Hierarchical porous nitrogen doped reduced graphene oxide prepared by surface decoration–thermal treatment method as high-activity oxygen reduction reaction catalyst and high-performance supercapacitor electrodes

Abstract

Nitrogen doped (N-doped) porous reduced graphene oxide (rGO) is successfully obtained by a two-step method, which includes a surface finishing of graphene oxide (GO) followed by thermal treatment. Distinct from many other methods, such as heat treatment, chemical vapor deposition or plasma treatment, a special atmosphere (NH₃) and special equipment is avoided using this method. The nitrogen atoms are introduced into the reduced graphene oxide by this method according to the X-ray photoelectron spectroscopy (XPS) results. Based on the high specific surface area, high porosity, and doped heteroatoms, the as-prepared N-doped reduced graphene oxide shows excellent electrocatalytic activity towards the oxygen reduction reaction (ORR) and high capacitance, compared with a pristine reduced graphene oxide sample. Therefore, this work demonstrates a good example of both electrocatalytic activity and capacitance properties of N-doped reduced graphene oxide prepared by the surface decoration of GO followed by heat treatment method, which opens the door for creating functional graphene materials with highly promising applications in high-performance renewable energy conversion and storage devices.

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1. Introduction

Graphene, a monolayer of sp²-bonded carbon atoms densely packed into a two-dimensional honeycomb crystal lattice, is recognized as an exciting material for its various superior properties such as high electrical and thermal conductivities, good transparency, great mechanical strength, inherent flexibility and huge specific surface area.^1–4 Since the first report in 2004, tremendous work has been carried out to apply graphene into various applications by adjusting its physicochemical properties. Chemical functionalization of graphene oxide, GO, is an effective approach to tailor the properties of graphene, since enriched reactive oxygen functional groups in GO can provide ample covalent bonding sites for the chemical functionalization.^5–7 Moreover, the most widely introduced heteroatom is nitrogen and it is known as N-doped graphene, which behaves differently from pristine graphene, and has been explored for many applications.^8–12 In supercapacitor applications, it is reported that N-doped graphene has an improved specific capacitance compared to the pristine material.^8–15 Furthermore, it also has considerable electrocatalytic activity for the ORR.⁷

Methods recently reported to synthesize N-doped graphene, include heat treatment under ammonia gas atmosphere at high temperature, chemical vapor deposition, plasma treatment and hydrothermal method.^7–18 But all these methods have their inherent shortcomings. For example, no matter heat treatment, chemical vapor deposition or plasma treatment, special atmosphere (NH₃) or special equipment is needed.^8,10 For hydrothermal method, a long preparation time is required, usually over 48 hours, and the aggregation of reduced graphene oxide is hard to avoid and dramatically decreases the surface area, leading to a poor specific capacitance.^7,17 All these are obstacles for high quality and mass productions of N-doped graphene. Therefore, a new strategy is needed to produce high quality N-doped graphene with a large scale.

In this work, we detailed a two-step method for doping nitrogen into graphene sheets with the use of melamine as the nitrogen source. At the meantime, the electrocatalytic activity to ORR and the capacitance performance of N-doped rGO were tested. It manifested that this novel strategy could produce high quality N-doped rGO and is suitable for mass production.

2. Experimental

2.1. Preparation of pristine rGO and N-doped rGO

Graphite oxide (GO) was synthesized using a modified Hummers method. 2 g of graphite and 1 g of sodium nitrate (NaNO₃) were mixed in 50 mL sulfuric acid (H₂SO₄) by rigorous stirring for 3 hours in an ice bath at 0 °C. Then 8 g of potassium permanganate (KMnO₄) was added slowly into the suspension, and the suspension was stirred until its color turned to dark gray. Next, 100 mL of DI water was slowly added to the suspension, and the suspension was maintained at 98 °C for 1 h. To dilute the suspension, 300 mL of DI water was added and followed by a 20 mL H₂O₂ addition to remove the residual KMnO₄. Finally, a bright yellow graphite oxide solution was obtained. The as-prepared GO was washed with DI water several times, followed by a drying step to obtain graphite oxide power. The next step was to obtain GO solution from the graphite oxide powder by sonication. Typically, 2.5 g of the graphite oxide powder was added to 200 mL of DI water, and this solution was sonicated at 300 W for 3 h. The sonicated solution was centrifuged for 30 min at 2000 rpm to remove the remaining graphite oxide powder. The supernatant solution was about 10 mg mL⁻¹ GO.

