Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors

Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors

Accepted Manuscript Title: Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors ´ Author: Agata Sliwak Bartosz Gr...

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Accepted Manuscript Title: Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors ´ Author: Agata Sliwak Bartosz Grzyb Noel D´ıez Gra˙zyna Gryglewicz PII: DOI: Reference:

S0169-4332(16)32777-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.12.060 APSUSC 34607

To appear in:

APSUSC

Received date: Revised date: Accepted date:

9-8-2016 6-12-2016 9-12-2016

´ Please cite this article as: Agata Sliwak, Bartosz Grzyb, Noel D´ıez, Gra˙zyna Gryglewicz, Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.12.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nitrogen-doped reduced graphene oxide as electrode material for high rate supercapacitors

Agata Śliwak, Bartosz Grzyb, Noel Díez, Grażyna Gryglewicz*

Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wrocław University of Technology, Gdańska 7/9, 50-344 Wrocław, Poland

E-mail

addresses:

[email protected]

(A.Śliwak),

[email protected]

(B.Grzyb), [email protected] (N.Díez) *Corresponding

author.

Tel:

+48

713206398.

E-mail

address:

[email protected] (G. Gryglewicz)

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Graphical Abstract

N6 NQ

NC NX

N5

300

100

250

90

6 200

5 4 3

80 150

2 50

1 0

0

-15 GO

r

N-

N-

r

0 -18 GO

0 -20 GO

r

N-

70

rGO-180 N-rGO-180 rGO-180 N-rGO-180

100

60

0

Rate capability / %

N1s

7

C / F g-1

Nitrogen content / at.%

8

50 0

20

40 60 80 Scan rate / mV s-1

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Highlights 

N-reduced graphene oxides were obtained by hydrothermal route using graphene oxide.



Amitrole was used as N-dopant.



Very high nitrogen amounts of 10.9 to 13.4 at.% were introduced to the graphenes.



N-rGO-180 exhibited a high capacitance value and superior cycling stability.



Excellent rate capability of 98% was achieved in a wide scan range.

Abstract Nitrogen-doped reduced graphene oxides (N-rGOs) have been synthesized at various temperatures by a facile hydrothermal route involving the doping of an aqueous graphene oxide dispersion with amitrole. The N-rGOs had a nitrogen content ranging from 10.9 to 13.4 at%, which is among the highest reported for this type of material. The predominant nitrogen species were pyridinic followed by amide/amine, pyrrolic, and quaternary nitrogen. Cyclic 2

voltammetry and impedance spectroscopy measurements performed on the N-doped and nitrogen-free samples revealed that nitrogen fixation provided the material with pseudocapacitive behaviour and improved ion diffusion and charge propagation. A high specific capacitance of 244 F g-1 was obtained at a high scan rate of 100 mV s-1 for the N-rGO with the highest nitrogen content. An outstanding rate capability for the N-rGO, with increasing scan rates, of 98% was obtained, while only 70% was obtained for the non-doped rGO. 92% of the initial capacitance was maintained over 5000 charge/discharge cycles due to the high stability of the electrochemically active nitrogen moieties. Hydrothermal synthesis using amitrole as a nitrogen dopant represents a simple route for the synthesis of graphene with very high nitrogen content and exceptional behaviour for use as electrode material in high-power supercapacitors.

Keywords: nitrogen-doped reduced graphene oxide; amitrole; hydrothermal treatment; supercapacitor; pseudocapacitance

1.

Introduction

Graphenes have brought researchers attention as a promising electrode active material for pseudocapacitors, owing to its high electrical conductivity, good electrochemical stability, and large theoretical specific surface area [1,2]. However, the reported specific capacitance values of graphene materials, when used as electrodes in 3

