## 4 Conclusions  
In this paper, we report a viable method for the synthesis of graphene materials and nitrogen-doped reduced graphene oxide derivatives with enhanced properties. These are based on the chemical oxidation of graphite flakes with different particle sizes and selecting the experimental conditions used during the subsequent thermal exfoliation process. The characterization results suggest that the starting particle size and thermal conditions applied during the exfoliation treatment remarkably affect the final surface properties of the prepared materials. The results point out that smaller particle sizes lead to higher surface areas. We achieved surface areas of 867 m\({}^{2}\)g\({}^{-1}\) for rGO. For NrGO samples obtained from graphite 10, 100, and 325, we observed BET surface areas of 236, 420, and 492 m\({}^{2}\)g\({}^{-1}\) respectively, which may provide a new way to control the surface area of N-doped graphene. The use of different reduction atmosphere (NH\({}_{3}\) versus inert) allow to successfully introduce nitrogen within the graphenic structure, enhancing the electronic properties and basicity of the dopedmaterials. For the N-doped samples, the amount of nitrogen introduced and surface areas could be also tailored. As a result of their tailored enhanced properties, these optimized NrGO and rGO samples were successfully applied as ORR electrocatalysts. The NrGO samples showed better ORR performance in alkaline medium with onset potentials ranging from 0.79 and 0.82 V versus RHE, low Tafel slopes (62-99 mV dec\({}^{-1}\)), and good \(j_{\mathrm{L}}\) values (\(-\)3.03--4.05 mA cm\({}^{-2}\)). The better performance of NrGO\({}_{325}\)-4 was attributed not only to the higher content of nitrogen but also to the smaller particle size of the starting material. Moreover, all the graphene materials presented good durability/stability and very low sensitivity to methanol. This work has led to a new class of metal-free ORR electrocatalysts with good efficiency and stability. The following are available online at [[http://www.mdpi.com/2079-4991/9/12/1761/s1](http://www.mdpi.com/2079-4991/9/12/1761/s1)]([http://www.mdpi.com/2079-4991/9/12/1761/s1](http://www.mdpi.com/2079-4991/9/12/1761/s1)), Experimental details; Figure S1 displays N\({}_{2}\) adsorption-desorption isotherm for all samples; Figure S2 contains XRD spectra for rGO and NrGO samples; Figure S3 contains TEM images of the different graphene materials; Figure S4 shows survey XPS for rGO\({}_{m}\)-2 and NrGO\({}_{m}\)-5 samples; Figure S5 contains TGA for graphene materials; Figure S6 contains the CVs of rGO\({}_{m}\)-3 and NrGO\({}_{m}\)-4 modified electrodes in N\({}_{2}\)-saturated and O\({}_{2}\)-saturated 0.1 mol dm\({}^{-3}\) KOH solution; Figure S7 contains CVs, LSVs, and K-L plots for Pt/C; Figure S8 contains the ORR polarization curves of rGO\({}_{m}\)-3 and NrGO\({}_{m}\)-4 modified electrodes at different rotation rates in O\({}_{2}\)-saturated solution; Figure S9 contains LSVs recorded with RRDE and the estimated percentage of H\({}_{2}\)O\({}_{2}\) formed for Pt/C and NrGO\({}_{325}\)-4. [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] carried out the synthetic experiments; [PERSON] and [PERSON] conducted the electrocatalytic measurements; [PERSON] performed the Raman characterization; [PERSON] and [PERSON] wrote the original draft manuscript; and all the authors discussed the results and contributed to the manuscript. This research was supported by the Spanish Agencia Estatal de Investigacion (AEI) under projects CTQ-2017-89443-C3-1-R and CTQ-2017-89443-C3-3-R. C.S.R.B. gratefully acknowledges financial support from Spanish Ministerio de Educacion, Cultura y Deporte, Grant N\({}^{\mathrm{s}}\) FPU15/01838. D.M.F. also thanks Project UNIRCELL - POCI-01-0145-FEDER-016422 funded by European Structural and Investment Funds (FEEI) through - Programa operacional Competitividade e Internacionalizacao - COMPETE2020 and by national funds through FCT - Fundacao para a Ciencia e a Tecnologia, I.P.