The pristine rGO, designated as rGO_A, was prepared by reducing 200 mL of the 6 mg mL⁻¹ GO solution with 6 g of ascorbic acid at 95 °C for 3 hours, as route (A) in Scheme 1. When the reduction process was completed, black rGO_A was filtered and washed with DI water and ethanol several times. Finally, rGO_A powder was obtained by vacuum drying at 70 °C overnight.

Scheme 1 Schematic illustrations showing the synthetic routes of rGO_A and MrGO_th.

The N-doped rGO, designated as MrGO_th, was prepared by first functionalizing GO sheets with melamine resin (M-GO), followed by a heat treatment under a vacuum, as route (B) in Scheme 1. For surface modification with melamine resin, melamine resin solution was firstly prepared by dissolving 2.5 g of melamine and 5 mL of 37 wt% formaldehyde solution into 60 mL of DI water at 70 °C with constant magnetic stirring in a flask. After the solution turned transparent, 20 mL of the prepared GO solution was added to melamine resin solution and stirred at 98 °C for 3 hours under continuous stirring. During the stirring, the condensation reaction between melamine resin and GO sheets took place, and bright silvery M-GO powder precipitated. After the reaction completed, M-GO was filtered and thoroughly washed with DI water and ethanol several times. Finally, M-GO powder was obtained by vacuum drying at 70 °C overnight. MrGO_th was obtained by a multistep heat treatment of M-GO (120 °C for 20 min, 800 °C for 2 h and 400 °C for 2 h) with a ramping rate of 5 °C min⁻¹.

2.2. Material characterization

The structures of samples were examined by X-ray diffraction (XRD, X'Pert-PRO MPD) using Cu Kα radiation with a wave length of 1.54 Å. The morphology was examined using scanning electron microscopy (SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2100F), respectively. The specific surface area was measured using the Brumauer–Emmett–Teller (BET) method based on the nitrogen adsorption–desorption isotherm at 77 K on a Micrometritics ASAP2020 sorption analyzer. Surface chemical information was collected by X-ray photoelectron spectroscopy (Thermo ESCALB 250Xi).

2.3. Electrode preparation

To prepare the working electrodes, 2 mg of MrGO_th was dispersed in 1 mL of 5 wt% Nafion aqueous solution. Then the mixture was ultrasonicated for 30 min to obtain a homogeneous catalyst ink. 0.5 μL of 2 mg mL⁻¹ MrGO_th dispersion was dropped onto a mirror like polished glassy carbon electrode with a diameter of 5 mm and dried in air at 70 °C. The rGO_A and Pt/C (20%) was deposited on the electrode under the same procedure.

2.4. Electrochemical characterization

All the electrochemical measurements were conducted on an electrochemical workstation system (CHI760D, Chenhua, Shanghai) with a three-electrode cell. Pt foil and Hg/HgO (1 mol L⁻¹ KOH) electrodes were used as the counter and reference electrodes, respectively. An aqueous solution of KOH (0.1 mol L⁻¹) was used as electrolyte for rotating disk electrode (RDE) voltammogram measurements. Before every test, an O₂ flow was used through the electrolyte for at least 30 min. For the rotating disk electrode test, the same amount catalysts were loaded on a rotating disk electrode. The polarization curves for ORR were conducted with a scan rate of 10 mV s⁻¹ at different rotating speeds from 400 to 1600 rpm from 0.2 to −0.8 V. Electrochemical impedance spectrums (EIS) were measured at −0.6 V from 100 kHz to 0.01 Hz and the perturbing AC amplitude was 5 mV.

The electron transfer number (n) can be calculated by Koutecky–Levich equations as follows:

B = 0.62nFC₀(D₀)^2/3v^−1/6

where J_K is the kinetic current density; J_L is the diffusion-limiting current density; J is the measured current density. B is the reciprocal of the slope; ω is angular velocity of the disk (ω = 2πN). N is the linear rotation speed. F is the Faraday constant; C₀ is the saturated concentration of O₂ in 0.1 mol L⁻¹ KOH at room temperature; D₀ is the diffusion coefficient of oxygen in water; v is the kinematic viscosity of the solution at room temperature. The durabilities of the N-doped graphene and Pt/C were tested by chronoamperometry (CA) measurement conducted at −0.6 V for 1800 s.

Furthermore, for capacitance performance measurement of MrGO_th, cyclic voltammogram (CV) was measured at different scanning rate between −1.0 and 0.0 V in a KOH aqueous electrolyte (1 mol L⁻¹). Meanwhile, galvanic charging–discharging (GC) tests were carried out between −0.95 and 0.0 V with different current densities. The stability of the as-prepared rGO samples was tested by CV measurements at 0.1 V s⁻¹ with 10 000 cycles.