supercapacitors, are not sufficient to fulfil the energy and power requirements for commercial devices [3]. This drawback principally arises from the bundling and restacking of graphene layers, which significantly reduces their specific surface area [4], and arises from the hydrophobic character of the material, which limits the interaction of its surface with aqueous electrolytes [5]. Chemical doping with electron-donating or electron-withdrawing atoms, such as N, B, S, or O, has been investigated in order to improve the electrochemical performance of the graphene materials [1,6,7]. The incorporation of these heteroatoms in the graphene structure is known to enhance the wettability of the graphene materials. Moreover, these heteroatoms can form electrochemically active species that undergo reversible faradaic redox reactions during charging/discharging, leading to an increased specific capacitance. Specifically, nitrogen can be easily introduced into the graphene structure, forming strong atomic bonds due to its similar atomic size compared with that of carbon atoms [4,5]. Moreover, doping with nitrogen increases the density of charge carriers in the material because of the contribution of nitrogen pelectrons to the graphene π-system. The type of nitrogen groups plays an important role in the electrochemical performance of the N-doped materials. Pyridinic (N6), pyrrolic/pyridonic (N5), and pyridine-oxide (NX) groups are responsible for increasing the overall energy density through reversible redox reactions, whereas quaternary nitrogen (NQ) improves the conductivity of the electrode material [3,8,9]. These features of N-graphenes lead to the consideration of these materials as promising candidates for active electrode material in supercapacitors. The performance of N-graphenes has been intensively studied using different electrolytes [5,10,11]. However, the relatively high values of the capacitance reported for N-graphenes decrease significantly at high current regimes [4,7,8]. Lee et al. [8] described the

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preparation of N-reduced graphene oxides by hydrothermal treatment of the graphene oxide solution and hexamethylenetetramine as a nitrogen source. Despite high nitrogen content (8.6 at%), the capacitance value was only 161 F g-1 at 0.5 A g-1 in 6 mol L-1 KOH and decreased to 141 F g-1 at a current density of 4 A g-1. The rate capability was 88%, while the current density was varied from 0.5 to 4 A g-1. Jeong et al. [12] also observed a large decrease of the capacitance of N-graphene material containing 1.9 at% of nitrogen from 280 to 200 F g-1 when the current densities were increased from 0.2 to 20 A g-1. A significant capacitance drop with increasing current density was also reported for N-doped graphene-based capacitor operating in organic electrolyte [10]. Thus, increasing the rate capability of these materials remains a challenge for many researchers. Many preparations of N-doped graphene have been reported. Chemical vapor deposition (CVD), nitrogen plasma, nitrogen-assisted arc discharge, and hightemperature ammonization constitute the most common methods to synthesize singleor few-layer N-graphene sheets [1,13,14]. However, the scalability of these techniques is limited by their complexity, the high cost of the equipment, and low process yield. In this regard, the hydrothermal treatment of graphene oxide in the presence of N-containing compounds such as ammonia [15,16], ammonium oxalate [17], hexamethylenetetramine [8], glucosamine [18], pyrrole [6], hydrazine [19], urea [9], and aminoacids [20] has recently been proposed as a simple and scalable procedure for the preparation of N-graphene materials. Our research group recently showed the suitability of guanidine, amitrole, and imidazole as nitrogen dopants for the synthesis of N-reduced graphene oxides via hydrothermal treatment [21]. Specifically, treatment with amitrole led to graphene materials with notably high nitrogen content and a high contribution of heterocyclic and quaternary nitrogen.

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In this work, we investigate the optimization of the hydrothermal synthesis of N-rGOs using amitrole as doping agent and their application as electrode material in supercapacitors. The optimum hydrothermal temperature of 180 °C led to the N-reduced graphene oxide with the highest nitrogen content. The electrochemical performance of this material was evaluated using galvanostatic charge/discharge, cyclic voltammetry, and electrochemical impedance spectroscopy. Nitrogen fixation upon hydrothermal treatment led to a very high capacitance value of 209 F g -1 at 20 A g-1 in 6 mol L-1 KOH and an excellent rate capability of 87% from 0.2 to 20 A g-1 and 98% from 1 to 100 mV s-1. This exceptional rate capability at high current density is reported for the first time.

2.

Materials and Methods

2.1.

Synthesis of materials

A coal-tar pitch-based graphite supplied by INCAR-CSIC (Oviedo, Spain) was oxidized by the Hummers method and exfoliated in an ultrasonic bath, as reported elsewhere [22], yielding a graphene oxide aqueous dispersion (GO) with a concentration of 1 mg ml-1. 2 g of amitrole (3-amino-1,2,4-triazole, Sigma-Aldrich) was dissolved in 200 ml of the GO suspension, resulting in an amitrole/GO mass ratio of 10:1. The mixture was hydrothermally treated in a steel autoclave at 150, 180 or 200 °C for 8 h. After the reaction, the autoclave was allowed to cool to room temperature. The resulting black precipitates were centrifuged and subsequently washed in deionized water five times. Subsequently, the products were washed with isopropanol and finally dried in an oven at 65 °C for 24 h. The N-rGOs obtained at different temperatures were labelled as N-rGO-T, where T refers to the temperature of reaction. The non-doped rGO (rGO-180) was independently synthesized as reference material

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by the hydrothermal treatment of GO at 180 °C for 8 h following the same experimental method. 2.2.

