results and Discussion
### ORR Electrocatalytic Activities
As discussed above in Section 3.1, usually a higher degree of oxidation is obtained in GO samples with smaller graphite particle size, and on the other hand, oxygen functional groups are responsible for reacting with NH\({}_{3}\), leading to the incorporation of nitrogen in the structure, which will favor ORR. Elemental analysis showed that NrGO\({}_{325}\)-4, NrGO\({}_{100}\)-4, and NrGO\({}_{10}\)-4 presented 5.0%, 3.8%, and 1.8% of N, respectively, whereas the XPS results indicated percentages of 3.4%, 3.3%, and 3.2%. The NrGO\({}_{325}\)-4 sample presents a higher N% at the surface and has also a higher percentage of pyridinic nitrogen. The latest have been reported to work as ORR electrocatalytic sites due to the electron donated to the conjugated \(\pi\)-bond of graphene and to the lone pair of electrons they have [61; 62]. Figure 8: \(n_{O2}\) at several potential values (**a**); ORR Tafel plots (**b**); chronoamperograms in O\({}_{2}\)-saturated KOH electrolyte at 1600 rpm and \(E\) = 0.50 V vs. reversible hydrogen electrode (RHE), with the addition of methanol (0.5 mol dm\({}^{-3}\)) at \(t\) = 500 s (**c**) and without for 20000 s (**d**). To confirm the excellent result obtained for NrGO\({}_{325}\)-4, RRDE tests were performed. The \(j\) values obtained at the disk and ring are depicted in Figure S9a. For comparison, the Pt/C data were also included. Equation (1) was used to estimate the percentage of H\({}_{2}\)O\({}_{2}\) produced, and the results are presented in Figure S9b at several potential values. The NrGO\({}_{325}\)-4 electrocatalyst presented a low percentage of H\({}_{2}\)O\({}_{2}\) (\(\approx\) 15%), which is in accordance with the high \(\bar{n}_{02}\) value (\(\bar{n}_{02}\) = 3.9), while a value of 3% was obtained for Pt/C. Tafel plots (Figure 8b) were obtained from LSV data in Figures S8 and S7b in KOH electrolyte purged with O\({}_{2}\) at 1600 rpm. The Tafel slopes obtained between \(E\) = 1.00 to 0.70 V versus RHE were 65, 48, 98, 78, 99, 62, and 89 mv dec\({}^{-1}\) for rGO\({}_{10}\)-3, rGO\({}_{100}\)-3, rGO\({}_{325}\)-3, NrGO\({}_{10}\)-4, NrGO\({}_{100}\)-4, NrGO\({}_{325}\)-4, and Pt/C, respectively. All electrocatalysts except for rGO\({}_{325}\)-3 and NrGO\({}_{100}\)-4 presented Tafel slopes lower than those obtained for Pt/C, which suggests that the graphene materials can easily adsorb O\({}_{2}\) onto their surface and activate it, promoting a robust electrocatalytic activity toward ORR. The different Tafel slopes obtained for rGO\({}_{325}\)-3 and NrGO\({}_{100}\)-4 are most likely associated to fluctuations in the oxygenated intermediates adsorption strength or to an incomplete electrocatalyst, which is used as a consequence of mass transport losses [63; 64]. It is recognized that the intermediates adsorption strength is dependent on the physical properties and on the chemical nature of the selected electrocatalyst, which rules the determining step rate. For these two electrocatalysts, the first discharge step upon the consumption of the MOOH with a high coverage of MOO\({}^{-}\) is the rate-determining step, whereas for the remaining electrocatalysts, the conversion of MOO\({}^{-}\) to MOOH rules the overall reaction rate (where M stands for an empty site on the electrocatalyst surface) [63]. Tolerance to methanol crossover is another parameter that is usually evaluated because in fuels cells run on methanol, this can be a problem, as the performance of the cathode can be drastically reduced if the catalyst in sensitive to methanol. Therefore, to evaluate this parameter, chronoamperometric tests were performed in a KOH electrolyte purged with oxygen for 2500 s, to which 0.5 mol dm\({}^{-3}\) of methanol were added at \(t\) = 500 s (Figure 8c). The presence of methanol led to an impressive decrease (\(\approx\) 49%) of Pt/C current. Oppositely, the graphene materials are much more stable and less sensible to methanol with current retentions between 78% and 92% suggesting higher selectivity toward oxygen reduction. The EC stability is also of extreme importance.