3. Results and discussion

The powder XRD patterns of the pristine graphite, GO and the resulted rGO samples are depicted in Fig. 1(a). In comparison to the pristine graphite, a typical broad (001) peak near 12.72° with an enlarged interlayer spacing of 0.822 nm was observed from the GO pattern. This should be resulted from the incorporation of oxygen functional groups during the oxidation process. After reduction by ascorbic acid and thermal treatment, the typical peak of parent GO around 12.72° disappeared in rGO samples, indicating high exfoliation and de-oxygenation of GO. Besides, weak peaks at around 24° (002) in rGO samples, with a corresponding d₀₀₂ space (0.35 nm) larger than that of the pristine graphite (0.34 nm), are attributed to the irregular stacking of wrinkled graphene nano-sheets. For the more weak intensity of the XRD pattern from MrGO_th, it is indicated that MrGO_th has a more irregular stacking structure than rGO_A.

Fig. 1 (a) XRD patterns of the pristine graphite, GO and the resulted rGO samples; (b) XPS spectra of GO and the resulted rGO samples; (c) XPS analysis of the C 1s peak of MrGO_th; (d) XPS analysis of the N 1s peak of MrGO_th; (e) different nitrogen functionalities in a graphitic plane.

To reveal the composition and the nature of carbon, nitrogen and oxygen bonds in the resulting rGO samples, XPS characterization was performed in this study and all the peaks were normalized by the intensity of C 1s peak (284.8 eV). For the XPS survey spectra from GO and as-prepared rGO samples shown in Fig. 1(b), the peaks at around 284.8 eV and 533.5 eV can be assigned to the binding energies of C 1s and O 1s respectively and it is apparent to find that the intensities of O 1s peaks of the resulting rGO samples have a great decrease comparing with that of GO sample. Furthermore, a N 1s peak at around 399.8 eV is observed in the XPS spectra of MrGO_th. The calculated atomic percentages of C, O and N from GO and as-prepared rGO samples are exhibited in Table 1. The atomic ratios of carbon to oxygen for MrGO_th is 19.09, which is much higher than that of GO (2.12), indicating a substantial elimination of oxygen containing groups in the thermal induced reduction, and moreover by the effect of melamine resin, 7.18% of N in introduced into the reduced graphene oxide sheet. To investigate the chemical state, the C 1s and N 1s XPS peaks of MrGO_th were fitted, as given in Fig. 1(c) and (d). The C 1s peak of MrGO_th can be fitted to four components, corresponding to the C=C/C–C, C–N, C=O and O–C=O species located at 284.8, 285.9, 286.2 and 291.0 eV, respectively. The peak for O–C=O (291 eV) is very weak in resulted rGO samples, indicating infrequent O–C=O groups. However, some oxygen still remains in graphene, which is typical for chemically reduced graphene. Similarly, the N 1s spectra of MrGO_th can be fitted into three peaks centered at 398.1, 399.7 and 401.1 eV, respectively. The peaks with lower binding energies located at 398.1 and 399.7 eV correspond to pyridinic- and pyrrolic-like nitrogen (as illustrated in Fig. 1(e)). The peak located at 401.1 eV in the high-resolution N 1s spectrum can be ascribed to “graphitic” nitrogen atoms (graphitic-center and graphitic-valley), indicating that nitrogen atoms substitute the carbon atoms in the graphene sheets and were incorporated into the carbon network.^11,12

Table 1 The atomic ratios of carbon to oxygen from GO and as-prepared rGO samples

Samples	Atomic percentage of C/%	Atomic percentage of O/%	Atomic percentage of N/%
GO	67.9	32.1	0
rGOA	91.2	8.8	0
MrGOth	88.2	4.62	7.18

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The SEM morphology of the as-synthesized MrGO_th sample in Fig. 2(a) shows a typical curved/wrinkled appearance as a result of the thermal expansion of GO and gasification of the melamine resin anchored at the surface of GO nanosheets with a disordered stacking. Notably, the disordered stacking could lower the total energy through van der Waals cohesion and stabilize the crumpled phase. Furthermore, TEM images also show the wrinkled structures of graphene nanosheets for MrGO_th.

Fig. 2 (a) SEM image of MrGO_th, (b) and (c) TEM images of MrGO_th.

