## 3 Results and Discussion
### Electrocatalytic Activity  
The LSV curves of 10% Fe/N-rGO, 15% Fe/N-rGO and 20% Fe/N-rGO are presented in Figure 5a,c,e, respectively and their corresponding K-L plots at different potentials are illustrated in Figure 5b,d,f.  
Figure 4: Surface morphology of (**a**) 10% Fe/N-rGO, (**b**) 15% Fe/N-rGO and (**c**) 20% Fe/N-rGO. (**d**) Element mapping of 20% Fe/N-rGO with its corresponding Fe, O and C.  
At 1600 rpm, 20% Fe/N-rGO exhibited the highest onset potential of 0.84 V (vs. RHE), whereas 15% Fe/N-rGO showed the lowest onset potentials 0.72 V (vs. RHE). The onset potential of 10% Fe/N-rGO was 0.74 V (vs. RHE). The onset potential of 20% Fe/N-rGO was comparable with those reported by [PERSON] et al. [7], [PERSON] et al. [24] and [PERSON] et al. [25]. A higher onset potential indicated a higher catalytic activity. No limiting current plateau is observed in Figure 5a,c,e, indicating that the ORR in all the three samples was dominated by the mixed kinetics-diffusion controlled mechanism. Fe nanoparticles are possibly confined in rGO layers, so the active sites of nanoparticles may be hindered from participating in ORR. The ORR of these samples is also affected by the degree of carbon layer graphitization [40,41].  
The number of electron transfers was estimated with the K-L plot at 0.34, 0.35 and 0.36 V (vs. RHE). A decent straight relationship could be observed between J\({}^{-1}\) and \(\omega^{-1/2}\) at  
Figure 5: (**a,c,e**): Rotating disk electrode (RDE) measurement of the oxygen reduction reaction (ORR) of Fe/N-rGO catalyst with Fe loadings of 10%, 15% and 20% in O\({}_{2}\)-saturated 0.1 M KOH, respectively. (**b,d,f**) Corresponding Routecky–Levich plots at 0.34, 0.35 and 0.36 V (vs. RHE), respectively.  
various cathode potentials. The number of electron transfers was calculated using Equation (2) at the different electrode potentials. The results showed that 10%, 15% and 20% Fe/N-rGO had average electron transfers of 1.37, 1.26 and 2.25, respectively, suggesting that the preferred reaction pathway for these catalysts was two-electron reduction. This pathway could result in an reduction of oxygen to form peroxide ions (HO2\({}^{-}\)) as the final product. [PERSON] et al. [42] have suggested that O2 prefers to adsorb on single atom catalyst in \"end-on\" binding mode that hinders O-O bond breakage, which may be the case in our catalyst, judging from the EDS result. The rate of generating OH\({}^{-}\) increases as the concentration of HO2\({}^{-}\) increases. The increasing trend of HO2\({}^{-}\) concentration leads to an increase in the dissolved Fe ion concentration [42, 43]. These results contradicted some of the findings that suggested the four-electron reaction pathway for this type of catalyst. Similar to other non-pyrolyzed Fe-based catalysts, these catalysts have a very low catalytic activity for fuel cell applications [44]. This observation could be attributed to the lack of an appropriate ratio of doped N and the contribution of Fe-Nx. rGO needs a high amount of N because of its large surface area. N should be added to more than 60 wt.% of rGO [45]. The weight percent of urea was one-third of rGO; as such, no strong C-N sites were detected in this work. Adding metallic N could improve electron densities and develop N-supported sites with carbon and Fe, directly participating in ORR. The intense interaction between Fe-Nx and O2 facilitates a fast electron transfer. Fe3C, N and Fe are important species to improve ORR [5, 24, 46, 47, 48].