Structural and chemical characterization

The morphology of the materials was evaluated by field-emission scanning electron microscopy (FE-SEM) with a Merlin Zeiss microscope with an accelerating voltage of 3 kV. The porous texture of resultant samples was determined by N2 sorption at 77 K using a gas sorption analyzer (Autosorb IQ, Quantachrome). The specific surface area (SBET) was calculated based on the BET method using the adsorption data in the relative pressure (p/p0) range from 0.02 to 0.2. The crystalline characteristics of the resultant materials were determined by X-ray diffraction (XRD) with an Ultima IV Rigaku analyzer using Cu Kα2 radiation, λ=1.54056 Å. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5000 VersaProbe (ULVAC-PHI) spectrometer. The surface atomic concentrations were calculated from the ratio of the corresponding peak areas after correction with theoretical sensitivity factors based on the Scofield’s photoionization cross-sections. The sample charging was corrected using the C1s peak at 284.6 eV as an internal standard. Deconvolution of the high resolution C1s, N1s, and O1s core-level spectra was performed using the CasaXPS software after a Shirley background subtraction. Peak fitting was performed using a Gaussian-Lorentzian (70/30) peak shape at the same FWHM for all fitted peaks. The C1s core level spectra were deconvoluted into four components, corresponding to graphitic sp2-hybridized C (C=C, 284.5 ± 0.1 eV), hydroxyl, epoxy, and C-N linkages (CO/C-N, 286.5 ± 0.3 eV), carbonyl (C=O, 287.6 ± 0.2 eV), and carboxylic groups (COOH, 289 ± 0.4 eV) [23-25]. The N1s core level spectra of the N-doped samples were deconvoluted into five components, corresponding to pyridinic-N (N6, 398.7 ± 0.3 eV), pyrrolic-N (N5, 400.3 ± 0.3 eV), quaternary-N (NQ, 401.4 ± 0.5 eV), amides/amines or lactams (NC, 399.7 ± 0.2 eV),

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and pyridine N-oxides (NX, 402-405 eV) [26]. The standard deviations for each peak area of the deconvoluted C1s and N1s spectra are given in Table S1. 2.3.

Electrode assembly and electrochemical testing The electrodes with a surface area of 0.9 cm2 were prepared by mixing 85 wt%

of active material (N-free or N-doped reduced graphene oxide), 5 wt% of carbon black (CB), and 10 wt% of polyvinylidene fluoride (PVDF, Kynar Flex 2801). The measurements were performed in the Swagelok ® three electrode assembly using Hg|HgO as the reference electrode. Activated carbon was used as the counter electrode. The electrodes were separated by a glassy fibrous membrane. 6 mol L -1 KOH aqueous solution was used as the electrolyte. Gold current collectors were used in order to preserve comparable experimental conditions. The electrochemical properties of graphene samples were determined using a potentiostat-galvanostat VSP Biologic (France). Cyclic voltammetry technique was applied at voltage scan rates from 1 to 100 mV s-1. The galvanostatic cycling method was used at current loads between 0.2 and 20 A g-1. The

electrochemical impedance spectroscopy was performed at

frequencies ranging from 10 mHz to 100 kHz. The capacitances were expressed in Farads per mass of active material in one electrode. The specific capacitance values (C, F g-1) are calculated from the galvanostatic discharge curves and from the cyclic voltammograms in accordance with equations (1) and (2), respectively. 𝐶=

∫ 𝐼𝑑𝑡 𝑈𝑚𝑒𝑙

(1)

𝐶=

∫ 𝐼𝑑𝑡 𝜈𝑚𝑒𝑙

(2)

where I is the current (A), U is the operating cell voltage (V), t is the time (s), ν is the scan rate (V s-1), and mel is the mass of the active material in electrode (g).

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3.

Results and Discussion

3.1.