The pore structure of the MrGO_th sample was quantitatively analyzed by the nitrogen adsorption and desorption isotherms as shown in Fig. 3(a). According to the IUPAC classification, the nitrogen adsorption isotherms of the as-prepared rGO was characterized as type IV with hysteresis loops, implying the presence of a large number of mesopores due to the formation of wrinkled and folded few-layered graphene sheets.¹⁴ The determined BET surface area of the as-synthesized rGO sample is 863.5 m² g⁻¹ and the pore volume is up to 1.986 cm³ g⁻¹. According to Fig. 3(b), most of the pores focus on that with diameters sub 10 nm and the average pore diameter is about 3.4 nm in the MrGO_th “fluffy” powder. The specific surface area value is lower than the theoretical surface area of 2630 m² g⁻¹, indicating an average overlap and stacking of ca. 3–4 layers of graphene sheets in the obtained rGO.

Fig. 3 (a) The nitrogen adsorption and desorption isotherms of the as-prepared MrGO_th and (b) surface area distribution with diameter.

All the above results demonstrate the unique porous, crumpled structure of MrGO_th and high nitrogen doping contents. In the thermal treatment of the surface functional GO, the melamine resin could react with GO matrix on the defects and vacancies. For melamine resin anchored on both sides of GO ultrathin sheet, it enabled the nitrogen doping reaction to proceed on both sides of GO, leading to uniform and high content nitrogen doping in graphene.¹³ Therefore, it differs with the nitrogen doped graphene using ammonia gas as the nitrogen source, in which the reaction mostly occurred on the exposed surface of carbon nanostructures, resulting in inhomogeneous and low nitrogen doping content, of the order of 2–4 at%.^15–18

The electrochemical activity for oxygen reduction reaction of the resulting rGO samples was investigated by linear scanning voltammogram on RDE with a rotation from 400–1600 rpm. According to the LSV results shown in Fig. 4(a) and S2,† it is apparent to see that both rGO_A and MrGO_th hold electrochemical catalytic activity to oxygen reduction reaction. Furthermore, LSV shows that limiting current density increases with higher rotation speeds as shown in Fig. 4(a) and (b) and S2.† The phenomenon can be explained by the shortened diffusion distance at high speeds, which is in accordance with other studies.^18–22 The ORR performance of MrGO_th in the diffusion and kinetically limited regions was evaluated using Koutecky–Levich (K–L) plots as shown in Fig. 4(c) and the K–L plots of RDE curves show a linear relationship between 1/j and ω^1/2 from −0.5 to −0.7 V. According to Koutecky–Levich equation, the number of electrons transferred (n) in the ORR process can be calculated and the n value was increased as the applied potential rose. As shown in Fig. 4(d), at the potential of −0.7 V, n of MrGO_th reaches to 3.4, and for 20% Pt/C catalyst is 3.9, indicating that MrGO_th follows an almost four-electron transfer pathway and has better ORR activity than rGO_A. At the 1600 rpm rotating speed, the LSV curves of rGO_A, MrGO_th and Pt/C catalysts were shown in Fig. 4(e) and it is obvious to see that the current density follow the order of rGO_A < MrGO_th < Pt/C. The onset potential of MrGO_th is about −0.12 V, which is only slightly more negative compared to that of Pt/C (−0.08 V). As shown in Fig. 4(f), no matter at the potential of −0.7 V, or at −0.6 V, even at −0.5 V, the current densities of MrGO_th are up to 80% of the current density of Pt/C. Fig. 4(g) reveals that the ORR electrochemical activity of MrGO_th is higher than most of the reported N-doped carbon materials.^23–32 Unlike MrGO_th, rGO_A exhibited a two-step two-electron process for oxygen reduction with the onset potential of about ∼0.26 V. The catalytic current density was found to be 52% less than that of MrGO_th at the potential of −0.8 V. These results indicate that the doping of N atoms in the carbon ring plays a key role in improving the catalytic activity. To further manifest the catalytic activity improvement of the MrGO_th, EIS was measured on a rotation disk electrode at −0.6 V with a rotation of 1600 rpm. Fig. 4(h) represents the Nyquist plots of different catalysts and the data were analyzed by the electric equivalent circuit shown in Fig. 4(i). In the equivalent electric circuit, R_s is the sum of resistance of electrolyte, electrode material and the contact resistance at the interface of the active material/current collector; Q₁ is a constant phase element, and Y₁ and n₁ are the parameters of Q₁; C₁, which represents the double layer capacitance, can be calculated by C₁ = [Y₁ × R₁^(1 − n₁)]^(1/n); R₁ is the resistance of the double layer; Q₂ represents the property of intermediate adlayer and R₂ is charge transfer resistance. The values of R_s, Q₁, R₁, Q₂ and R₂ were calculated from the CNLS fitting of the experimental impedance spectra and their resulting values are listed in Table 2. According to the fitting results of C₁ and R₁, it can be concluded that rGO_A has the weakest ability to absorb O₂, which induced the largest resistance in O₂ absorbing process and MrGO_th has a similar ability to absorb O₂ with Pt/C catalyst. The results of C₂ indicate that both MrGO_th and Pt/C catalysts have fewer amounts of intermediates than rGO_A, due to their nearly direct 4 electron transfer in O₂ reduction process. Moreover, the charge transfer resistance has a great decrease from rGO_A to MrGO_th. Therefore, all these EIS results manifest that MrGO_th has a better electrocatalytic activity than rGO_A by N-doping. Furthermore, the stabilities of MrGO_th and Pt/C were tested by chronoamperometry (CA) measurements conducted at −0.6 V for 1800 s. All the data collected were normalized by the initial current density and shown in Fig. 4(j). It is apparent to see that the current density of Pt/C decreased to 88% of the initial current density after 1800 s test, which means the deterioration of the electrocatalytic activity. But for MrGO_th, the current density has an increase before 400 s and then a decrease. After 1800 s test, the current density is still 110% of the initial one. Therefore, it manifests that MrGO_th has a better durability than Pt/C.

Fig. 4 (a) ORR polarization curves (at different rotation speeds) of MrGO_th; (b) ORR polarization curves (at different rotation speeds) of Pt/C; (c) Koutechky–Levich plots at different potentials of MrGO_th; (d) comparison of electron-transfer number from different catalysts; (e) ORR polarization curves of different catalysts at 1600 rpm; (f) comparison of current densities at different potentials from different catalysts; (g) current density comparison with results previous reported;^23–32 (h) Nyquist plots of different catalysts at −0.6 V; (i) electric equivalent circuit; (j) proportion to the initial current density.

Table 2 Fitting results of EIS from different catalysts

	Rs (Ω)	C1 (F)	R1 (Ω)	C2 (F)	R2 (Ω)
rGOA	3.645	6.228 × 10^−6	782	7.71952 × 10^−5	8044
MrGOth	3.868	9.16284 × 10^−6	88.54	4.30614 × 10^−5	7170
Pt/C	3.513	9.58932 × 10^−6	189.4	3.99385 × 10^−5	5619

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To have a deep insight in ORR catalytic activity improvement from rGO_A to MrGO_th by N-doping, the oxygen adsorptions on graphene and N-doped graphene were studied based on Density Functional Theory (DFT) by using Materials studio. According to results shown in Fig. 5, it is apparent to know that for graphene, the oxygen molecule is physically adsorbed on the center position above the C6-ring, which is 2.824 Å from the graphene sheet. The length of the chemical bond in oxygen molecule is about 1.263 Å, which is longer than that of oxygen molecule in vacuum (1.235 Å). Furthermore, for N-doped graphene, the adsorption position of oxygen molecule is changed by the effect of the doped nitrogen atom. One of the atoms in O₂ is located on the top of carbon atom neighbored nitrogen atom and it is 2.461 Å apart from this carbon atom. The other oxygen atom is above the center of C5N-ring and 2.775 Å apart from. The bond length in oxygen molecule enlarges to about 1.280 Å. It indicates that the doped nitrogen atom in graphene could increase the bond length of the adsorbed O₂ and decrease the bond energy.^33,34 Therefore, O₂ absorbed on N-doped graphene is more active than that on pure graphene.

Fig. 5 O₂ adsorbed on pure graphene (a) and on N-doped graphene (b).