Influence of hydrothermal treatment temperature on the N-rGO surface

chemistry The GO was reduced under hydrothermal treatment in the presence of amitrole at various temperatures ranging from 150 to 200 °C. The XPS N1s and C1s spectra of the initial GO and hydrothermally reduced materials are presented in Fig. 1. Table 1 shows the elemental composition and functional groups distribution, determined by XPS, for GO and the hydrothermally treated samples. Very high nitrogen contents were found in the samples treated with amitrole. The maximum content of 13.4 at% was found in the sample treated at 180 °C. An increase in the reaction temperature to 200 °C led to a decrease in the nitrogen content of the reduced graphene oxide to 10.9 at%. Nitrogen speciation in the samples treated with amitrole was investigated by deconvolution of the high resolution N1s core level spectra (Fig. 1a). The content of each type of nitrogen was calculated by multiplying the total nitrogen content by the contribution of each deconvoluted peak to the overall N1s spectrum (Table 1). Pyridinic-N (N6) was the predominant type of nitrogen in all of the N-graphenes with a contribution of 4648% to the total N1s signal (Table 1). It is worth mentioning that the highest content of pyridinic-N was in the sample N-rGO-180 (6.4 at%), which also has a relatively high content of graphitic nitrogen (NQ, 1.9%). These nitrogen species are responsible for pseudocapacitive behaviour and enhanced conductivity, respectively [27], and N-rGO180 can therefore be considered a promising material for use as electrodes in supercapacitors. The oxygen content decreased upon conventional hydrothermal reaction (reaction without amitrole) from 37.2 at% in GO to 16.2 at% in rGO-180, due to the evolution of oxygen moieties and organic fragments that takes place during hydrothermal reduction [28]. 9

We previously showed that the oxygen content in the reduced graphene oxide can be reduced to 11.2 at% by applying a high hydrogen pressure in the headspace of the autoclave [29]. It should be noted that hydrothermal treatment of GO with amitrole using the same experimental conditions led to a higher oxygen removal. Thus, N-rGO-180 exhibited an oxygen content as low as 9.4 at%. The ability of N-compounds to promote oxygen removal has been reported in other works [18,20]. Deconvolution of the high resolution C1s core-level spectra confirms the removal of the oxygen groups upon hydrothermal treatment (Figure 1b, Table 1) [19]. Despite having lower oxygen content than rGO, the N-doped materials exhibited a more intense deconvolution peak at 286.5 eV (ascribed to C-O and C-N bonds), due to the large incorporation of nitrogen into the carbon structure. The intense and welldefined peak ascribed to C=C bonding indicates a significant restoration of the sp2–hybridized graphitic structure [19,30]. The restoration of the graphene structures in the hydrothermally treated samples is corroborated by their XRD patterns, shown in Fig. 2a. The structural parameters of the prepared materials determined by XRD are presented in Table S2. GO shows a 001 diffraction peak at 10.5° corresponding to an interlayer distance of 0.8423 nm, typical of graphene oxide [31]. The peak disappears in the patterns obtained from the reduced samples, but a new 002 peak at ~25° is observed. The interlayer distance decreased with the temperature of treatment, from 0.3647 to 0.3526 nm for N-rGO-150 and N-rGO-200, respectively, indicating a strong impact of the process temperature on the restoration of the graphene structure. The height of crystallite (Lc) is in the range of 1.6-2.1 nm for the reduced samples, which is more than twice lower compared with GO. The introduction of nitrogen into the graphene nanosheets decreases slightly an average number of stacking graphene layers from 7 for rGO to 5-6 for N-rGOs. The interlayer distance calculated for rGO-180 (0.3585 nm) was highly similar to that of N-rGO-180 (0.3586 nm). FE-SEM analysis of rGO180 and N-rGO-180 revealed an aggregated structure of free-standing wrinkled graphene

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nanosheets (Fig. 2 b,c). Sharper graphene edges were observed in rGO-180, whilst the Ndoped sample showed a smoother surface. As mentioned above, N-rGO-180 exhibited the highest nitrogen content (13.4 at%), including the highest contribution of pyridinic and quaternary nitrogen. Based on this, N-rGO-180 was selected and tested as an active material for electrodes in supercapacitors. 3.2.