In order to investigate the effects of N-doping on supercapacitance of reduced graphene oxide, cyclic voltammetry (CV) and galvanostatic charge–discharge tests were carried out using a three electrode system. Fig. 6(a) shows the CV curves of rGO_A and MrGO_th at the scan rate of 0.05 V s⁻¹. The rectangular CV curves with a pair of broaden peaks were observed, which indicate the pseudo-capacitance was introduced into the double layer capacitance. Additionally, MrGO_th electrode exhibits much higher current density than the electrode of rGO_A, indicating increased capacitance due to N-doping. Cyclic voltammetry of rGO_A and MrGO_th at various scan rates are shown in Fig. S3.† As the scan rate increases, the CV curves of rGO_A have more obvious deviations from rectangularity than that of MrGO_th. It indicates MrGO_th has a more excellent rate capability than rGO_A. The improved capacitance of MrGO_th can also be observed from the galvanostatic charge–discharge curves at a current density of 2 A g⁻¹ as shown in Fig. 6(b). The galvanostatic charge–discharge curves of as-prepared rGO samples at various current densities are shown in Fig. S4.† All these charge–discharge curves at different current densities show linear responses and good symmetry, demonstrating the excellent double layer capacitive behavior. Fig. 6(c) shows the dependence of the specific capacitance of rGO_A and MrGO_th on the discharge current densities. It is clearly to conclude that MrGO_th has a better rate capability than rGO_A. This may be attributed to the synergistic effects of high specific surface area and N-doping of MrGO_th. As Fig. 6(c) shown, the specific capacitance of MrGO_th is 163 F g⁻¹ at the current density of 1 A g⁻¹. Due to the synergistic effects of high surface area and N-doping, the specific capacitance is still as high as 108 F g⁻¹ even at the current density of 100 A g⁻¹. When the current density increases to 200 A g⁻¹, the specific capacitance also remains to 78 F g⁻¹, indicating the MrGO_th shows the best rate capability to our known.^35–37 The electrochemical stability of MrGO_th electrode for supercapacitor application was also evaluated using the cyclic voltammetry at a scan rate of 0.1 V s⁻¹. As shown in Fig. 6(d), after 10 000 cycles there is no significant change in the shape of CV curves. The capacitance retention of MrGO_th is 92.68% after 10 000 cycles as shown in Fig. 6(e). Therefore, it demonstrates that MrGO_th has excellent electrochemical stability and a high degree of reversibility.

Fig. 6 (a) CVs of as-prepared samples at the scan rate of 0.05 V s⁻¹; (b) galvanostatic charge–discharge curves of as-prepared samples at charge current density of 2 A g⁻¹; (c) specific capacitance of as-prepared samples at different discharge current densities; (d) CV cycling performance of MrGO_th; (e) CV cycling stability at a scan rate of 0.1 V s⁻¹.

The above results clearly reveal that MrGO_th exhibits significantly improved electrochemical capacitance compared with rGO_A. It should be noted that MrGO_th has a high nitrogen doping content (the ratio of C to N is 12.28) with apparent crumpled structures which can help it achieve high capacitance, excellent rate capability and cycle performance. As reported by previous works, there are four reasons for the improved supercapacitance: firstly, pyridinic N and pyrrolic N are assumed to be the main configurations constructing to the faradic reaction based pseudo-capacitance. It is also reported that the presence of quaternary N can enhance the conductivity of the materials, which is beneficial for rate capability of the supercapacitor performance.^34,38,39 Additionally, the heterogeneous nitrogen atom, which is introduced into carbonaceous structures, could change the electron distribution of the materials, which can further enhance the wettability between electrode materials and electrolyte, leading to a significant increase of active surface area accessible to the electrolyte.³⁴ Furthermore, the remaining oxygen-containing surface functional groups such as hydroxyl and carbonyl groups can provide pseudo-capacitance as a complement to the capacitance. Finally, the high temperature thermal-treatment can increase the conductivity of carbon-based materials and give a highly crumpled structure to the reduced graphene oxide. Therefore, it can provide a high accessible specific surface area for adsorbing ions and accelerating electron transfer. Therefore, it resulted in an excellent rate capability of MrGO_th compared with many other reported samples.^35–37

4. Conclusions

N-Doped porous reduced graphene oxide was successfully obtained by a melamine resin surface finishing and thermal treatment method. The physical characterizations demonstrated that the nitrogen atoms were introduced into the reduced graphene oxide by this method. Based on the high specific surface area, high porosity, and doped hetero-atoms, the as-prepared MrGO_th shows excellent electrocatalytic activity towards the ORR and high capacitance compared with reduced graphene oxide sample. Therefore, this work demonstrates a good example of both electrocatalytic activity and capacitance properties of N-doped rGO prepared by surface decorating of GO and followed heat treatment method, which opens the door for creating functional graphene materials with highly promising applications in high-performance renewable energy conversion and storage devices.

Acknowledgements

The authors are grateful for the financial support by One Hundred Talent Program of Chinese Academy of Sciences, Chinese National Programs for High Technology Research and Development (2014AA06A513), as well as by the NSFC (51302264, 51404230) of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09402c

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