Pseudocapacitive effect of nitrogen

Cyclic voltammetry, galvanostatic charge-discharge cycling, and electrochemical impedance spectroscopy were conducted in order to evaluate the supercapacitor properties of N-rGO-180 in comparison to rGO-180, prepared without N-dopant. Fig. 3 shows the cyclic voltammetry curves of rGO and N-rGO-180 in a three electrode system at different scan rates. rGO-180 shows reversible humps (at approximately -0.6 V vs. Hg|HgO), which are related to the pseudofaradaic contribution of surface oxygen groups. Quinone and hydroxyl groups (3.9 and 10.1 at%, respectively) are responsible for the reversible redox reactions taking place during the charging and discharging (Fig. 4) [12,32]. In the case of N-rGO-180, reversible humps are also visible, although they are shifted to lower potential values. These peaks might be associated with the pseudocapacitive effect of electrochemically active nitrogen groups, such as pyrrolic and pyridinic groups (Fig. 4). Very high capacitance values of 249 F g-1 and 250 F g-1 were calculated for rGO-180 and N-rGO-180, respectively, from the CV curves obtained at a scan rate of 1 mV s-1. At 100 mV s-1, rGO-180 exhibited a capacitance value of 175 F g-1. In contrast, N-rGO-180 maintained a very high capacitance of 244 F g-1 at this scan rate. This outstanding rate capability from 1 to 100 mV s -1 (98%) has never been reported for N-graphenes to date (Fig. 5a). The high content of quaternary 11

nitrogen in N-rGO-180 improves the conductivity of the electrode material, leading to faster charge propagation [33] and efficient charge storage at very short times. Fig. 6 shows the galvanostatic charge/discharge curves of rGO-180 and N-rGO-180 at different current densities. A nearly symmetric triangular shape and small voltage drop can be observed even at a current density of 20 A g-1, which indicates a double-layer capacitive behaviour and fast ion and electron transport through the electrode material [11]. At a current load of 0.2 A g-1, N-rGO-180 and rGO-180 reach high capacitance values of 239 and 228 F g-1, respectively. However, the capacitance of rGO-180 decreases with the current load with a capacitance value of 142 F g -1 obtained at 20 A g-1 from the discharge curve. Alternatively, N-rGO-180 exhibited very high rate capability of 87% from 0.2 to 20 A g-1, maintaining a capacitance of 209 F g-1 at 20 A g-1, while rGO-180 reaches only 62% (Fig. 5b). It must be noticed that N-rGO-180 has a higher capacitance than rGO-180, despite having a lower specific surface area (354 m2 g-1 and 530 m2 g-1, respectively). Li et al. [11] obtained a N-rGO with a specific surface area of 677 m2 g-1, but the capacitance value at 100 mV s-1 was only ~200 F g-1. However, Lee et al. [9] synthesized a N-rGO with a BET surface area similar to that of our N-rGO-180 (355 m2 g-1), but with a much lower nitrogen content of 3 at%, exhibiting a significantly lower capacitance value of 182 F g-1 at 25 mV s-1 in KOH electrolyte. The presence of pyridinic and pyrrolic groups [34] and quaternary nitrogen combine to enhance the conducting properties of N-rGO-180 [35], resulting in superior electrochemical properties. Introducing large amounts of nitrogen in the graphene structure, therefore, appears to be a good strategy to reach high capacitance values, even with low specific surface areas.

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The EIS data, expressed as Nyquist plots over the frequency range of 10 mHz to 100 kHz, are shown in Fig. 7a. The equivalent series resistance (ESR) measured at 100 kHz is very low (0.1 Ω) for both materials, indicating their excellent conductivity properties. The semicircle in the high frequency region is larger in the case of N-rGO180, which can be ascribed to the charge-transfer resistance related to the faradic processes derived from its high nitrogen content. In the middle-frequency region, a 45° slope in the spectra of rGO-180 is indicative of the resistance associated with the diffusion of electrolyte ions into the electrode material. This feature is considerably less prominent in the spectra of N-rGO-180, probably due to the enhanced conductivity of the material. The knee frequency delimits this sloppy region and the vertical branch at low frequencies. It can be observed that in the N-doped sample, the electrolyte ions are able to access the whole surface of the active material at higher frequencies. Fig. 7b shows the capacitance as a function of frequency. Higher capacitance values were obtained for N-rGO-180 compared to rGO-180, which reflects the unique structure of the N-doped graphene. The cycling stability of rGO-180 and N-rGO-180 is plotted in Fig. 8. After 5000 cycles, rGO-180 suffers from decay in the capacitance value of approximately 29%, probably due to the presence of oxygen groups in the graphene structure that are not stable during long-term cycling. Very high capacitance values (over 215 F g-1) were observed for N-rGO-180 after 5000 cycles. The high retention of 92% after 5000 cycles proves the stability of the nitrogen functionalities, as well as the reversibility of their faradaic reactions.

4.

Conclusions

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Nitrogen-doped graphenes with a nitrogen content as high as 13.4 at% were prepared by a simple hydrothermal reduction of graphene oxide in the presence of amitrole. Despite a low surface area (354 m2 g-1), N-rGO-180 reached very high capacitance values of 250 F g-1 at 1 mV s-1 when tested as electrode material in supercapacitors. Nitrogen fixation occurred mostly in the form of heterocyclic and quaternary nitrogen and was responsible for pseudocapacitive behaviour and enhanced charge propagation and ion diffusion compared to the N-free reduced graphene oxide. The superiority of the N-doped material was especially evident at high current densities and scan rates. An uprecedented rate capability of 98% with increasing scan rates, maintaining 244 F g-1 at 100 mV s-1, was observed in N-rGO-180, while the capacity of the non-doped graphene was only 175 F g-1. Also, 92% of the initial capacity was maintained after 5000 charge/discharge cycles due to the good stability of the nitrogen groups. Accordingly, amitrole can be regarded as an effective nitrogen dopant for the synthesis of highly doped N-graphenes with excellent performance when used in high-power systems.

Acknowledgements The research leading to these results has received funding from the European Union´s Research Fund for Coal and Steel (RFCS) research program under grant agreement RFCR-CT-2013-00006.

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19

8

a)

6

GO

b)

C-O

GO

4

2

Csp2 C=O

COOH 0 8

rGO-180

rGO-180 6

4

2

N-rGO-150

400

8

N6 6 N5

NQ NX

N-rGO-150

Intensity, a.u.

Intensity, a.u.

1000

4

NC 2

C-N/C-O 4

Csp2

6

8

10

COOH C=O

200

N-rGO-180

N-rGO-180

0 1800

N-rGO-200

1600

N-rGO-200

1400

200

410

0

0

405

400

Binding energy, eV

0

395

292 290 288 286 284 282 280

Binding energy, eV

Fig. 1. N1s (a) and C1s (b) XPS spectra of GO, rGO, and N-rGOs.

20

b)

Intensity / a.u.

a)

GO 100 nm

rGO-180

c) N-rGO-200 N-rGO-180 N-rGO-150 10

20

30

40

50

60

100 nm

2 theta / °

Fig. 2. XRD patterns of GO, rGO-180, and various N-rGOs (a); FE-SEM images of rGO-180 (b); and N-rGO-180 (c).

21

Fig. 3. Cyclic voltammograms of rGO-180 (a) and N-rGO-180 (b) in 6 mol L-1 KOH at different scan rates.

22

O

OH

+

2H2O

+ 2e-

O

+ 2OHOH H N

N

+

H2O

+

+ OH-

e-

H N

N

+

H2O

+ e-

+ OH-

Fig. 4. Redox reactions of electrochemically active oxygen and nitrogen groups.

23

Fig. 5. Rate capability in the function of scan rates (a) and current densities (b) for rGO-180 and N-rGO-180.

24

Fig. 6. Charge/discharge galvanostatic curves of rGO-180 and N-rGO-180 at 0.2 A g-1 (a), 1 A g-1 (b) and 20 A g-1 (c).

25

a)

b)

Fig. 7. Nyquist plots (a) and capacitance as a function of frequency (b) for rGO-180 and NrGO-180 in 6 mol L-1 KOH.

26

Fig. 8. Cyclic stability of rGO-180 and N-rGO-180 at a current density of 1 A g-1 in 6 mol L-1 KOH.

27

Table 1 XPS elemental composition and speciation of GO and hydrothermally treated samples Elemental composition (at%)

N1s peak deconvolution (at%)

C1s peak deconvolution (at%)

C

N

O

N6

NC

N5

NQ

NX

C=C

C-O C-N

C=O

COOH

GO

62.8

-

37.2

-

-

-

-

-

20.3

33.7

5.8

3.0

rGO-180

83.8

-

16.2

-

-

-

-

-

68.2

10.1

3.0

2.5

N-rGO-150

78.1

11.1

10.8

5.1

1.5

1.9

2.2

0.4

56.5

14.6

5.5

1.5

N-rGO-180

77.2

13.4

9.4

6.4

2.5

2.1

1.9

0.5

54.9

14.6

6.2

1.5

N-rGO-200

79.3

10.9

9.8

5.0

2.7

1.0

1.8

0.4

57.1

13.0

6.3

2.9

Sample

28